AUTOMATED HOLLISTER THERMAL INDEX
PREDICTION (TIP) PROGRAM
The Thermal Index (TI) at a given altitude is the difference between the observed air temperature and the temperature that a parcel of air would have if it started at the surface, as occurs in a thermal, and rose adiabatically (that is, without losing heat) to that altitude. The TI=0 height approximates the height of the tops of the thermals for the day, but the maximum thermalling height is expected to be somewhat lower due to the sailplane's descent rate and because the thermally mixed layer, or Boundary Layer (BL), is not truly adiabatic. At present the program computes heights associated with two TI criteria, -4°F and 0°F, to judge the thermal conditions. Ideally the maximum thermalling height could be estimated from a single TI criterion, but such a criterion is not well established. For example, traditionally reported TI criteria for the maximum thermalling height include the TI=-5.4°F(-3°C) criterion mentioned on Kevin Ford's TI calculation website and the TI=0°F(0°C) criterion used for the Reno NWS Soaring Forecast. Also, the maximum thermalling height will depend upon glider performance and pilot proficiency. Actual soaring experience at Hollister may help establish a reasonable TI criterion for that site. The uncertainty in the TI criterion is likely smaller than the uncertainly due to the average error in the forecast surface temperature maximum, which is about 4°F for Hollister as discussed below. A more detailed explanation of soundings and the theory behind the Thermal Index is available at the TIP Sounding Analysis webpage. [Note: The TI has units of temperature and when used on other websites is generally given in degrees Celsius, °C, not Fahrenheit, although that fact may be obscured when a website neglects to provide units when referring to the TI. For this program, however, all TIs will be given in Fahrenheit,°F, and this is implicit when the TI units are omitted for convenience. This choice more easily allows a change in the surface temperature maximum Tmax, which is reported in °F, to be translated into a change in the predicted TI aloft, since there is then an inverse 1:1 correspondence between the two; that is, a 1°F increase/decrease in Tmax will decrease/increase the predicted TI at a given level by 1°F.]
The following sections describe the Hollister
TIP email and the methods used to obtain its predictions - more
information than you likely want to know! The only section I
consider mandatory reading is the following SUMMARY DESCRIPTION, which describes
information provided in the email's Subject line and in the
SUMMARY NUMBERS section at the top of the TIP. The
other sections can be read when the user desires further information.
Predictions based upon th afternoon (PM) sounding are also provided because most flights occur in the afternoon and conditions then may have altered from those of the morning sounding, particularly near times of frontal passage. However, calculation of PM TI=-4 and TI=0 heights is subject to mismatches between the TI-method assumptions and the model-predicted soundings, as described below, so misleading heights can result. Therefore, PM conditions are reported as a change in the thermalling conditions, specifically as an increase or decrease of the TI=+4 height since it is less subject to such mismatches. PM predictions are thus made in a relative fashion and other thermalling parameters are expected to similarly change for the better/worse. This number provides a ballpark estimate of the changes in the maximum thermalling height, but because it must be computed at the TI=+4 level instead of the TI=-4 or TI=0 level this change is only an approximation to the change at those levels. [A subtle and perhaps confusing point is that both AM and PM soundings are used to predict predict soaring conditions at the time of Tmax (which is why the same Tmax is used for both predictions) - the AM/PM designators refer to the sounding used for the prediction and do not indicate that the predictions are intended to be for the morning and afternoon hours respectively.]
The summary numbers given in the email's Subject line are the TIP results of primary interest, typically giving enough information that the email can be deleted without needing to open it. Alternatively, these numbers are also given in the SUMMARY NUMBERS section at the top of the TIP, together with a textual description for each. The Subject line will look something like:
HolTIP: am>6178,8058|6|610 (-2) pm>-1373 MONam>5000,7000 MONpm>-1218 TUEam>4833,6636
and the corresponding SUMMARY NUMBERS section, which includes additional information, would be
AM Avg. TI HEIGHTS: 6178,8058 ftMSL @TI=-4,0degF AM Avg. Hcrit HGT: 6069 ftMSL (Max. _flat_terrain_ thermalling height) AM HGT VARIABILITY: 1870 ft (from TI=0-+4degF) AM Avg. Buoy/Shear: 6 (thermals may be unworkable if 5 or less) AM Avg UPDRAFT W* : 610 fpm (subtract glider sinkrate to get vario) PM Avg HGT CHANGE : -1373 ft @TI=+4degF Tmax UNCERTAINTY : -2 degF (deviation between NWS & WxC Tmax forecasts) BL Max. Rel.Humid.: AM= 87% PM= 95% AM Extensive CLOUD: Dif= 3733 ft LCL= 4318 ftMSL (if Dif>0, expect OD at LCL) PM Extensive CLOUD: Dif= 915 ft LCL= 4653 ftMSL (if Dif>0, expect OD at LCL) THUNDERSTORM CAPE : AM= 0 PM= 0 (if CAPE>0, thunderstorms possible) MON AM HEIGHTS: 5000,7000 ftMSL @TI=-4,0degF & Hcrit= 5144 ftMSL MON PM CHANGE: -1218 ft @TI=+4 TUE AM HEIGHTS: 4833,6636 ftMSL @TI=-4,0degF & Hcrit= 4955 ftMSL
Both indicate that today's estimated thermalling height over flat terrain is 6178 ftMSL (the TI=-4 height) based upon the AM sounding (a large height for Hollister due to passage of a front just a few hours before). Conditions are expected to weaken in the afternoon since the TI=+4 height decreases by -1373 ft. For tomorrow (MON), condiitions are expected to be weaker, the predicted TI=-4 thermalling height being 5000 ftMSL for the AM and again lowering in the PM (-1218 ft). For the following day (TUE), the AM estimated thermalling height (TI=-4) is 4833 ftMSL, slightly lower but comparable to conditions on MON.
The number 6 following today's AM predicted TIs is the estimated Buoyancy/Shear "B/S" ratio: if this is 5 or less the thermals may be broken up by wind shear. The following 610 is the estimated average upward velocity of thermals "W*" in fpm (subtract the sailplane's descent rate to get the actual average rate of climb). Following that, in parenthesis, is the difference between the NWS (National Weather Service) and WxC (WeatherChannel) Tmax predictions as an indication of the Tmax forecast uncertainly for that day: today the predicted WxC Tmax is 2°F cooler than the predicted NWS Tmax.
The SUMMARY
NUMBERS section also provides additional information
not given in the Subject line.; Hcrit provides an
alternative (and more theoretically defensible) estimate of the
maximum thermalling height over flat terrain; here Hcrit is
close to the TI=-4 height, as is usually the case, but under some
circumstances the two can differ significantly - which is the more
accurate number in such cases is presently undetermined (and I would
like to hear of any flights which would help such a
determination). Today's thermalling height variability, which is
also the sensitivity of today's predicted TI heights to error in the
forecast maximum temperature, Tmax, is rather large
(
The AM and PM prediction summary numbers each have strengths and weaknesses. Both should be considered when forecasting expected thermalling conditions for a given day, but the PM forecast is particularly important due to its proximity to the flight period. AM and PM predictions in the Subject line are based upon the NWS Tmax forecast. Today's AM values are based upon the morning observed Oakland sounding while all other values represent the average of the MAPS and ETA model TI predictions (when both model forecasts are available).
Note that a large (greater than 2000 ft, say) difference between the TI=-4 and TI=0 heights indicates that the "maximum thermalling height" for that day is especially sensitive to changes (or errors) in the maximum surface temperature. Also, in such cases it is more likely that better thermalling conditions will occur over elevated terrain.
When utilizing and evaluating the TI predictions, it is important to recognize that these predictions assume relatively simple thermalling conditions, i.e. cloudless thermals developing over flat terrain, and thus the TIP cannot numerically forecast all thermalling conditions. But relative predictions of "better" or "worse" conditions should still be useful even when complicating factors occur.
One complicating factor is terrain. Thermals over elevated terrain will have higher tops than those over the nearby valley. Often this difference can be allowed for by simply using a larger TI criterion to estimate the thermal top over elevated terrain. Such adjustments are necessarily empirical, depending for example on the height of the terrain. So the criterion value to be associated with a specific terrain feature must be established through previous experience at that feature.
A complex complicating factor is cloud formation. As discussed below, cloud formation produces stronger updrafts than predicted by the TIP due to additional buoyancy produced aloft, particularly for deeper convective clouds. The thermal strength W* and the Buoyancy/Shear ratio B/S will then be larger than the TIP predicted values. The height of the thermal also increases. becoming the cloud top height, but the maximum soaring height is now limited by the cloud base instead of the thermal top. In such cases the maximum soaring height is often below the maximum thermalling height predicted by the TIP, since the condensation occurs below the height reached by a dry thermal. In the presence of convective clouds the TIP is probably best thought of as providing a minimum estimate of thermalling conditions, with thermalling conditions expected be better than predicted - here "thermalling conditions" is to be interpreted as meaning updraft strength, since the maximum soaring height is now decoupled from the maximum thermal height, which is what the TIP TI heights predict.
Please note that the TIP is not a complete "Soaring Forecast". The TIP is a tool which provides one important piece of the soaring forecast puzzle, that of forecasting thermalling conditions, in an easily digested package. It is a tool which uses computer automation to take some of the drudgery out of soaring predictions and provide information which pilots generally do not have the time or knowledge to obtain individually. But the TIP only considers one aspect of soaring weather - that of thermal lift. The TIP says nothing about lift from another sources, such as wave or shear, so pilots cannot just look at the TIP forecast and expect to know whether today is a "good" soaring day in the general sense of having some kind of strong lift. In addition, other weather factors affecting soaring, such as ceiling height and precipitation, are not considered by the TIP's summary numbers.
Forecasting the weather will never be like reading an airspeed indicator. Intelligent interpretation of the TIP, based upon an understanding of its methodology and limitations, will give a more realistic expectation of expected conditions than is obtained by simply accepting its numbers without evaluation. And of course the TIP forecasts are no better than the Tmax and sounding information from which they are calculated. If, for example, the pilot judges that the actual Tmax will be warmer/cooler than predicted, he should then expect thermalling conditions to be better/worse than predicted by the TIP.
Notes:
· The email is intended to be read with a fixed font so
that similar columns align vertically but can be read with a
proportional font (except for the temperature profile plot,
which requires a fixed font for proper presentation).
· Since email recipients will often not be reading the
body of the email, if a new note is added the email Subject
line will be begin with the message "NEW_NOTE" for that day
only.
The email body consists of the following main sections:
The "NEAREST NWS TAF FORECAST" email section [not included in the sample email provided below] gives the NWS Terminal Aerodrome Forecast (TAF) nearest to the TIP location, which hopefully will be relevant to the TIP location. This section provides information on cloud bases (notably for clouds above the BL, which the TIP does not predict), surface winds, and precipitation.
The "WxC HOURLY FORECASTS" email section gives hourly weather conditions predicted by the Weather Channel. It is particularly useful for judging the time of maximum heating, which generally occurs when Tmax is largest, and whether sky conditions or wind speeds are expected to change significantly.
The "THERMAL INDEX ANALYSES" email section consists of several subsections. The "TODAY's ANALYSIS OF MORNING SOUNDING" subsection gives an extensive analysis of the morning atmospheric sounding observed at Oakland, providing information for those who want to gauge the uncertainty in today's TI predictions. It gives the TI heights forecast by several possible Tmaxs: the NWS prediction, the WxC prediction, and their AVeraGe. The resulting TI variation is an indication of TI uncertainty due to uncertainty in Tmax prediction. The second and third subsections, titled "MAPS MODEL FORECAST TIs" and "ETA MODEL FORECAST TIs" give individual TI predictions for this afternoon and the following two days from the two different models. For the AM soundings the TI=-4 and TI=0 heights are given, while for the PM soundings the change in the TI=+4 height is given.our summary sections are ordered by time. If enabled, there will be an additional subsection titled "POST-ANALYSIS OF YESTERDAY's ACTUAL CONDITIONS" [not included in the sample email provided below] giving yesterday's observed Tmax at Hollister and yesterday's NWS and WxC forecast Tmax as an indication of the degree of Tmax error in yesterday's TI forecast.
The next two email sections are titled "TODAY's *AM* TI LISTING..." and "TODAY's *PM* TI LISTING...". They provide detailed listings of the "best guess" AM and PM model soundings, normally MAPS model soundings using the NWS Tmax (when available), and winds are given for different heights. The feature of interest is often the plot depicting the atmospheric temperature structure in an "unskewed-T" form, which is more easily readable than a skew-T plot, and also plots the adiabat corresponding to Tmax. For the AM sounding, the point at which the two curves cross is the predicted TI=0 height. The best estimate of the PM thermalling height is obtained through analysis of the PM sounding - however, to be effective such interpretation requires knowledge of the PM mixing layer structure, as discussed briefly in the PM analysis section below At the end of each of these listing sections is "CLOUD FORMATION PARAMETERS", which gives cloudiness estimates.
If enabled, an email section titled "POST-ANALYSIS TI LISTING" [not included in the sample email provided below] gives yesterday's predicted PM sounding with yesterday's observed Tmax at Hollister (when available).
The final email section "MISCELLANEOUS NOTES" can include
updated information which supersedes the descriptions given on this
web page.
The sample email is the same as used for the
Subject line and SUMMARY
NUMBERS examples used previously. To
recapitulate, the Subject line for this example is:
HolTIP: am>6178,8058|6|610 (-2)
pm>-1373 MONam>5000,7000 MONpm>-1218 TUEam>4833,6636
The first pair of numbers gives the prediction for today's heights
at which TI=-4°F and TI=0°F, the buoyancy/windshear
ratio B/S follows the first vertical bar, and the predicted thermal strength
W* follows the second vertical bar. These predictions are based upon
the NWS temperature prediction, and the temperature difference between
the NWS and WxC predictions is given in parentheses. These predictions
for the current day are followed by TI height forecasts for this afternoon,
for tomorrow, and for the following day.
Sample Email
A sample email example is (you can open a
new browser window to keep this example in view when reading the descriptions
given below - you may need to widen the window to see the entire
line. A fixed font is used to display the email so that plotted values
will be correctly aligned):
DrJack's TIP (Thermal Index Prediction) for HOLLISTER on SUN Nov 25 ***************************************************************************** ****************** SUMMARY NUMBERS - based on NWS Tmax only ***************** AM Avg. TI HEIGHTS: 6178,8058 ftMSL @TI=-4,0degF AM Avg. Hcrit HGT: 6069 ftMSL (Max. _flat_terrain_ thermalling height) AM HGT VARIABILITY: 1870 ft (from TI=0-+4degF) AM Avg. Buoy/Shear: 6 (thermals may be unworkable if 5 or less) AM Avg UPDRAFT W* : 610 fpm (subtract glider sinkrate to get vario) PM Avg HGT CHANGE : -1373 ft @TI=+4degF Tmax UNCERTAINTY : -2 degF (deviation between NWS & WxC Tmax forecasts) BL Max. Rel.Humid.: AM= 87% PM= 95% AM Extensive CLOUD: Dif= 3733 ft LCL= 4318 ftMSL (if Dif>0, expect OD at LCL) PM Extensive CLOUD: Dif= 915 ft LCL= 4653 ftMSL (if Dif>0, expect OD at LCL) THUNDERSTORM CAPE : AM= 0 PM= 0 (if CAPE>0, thunderstorms possible) MON AM HEIGHTS: 5000,7000 ftMSL @TI=-4,0degF & Hcrit= 5144 ftMSL MON PM CHANGE: -1218 ft @TI=+4 TUE AM HEIGHTS: 4833,6636 ftMSL @TI=-4,0degF & Hcrit= 4955 ftMSL ******************************************************************************* ******************** WxC HOURLY FORECASTS ************************************* 95023 WxC hourly for 11/25 @ 9AM 10AM 11AM 12PM 1PM 2PM 3PM 4PM Sfc. Temperature (F) = 46 49 53 56 58 58 57 55 Sfc. Wind Speed (kt) = 6 6 6 6 6 6 6 6 Sfc. Wind Direction = S S S SW W SW W W Precip.Probabil. (%) = 20 20 20 20 20 20 30 40 Weather Type = Prtly Prtly Prtly Prtly Prtly Prtly Prtly Prtly Cldy Cldy Cldy Cldy Cldy Cldy Cldy Cldy ******************************************************************************* ******************** THERMAL INDEX ANALYSES *********************************** TODAY's ANALYSIS of MORNING SOUNDING at OAK @ 11/25:12Z for Sfc= 230 ftMSL --------------------------------------------------------------------------- NWS Tmax= 59 > TI=-4,0@ 6178, 8058 Hcrit= 6069 HgtVar=1870 B/S= 6 W*= 610 WxC Tmax= 57 > TI=-4,0@ 4833, 6843 Hcrit= 4913 HgtVar=1907 B/S= 6 W*= 509 AVG Tmax= 58 > TI=-4,0@ 5642, 7357 Hcrit= 5422 HgtVar=1920 B/S= 6 W*= 558 MAPS MODEL FORECAST TIs at OAK for Sfc= 230 ftMSL -------------------------------------------------- am NWS Tmax= 59 > TI=-4,0@ 5000, 7450 Hcrit= 5511 HgtVar=2216 B/S= 5 W*= 566 MAPSanl:11/25:12Z pm NWS Tmax= 59 > Change@TI=+4: -1726 -4.5F MAPS12h:11/26:00Z ETA MODEL FORECAST TIs at OAK for Sfc= 230 ftMSL ------------------------------------------------- am NWS Tmax= 59 > TI=-4,0@ 5142, 7875 Hcrit= 5889 HgtVar=2513 B/S= 8 W*= 592 ETAanal:11/25:12Z pm NWS Tmax= 59 > Change@TI=+4: -1020 -2.4F ETA12hr:11/26:00Z MONam NWS Tmax= 60 > TI=-4,0@ 5000, 7000 Hcrit= 5144 HgtVar=1281 B/S=33 W*= 551 ETA24hr:11/26:12Z MONpm NWS Tmax= 60 > Change@TI=+4: -1218 -4.2F ETA36hr:11/27:00Z TUEam wxc Tmax= 64 > TI=-4,0@ 4833, 6636 Hcrit= 4955 HgtVar=1721 B/S=13 W*= 584 ETA48hr:11/27:12Z ******************************************************************************* **************** TODAY's *AM* TI LISTING using RAOB+NWStmax *************** 25-NOV-2001 12 UTC TI report from OAK OBSraob upper air data. Forecast max Temp.: 59.0 F Forecast max VirtT: 60.6 F Forecast sfc VirtT: 60.8 F Raob est. max temp: 54.1 F Surface elev Temp: 48.8 F Surface elev VirtT: 50.6 F Surface elevation: 230 ftMSL SfcAdjust:1 Station elevation: 10 ftMSL Lowest elevation: 404 ftMSL Lowest elev Temp: 48.6 F Lowest elev VirtT: 50.3 F Lifted Index @700mb: 0.9 C Sfc-Lift Cond Lev: 1342 ftMSL Lift Cond Lev (LCL): 2657 ftMSL Conv Cond Lev (CCL): 2967 ftMSL Ford est. base of any clouds: 4369 ftMSL Lowest elev windspeed: 5 kt Mixing Layer windspeed: 18 kt Convection overcast height: 6383 ftMSL Convection TI=0 height: 8051 ftMSL Height *TI* Wind TI=0 Tv ------ Temperature Profile Plots ------ ftMSL degF deg kts trig degF o=Tv .=DewPt *=Adiabat 1.2 degF/division ----- ---- ------- ---- . ---- --------------------------------------------- 12000 5.6 65 | 3.3 11500 5.3 300 28 64 | 5.7 11000 4.9 64 | 8.0 10500 4.5 63 | 10.2 o 10000 4.1 63 | 12.5 * o 9500 3.4 300 26 62 | 14.5 * o 9000 2.5 61 | 16.2 * o 8500 1.5 289 27 61 | 18.0 * o 8000 -0.2 286 27 59 | 18.9 . o 7500 -0.8 58 | 21.0 . o* 7000 -1.5 290 26 58 | 23.0 . o* 6500 -3.1 56 | 24.0 . o * 6000 -4.5 299 23 54 | 25.3 . o * 5500 -5.2 54 | 27.3 . o * 5000 -5.8 53 | 29.3 . o * 4500 -6.4 305 21 53 | 31.4 . o * 4000 -6.9 52 | 33.6 . o * 3500 -7.3 52 | 35.8 . o * 3000 -7.8 51 | 38.0 . o * 2500 -8.3 295 15 51 | 40.2 . o * 2000 -8.5 50 | 42.6 . o * 1500 -8.8 50 | 45.0 . o * 1000 -9.1 50 | 47.4 . o * 500 -9.3 235 5 50 ! 49.9 . o * CLOUD FORMATION PARAMETERS from AM sounding ------------------------------------------- Expect extensive cloud formation at LCL when LCLdiff positive or over mountains when LCL is (approx) below mt top (1stLine=_no_warming, 2ndLine=_with_warming) Thunderstorms possible when CAPE is positive BLmaxRH LCLdiff LCL CCL CAPE Zsfc= 230 TI=0@ 8051 87 3733 4318 4574 0 83 2285 5766 5880 0 ******************************************************************************* **************** TODAY's *PM* TI LISTING using MAPSpm+NWStmax ***************** *NB* TI values below pm model-predicted mixing height are meaningless ! 26-Nov-2001 00 UTC TI report from OAK MAPS12h upper air data. Forecast max Temp.: 59.0 F Forecast max VirtT: 60.9 F Forecast sfc VirtT: 61.0 F Surface elev Temp: 52.9 F Surface elev VirtT: 54.9 F Surface elevation: 230 ftMSL SfcAdjust:1 Station elevation: 518 ftMSL Lowest elevation: 230 ftMSL Lowest elev Temp: 52.9 F Lowest elev VirtT: 54.9 F Ford est. base of any clouds: 4033 ftMSL Lowest elev windspeed: 5 kt Mixing Layer windspeed: 7 kt Convection overcast height: 4830 ftMSL Convection TI=0 height: 5568 ftMSL Height *TI* Wind TI=0 Tv ------ Temperature Profile Plots ------ ftMSL degF deg kts trig degF o=Tv .=DewPt *=Adiabat 1.2 degF/division ----- ---- ------- ---- . ---- --------------------------------------------- 12000 14.3 73 | 12.3 o 11500 12.9 312 26 72 | 13.5 o 11000 11.9 71 | 15.3 o 10500 11.0 70 | 17.0 o 10000 10.1 319 22 69 | 18.8 * o 9500 8.8 68 | 20.2 * o 9000 7.6 67 | 21.6 * o 8500 6.4 65 | 23.1 * o 8000 5.2 319 13 64 | 24.6 * o 7500 4.1 63 | 26.2 * o 7000 3.2 62 | 27.9 . * o 6500 2.2 61 | 29.6 . * o 6000 1.2 60 | 31.2 . *o 5500 -0.2 264 8 59 | 32.5 . o 5000 -1.7 240 8 57 | 33.7 . o * 4500 -2.7 229 8 56 | 35.4 . o * 4000 -3.3 210 9 56 | 37.4 . o * 3500 -4.1 207 8 55 | 39.3 . o * 3000 -4.5 210 7 54 | 41.6 . o * 2500 -4.6 211 7 54 | 44.2 .o * 2000 -4.7 215 7 54 | 46.7 . o * 1500 -4.6 217 7 54 | 49.5 . o * 1000 -4.7 221 7 54 | 52.1 . o * 500 -5.0 224 6 54 ! 54.4 . o * CLOUD FORMATION PARAMETERS from PM sounding ------------------------------------------- Expect extensive cloud formation at LCL when LCLdiff positive or over mountains when LCL is (approx) below mt top (1stLine=_no_warming, 2ndLine=_with_warming) Thunderstorms possible when CAPE is positive BLmaxRH LCLdiff LCL CCL CAPE Zsfc= 230 TI=0@ 5568 95 1726 3842 4171 0 95 915 4653 4693 0 ****************************************************************************** ******************** MISCELLANEOUS NOTES ************************************* Addtional local weather information is available at the Tmax data sites: http://www.wunderground.com/cgi-bin/findweather/getForecast?query=95023 http://www.weather.com/weather/local/95023 and at the hourly data site: http://www.weather.com/weather/hourbyhour/95023 A description of the Hollister Thermal Index Prediction (TIP) program is given at: http://www.drjack.info/INFO/holti.htm First-time TIP users are strongly encouraged to read the second section of that description, titled "SUMMARY DESCRIPTON", for information on how to interpret the email's Subject line, which gives the basic TIP forecasts. Descriptions of how some pilots use the TIP for their own soaring flights are given at: http://www.drjack.info/TIP/TIPexperience.html The latest TIP forecasts are also available on the web at: http://www.drjack.info/TIP/index.html The hourly-updated BLIPSPOT forecasts of thermal soaring parameters at specific locations for various times throughout the day are available on the web at: http://www.drjack.info/BLIP/RUC/SPOTindex.html The latest BLIPMAP area forecasts of thermal soaring parameters for the current day over CA/NV are available on the web at: http://www.drjack.info/BLIP/index.html Currently the TIP seems stable and I am only looking at it for my own soaring needs. If you do notice a continuing problem, please email me. UPDATES to the description at http://www.drjack.info/INFO/holti.htm:
Subsection: TODAY's ANALYSIS of MORNING
SOUNDING
This subsection predicts the current day's
thermalling conditions based upon the observed morning sounding released
from Oakland for predicted temperature maxima obtained from NWS
and WxC.
[For sites with no local observed sounding, or for Hollister if the
Oakland observed sounding is missing, the predictions here will employ
model-predicted soundings rather than an observed sounding.]
The subsection title is followed by the date and time (in UCT) of the sounding
analyzed, as a check that that morning's observation has actually been
used. For the sample case, the Oakland sounding is for November 25
at 12Z=4AMpst.
The next lines give the TI heights calculated
for each of three different surface temperature predictions. The
identifier for the surface prediction source, either "NWS"
or "WxC" or their average
"AVG", begins each TI estimation
line, followed by the predicted surface temperature maximum Tmax in degrees
Fahrenheit ("Tmax=").
In the example, the predicted
NWS
temperature is 59°F.
The number pair on each line, following "TI=-4,0@",
give the primary program result, the calculated heights in feet MSL for
TI values of -4°F and 0°F respectively. These heights
are 6178 and 8058 ftMSL for the NWS
case. Heights for two criterion values are provided to help each
pilot make his own prediction of the maximum thermalling height, based
upon his own criterion - however, I would suggest use of the TI=-4
height as a starting point for thermalling over flat terrain. Comparison
of the two different TI heights, computed for a for a single Tmax,
will help the user evaluate the sensitivity of the maximum thermalling
height prediction to the assumed TI value and also its sensitivity to the
predicted Tmax. If the TI=0 height is much larger than the TI=-4
height, such as over 2000 ft larger, then conditions in different locations,
such as over the hills, have a good chance of being substantially better
than those over the valley. Further, the TI forecast will then be more
sensitive to any error in the surface temperature prediction. On
days having a strong temperature inversion aloft the two numbers will both
be low and relatively close together (within 500 feet for example), indicating
that there is not much hope of good thermalling conditions.
The next number on the same line ("Hcrit=")
gives an alternative prediction of the maximum thermalling height expected
over flat terrain. often Hcrit is roughly comparable to the TI=-4
height, as is true for its present value of 6069 ftMSL. This parameter
is described in further detail below.
The next number indicates the expected variability
of the thermalling height, which is assumed to depend upon the atmospheric
stability above TI=0. This number is the difference between the TI=0
and TI=+4 heights in feet, here 1870 ft. A larger number indicates
that the maximum thermalling height will have greater variation from one thermal
to another.
The next number ("B/S=") is the Buoyancy/Windshear ("hot
air") ratio, here being 6. A small B/S ratio means that
windshear is more likely to be significant. At present, the best
estimate, based on limited pilot reports, is that for a B/S of 5 or
less the thermals are likely to be torn apart by windshear and
broken. Further discussion is given below.
The next
number ("W*=") gives the
predicted thermal strength, the upward velocity of air within
the thermal in feet per minute, 610 fpm for the NWS case. Note
that a sailplane's thermalling descent rate must be subtracted to give
its vertical velocity while thermalling. W* is calculated from
convective theory, as described further below.
Providing TI calculations for two different Tmax predictions, NWS and
WxC, aids in evaluating the uncertainty of the maximum thermalling
height resulting from uncertainty in the surface temperature
prediction. This uncertainty increases if the heights for
TI=-4 and TI=0 differ greatly, since this indicates
large sensitivity to a 4 °F error in the surface temperature
prediction. Note that there is an inverse 1:1 relationship
between the TI index at a given height and the predicted Tmax.
Subsection: MAPS MODEL FORECAST
TIs
This subsection gives
TI estimates based upon soundings from NOAA FSL's MAPS model,
providing forecasts for the morning and afternoon of the current
day. The format of the morning prediction is similar to that of
the previous subsection. For Hollister, comparison between the
AM model prediction and the prediction obtained from the observed AM
Oakland sounding in the previous section provide an estimate of the
model's initial state.
The afternoon prediction gives the relative
change expected, based upon the change in TI=+4 heights. This technique
is employed because TI=-4 values are often invalid for PM soundings, as discussed
further below.
In the above sample, thermalling conditions
are expected to significantly diminish in the afternoon, the change in the TI=+4
height being -1726 ft.
Subsection: ETA MODEL FORECAST
TIs
This subsection is similar to the
previous one except that a different numerical model is utilized, the
NWS's ETA model. An advantage of this model is that it provides
forecasts for tomorrow and for the day after, but this is achieved
through use of a coarser grid mesh so ETA predictions are expected to
be less accurate than those of the MAPS model. The ETA 24 hour
prediction likely reasonable, but the 48 hour prediction is more
speculative. Since models tend to have biases, qualitative
results - such as whether tomorrow's thermal conditions are forecast
to be better or worse than today's, is likely to be more valid than
the actual numbers produced. The ETA model is intended primarily
to forecast broad scale weather events and its resolution is marginal
for evaluating changes in the Boundary Layer. Further details
and more caveats are given below.
Subsection: POST-ANALYSIS OF
YESTERDAY's ACTUAL CONDITIONS
At present no TIP contains this subsection, due to a disk
failure on my machine
This subsection is
not included in the sample email presented above.
This subsection is only present when a
separate program is utilized to obtain the actual surface temperature
maximum observed at Hollister on the previous day. It gives a TI
analysis of the previous day for those who actually flew yesterday and
want to find out why yesterday's thermals did not agree with that
forecasted or for those interested in the scientific accuracy of the
TI predictions, such as prediction dependence upon Tmax accuracy or
how well the morning sounding represents that afternoon's observed
atmospheric conditions. The first sentences give yesterday's
observed Tmax and reviews the previous day's predictions from that
morning's sounding with the NWS and WxC temperature predictions.
The actual observed Tmax is then used to provide two TI analyses,
using yesterday's morning and yesterday's afternoon observed Oakland
soundings. The former indicates the "best" prediction that could
have been obtained from the morning sounding while the latter gives
the prediction that might have been obtained if clairvoyance allowed
the afternoon atmospheric conditions to be known early in the
morning. Differences between the two analyses indicate the
effect of atmospheric changes during the day, since the same Tmax is
used for both.
Afternoon
Predictions
The afternoon (PM)
predictions are of particular interest because most soaring flights
occur in the afternoon. Use of a PM sounding for TI predictions
allows the inclusion of atmospheric changes which occur after the AM
sounding time. Although the PM forecasts must be based upon
model forecast soundings, rather than observations, in general the
forecast PM soundings have agreed well with the observed PM sounding
and also with thermalling experience. However, difficulties
arise in the computation of summary numbers from those soundings, of
which users should be aware when utilizing the PM forecasts.
The TI method assumes that the PM
temperature profile will be an adiabat, representing the layer mixed
by the thermals, extending upward from the surface Tmax to the
unmixed atmosphere and all TI values are intended to be
referenced to an unmixed sounding. Numerical TI estimates
are therefore invalid for heights below the top of the model-predicted
mixing layer. The model calculates its own PM Tmax, which
can differ from the TI-assumed Tmax, and its own PM mixing
layer, with a temperature lapse rate which may not be exactly
adiabatic. As a result, the TI-assumed adiabat can intersect the
PM sounding in the model-predicted mixed layer or even not intersect
the model PM sounding at all. Predicted PM TI=-4
and TI=0 heights, particularly the former, are susceptible to being
below the model-predicted mixing height should that occur invalid and
misleading PM height estimates result. The PM TI=+4 height,
being higher in the atmosphere, generally is above the model-predicted
mixing layer and remains valid. Therefore the PM predictions are
given as the change of the TI=+4 height from AM to PM, in feet.
Thus PM predictions must made in a
relative fashion, by assuming a bettering/worsening of all
thermalling conditions (i.e. the TI=-4 and TI=0
heights and the B/S and W* parameters) similar to the change in the
TI=+4 height. The PM height change at the TI=-4
and TI=0 heights obviously will differ from that at the TI=+4 height,
but the TI=+4 height change is at least a reasonable approximation to
those height changes.
An additional
difficulty with the PM prediction is that calculation of both the B/S
and W* parameters require knowledge of the surface heat flux and at
present this cannot be estimated from the PM sounding, so no B/S or W*
parameters are given for the PM times.
A large change between AM and PM
forecasts generally reflects unsettled and changing atmospheric
conditions, indicating that the forecast uncertainty is
large. In such cases examination of the detailed PM
sounding plot is suggested to evaluate visually the forecast depth of
the mixing layer, within which the temperature profile is
approximately adiabatic. This requires some knowledge on the
part of the user, but this is a case where human intelligence will
provide a better evaluation than the simplicities used in a computer
analysis. The information necessary for such interpretations is
provided in the TIP
Sounding Analysis webpage.
TI Uncertainty
The
uncertainty in a given prediction of maximum thermalling height
results primarily from uncertainty in:
(1) the predicted surface
temperature maximum,
(2) the
difference between the sounding and actual conditions while soaring,
(3) the TI criterion used to
estimate the maximum thermalling height.
Prediction error can
also result from assumptions inherent in the analysis. For
instance, the TI prediction method assumes flatland conditions.
Further, the Hollister TI prediction is based upon the Oakland
sounding released over the coastal plain. Atmospheric profiles
over the mountains differ from those over the plain since, for
example, upslope flow alters environmental conditions there.
Thermalling heights in mountainous terrain are often significantly
higher are than estimated by a simple TI criterion due to local
variations induced by the terrain. The TI also assumes
non-condensing convection (though the density effect of water vapor
is considered). Cloud formation during the day releases
latent heat which provides additional buoyancy aloft and increases
updraft intensity, producing higher thermalling heights than
calculated by TI prediction. Large cloud systems can also
significantly alter the atmospheric environmental conditions.
Use of a single sounding also assumes time-invariant conditions.
During rapidly changing conditions, such as occurs with a frontal
passage, the atmospheric profile during the day may differ
considerably from that of the early morning hours. Of course,
the TI estimates consider only thermal lift, not frontal or
convergence lift.
Tmax Uncertainty
Evaluation of TI Prediction Uncertainty
Critical Height (Hcrit)
Buoyancy/Shear Ratio (B/S)
Thermal Strength (W*)
Cloud Parameters
Neglect of Cloud-Generated
Buoyancy
Model Forecasts
Adjustment of Surface Elevation
Day After Tomorrow Tmax
TI Analysis Method
Program Description
ToDo? Contacting the Author
The degree of uncertainty of the
predicted temperature maximum is important for evaluating TI
prediction uncertainty. By noting differences between the NWS
and WxC temperature predictions over several weeks, the user will gain
some appreciation of the amount of error inherent in such predictions
at Hollister. I have made a statistical comparison of observed
(as reported by Hollister's Maze
middle school website) to predicted Hollister temperatures for a 8
month period. The NWS predictions average 1.6°F warmer than
the the observed temperature maxima and thus tend to predict
thermalling heights that are too high; however, their
day-
An overall conclusion is that the Tmax
forecast error is large compared to typical temperature
variations. A historical analysis
of Hollister TIs finds that the average Tmax forecast error
corresponds to a TI forecast error of roughly 850 ft. It should
be noted that the historical analysis only considers 8 months,
including summer months when Tmax can be difficult to forecast because
it is greatly affected by non-local phenomena such as the sea breeze
and fog/stratus created by the ocean - so the average error
during the other months may be lower. Still, we need better Tmax
predictions! Tell your congressman to give the NWS more
money! It is likely that the principal error in the TI forecasts
is due to Tmax error.
The source of the
NWS temperature prediction for Hollister is a NWS
website while the source of the WxC prediction is a Weather
Channel website. I have found that the WxC often "updates"
it's surface temperature prediction around 8:15 AMpst, after the TI
prediction program has run, so on some days a later calculation would
give a different TI result based upon a possibly more accurate
temperature prediction. Should be TI program be run at a later
time, such as 8:30, to incorporate this update? At present I
believe it best to have as early a prediction as possible, but that
may change.
The TIP email provides many outputs to aid
in the evaluation of TI uncertainty. For the same sounding, comparison
of TI predictions from differing Tmax values indicates the TI uncertainty
due to Tmax uncertainly. For the same Tmax value, comparison of TI
predictions from differing model soundings indicates the TI uncertainty
due to model error. In addition to those uncertainties, there is
uncertainty resulting from possible cloud formation or terrain effects
-
but these cannot be presently evaluated.
More exactly described as the Height of
Critical Updraft Velocity, this parameter estimates the maximum
thermalling height over flat terrain under cloudless conditions.
Hcrit is obtained from an averaged profile of thermal updraft velocity
vs. height (as obtained from aircraft measurements by Lenschow and
Stephens) by assuming the thermal strength W* to be the maximum
updraft velocity in the BL and computing the height at which the
updraft velocity drops below 225 fpm (a rough estimate of the sink
rate of a sailplane or hang glider actively turning and maneuvering to
remain with in a thermal). The intent is to obtain a better
estimate of the maximum thermalling height than is provided by the
TI=-4 height, since the latter often has no meaning for a PM sounding; in
addition, the TI=-4 height seems to overestimate the maximum
thermalling height under weaker conditions, and Hcrit is expected to
be smaller than the TI=-4 height under such conditions. But
the superiority of Hcrit over the TI=-4 height as a predictor of the
maximum thermal soaring height has not yet been been demonstrated in
practice so I would be interested to learn of any flights under
conditions when the two differ significantly, which would help
such an evaluation. Note that if W* is less than 225 fpm, then
Hcrit is predicted to be the surface.
A shortcoming of the TI index is that it
indicates the height to which mixing should occur but not all mixing
is equally useful to glider pilots. Mixing can be produced both
by thermals and by wind shear, but only thermals produce the
relatively large updrafts needed for soaring. To help evaluate
the degree to which the day's mixing is convectively driven, a thermal
"hot air" parameter "B/S" (sic) has been added to predictions in the
email body, representing the ratio between Buoyancy and Shear
production of turbulence. A small B/S value indicates shear is
likely a significant problem - at present the best guidance I
have, based upon a limited number of pilot reports, is that on days
with B/S of 5 or less the thermals are likely to be too broken to be
usable. At B/S of 10 or above shear is likely not a significant
factor. Actual experience which would indicate those criteria
should be altered, or confirming those estimates, would be
appreciated. Because I believe the "B/S" ratio can be a
significant modifier of thermalling conditions, it is included in the
Subject line following the two TI heights.
For those interested in more scientific
detail, the B/S ratio is not an empirical approach but is based
upon the non-dimensional number used to distinguish between "buoyancy
dominated" and "shear dominated" BLs (and those in between). It
is the ratio of the "buoyant production of turbulent kinetic energy"
to the "shear production of turbulent kinetic energy" with both being
well defined terms. However, the cross-over criterion between
"workable" and "unworkable" thermals must be determined empirically
(and for that matter there is no sharp cut-off between the two cases).
The program's method of estimating the
thermal strength W* is unique within the soaring community, to my
knowledge, and so will be discussed in some detail. Note that
this prediction is intended to forecast the upward velocity of
air within the thermal and will never be negative. It is
well established, both theoretically and experimentally, that vertical
motion in a cloud-free convective boundary layer is
proportional to the cube root of the product of a buoyancy constant,
the boundary layer depth (or thermal depth), and the surface heat
flux. This produces a quantity with the units of velocity, known as
the convective velocity W* ("w star"). It makes physical sense
that a thermal's strength will depend upon the amount of heat entering
into the atmosphere at the ground, and the thermal height is an
important factor because a rising thermal bubble will achieve a higher
velocity if it accelerates for a longer time. The
proportionality constant between W* and vertical velocity in a thermal
should be approximately one, its exact value depending upon such
factors as the area over which the vertical velocity is averaged
(since a thermal's core is stronger than its periphery) and thus will
depend upon the thermalling radius of the glider, for example.
Of the needed factors, the buoyancy
constant is known and the thermal depth can be assumed to be the TI=0
height. [Note: the remainder of this paragraph descibes the
surface heating estimate utilized by TIPs and does not apply to
BLIPMAPs, whcih utilize the known model surface heating so no such
calculation is required.] However, surface heat flux
predictions are not available on the web for input to the TIPs
(although the forecast models do need to compute it when predicting
the maximum temperature). Therefore I have attempted to back
that number out of the available data, though there will be some error
in the result, by using the fact that the total heat input into the
boundary layer can be determined by vertically integrating the
temperature difference between the early morning sounding and the
afternoon temperature resulting from adiabatic mixing produced by the
predicted surface temperature (this requires the assumption that there
is no net transport of heat by either horizontal or vertical mean
winds and will be in error if that assumption is not met, such as when
atmospheric conditions above the mixing layer are changing). A
relationship between that total heat flux and the maximum heat flux
during the day, the factor of interest, is needed so I have assumed
the heating to be a sinusoid extending over the daylight hours.
For now I have simply set the problematic
proportionality factor to one and it is gratifying to find that
without having to resort to any empiricism whatsoever the predicted
vertical velocities are already quantitatively very realistic, when
compared to the "Soaring Index" (SI) results presently accepted for
the Reno area . With
further experience the proportionality constant might be adjusted
slightly, but the present results are considered very reasonable and
more accurate than the "SI" predictions for Hollister under weak
convective conditions. I expect the present predictions to be
accurate within a factor of two, though of course a range of thermal
strengths occur at any one time. One should note that W* follows
the rule that deep thermals tend to be strong thermals, so larger TI=0
heights will generally be associated with stronger thermal lift.
Several cloud parameters are provided in the
header accompanying the detailed sounding analysis. There is great
potential to misunderstand these predictions! All these parameters
apply only to clouds which develop locally due to convection, not to clouds
which move into the area or which occur above the Boundary Layer. The "Sfc-Lift Cond Lev" is the height at
which small clouds might develop due to thermal convection from the surface;
this is the lowest level at which cloud bases might occur, but usually
unreasonably low because it considers only the surface dew point. The Lifting
Condensation Level, LCL, is a height at which the base of more extensive
clouds would develop due to surface heating, since the moisture is averaged
over a layer thickness near the surface. The Ford cloud base estimate
is a similar prediction using a layer of different depth and generally
predicts a somewhat higher cloud base than the LCL. The Convective Condensation
Level (CCL) is similar to the LCL but also evaluates limitations resulting
from the actual atmospheric temperature profile and so should always be
higher than the LCL.
Also included is an index of moist instability
called the "Lifted Index (LI)": the smaller, or more negative, the number
the greater the chance of strong convective clouds ("towering cumulus")
developing. Of the LI values available on the web, I chose the LI
for 700 mb (around 11,000 ftMSL) because it is more appropriate to local
conditions than the "normal" LI, which uses a temperature difference evaluated
at 500 mb (around 18,000 ftMSL) to forecast the large thunderstorms that
develop in the Midwest. But while there is guidance available on
use of the "normal" LI, since that has been "calibrated" by experience -
but note that all stability indexes described there are for very large
convective systems). I am not aware of any guidance on what LI@700
values are associated with what kinds of clouds, even though LI@700 is
a commonly reported parameter.
The above paragraphs apply to scattered/broken
clouds which develop locally due to convection. It is also possible
to use sounding profiles of temperature and dew point to predict overcast/broken
clouds which either move into the the area or develop locally, as described
in the TIP Sounding Analysis
webpage.
Clouds mark thermals, but they also add buoyancy
to the thermals through the release of latent heat of condensation.
The TIP predictions, however, assume that thermals are driven entirely by
heating at the earth's surface, so this release of heat aloft is not included
in the TIP buoyancy estimates. With cloud formation the maximum height
to which a glider can climb now becomes limited by the cloud base height,
not by the top of the thermal (which is the top of the cloud), so the actual
soaring height cannot be equated to the thermal height, as the TIP TI height
prediction assumes. This dissociation is particularly apparent when
maximum lift is found at cloud base, trying to suck the glider into the
cloud - clearly the glider is then not at the top of the thermal!
(It should be noted that the above description applies to convective clouds
in their growth stage - at later times the clouds can "overdevelop", forming
an overcast which blocks sunlight from reaching the surface, which in turn
reduces the surface buoyancy and weakens the thermals.)
When convective clouds form, therefore, the
actual thermal top, the updraft strength W*, and the Buoyancy/Windshear
ratio B/S are all larger than the TIP prediction due to buoyancy generated
aloft. Further, these parameters are increased more by deep cloud
convection than by shallow puffy cumulus. However, the cloud base
is expected to be below the maximum thermalling height predicted
by the TIP since the condensation initially occurs in a dry thermal, below
the thermal top; the deeper the cloud, the larger the difference between
the cloud base and the predicted maximum thermalling height.
Because cloud-generated buoyancy is so significant,
the best soaring conditions often occur when clouds form so neglect of
this effect is a significant deficiency in the TI method. Unfortunately,
inclusion of cloud-generated buoyancy would be difficult since cloud formation
is hard to forecast accurately and since even relatively small amounts
of condensation can significantly affect the thermal strength. It
is probably best to regard the TI forecasts as a forecast of "minimum"
thermalling conditions in the absence of clouds, with cloud formation generally
expected to strengthen thermalling conditions. Here "thermalling
conditions" should be interpreted as meaning updraft strength, since the
maximum soaring height is now determined by the cloud base rather than
by the maximum thermal height.
Forecast soundings are obtained from the
research-mode FSL "MAPS"
model, which predicts out to 36 hours at best, and the operational
NWS
"ETA" model, which predicts out to 48 hours. Both models are
intended to provide predictions of larger scale weather systems and
have limitations which affect their ability to predict accurate
soundings near the surface. For example, the vertical resolution of the available
ETA model data is around 500 feet at 3000 ftMSL, so changes in TI
heights of less than 500 ft do not have great significance (the Eta
model actually uses a finer grid resolution, but its output is
degraded to supply values at constant pressure levels). MAPS has
a somewhat better vertical resolution of 350 ft at 3000 ftMSL
At present the program analyzes soundings
forecast for Oakland's coordinates. I will discuss this choice
in detail, since it might seem that a location closer to Hollister
would be more appropriate. In addition, this illustrates
pitfalls that can result from unwary acceptance of information
available on the web. For the NWS models forecast soundings are
only available at certain coordinates, coinciding with airport
locations. For the MAPS model, however, sounding data is
available for any grid point location at the FSL sounding
website, which is the program's MAPS data source. The first
point I wish to emphasize is that the model horizontal resolution is
20 km for MAPS and 32 km for Eta, which greatly limits their ability
to resolve the influence of our local terrain. Use of a 20 km
grid, for example, means that a forecasted variable is expected to be
the average of that variable over a 20x20 km area. However, to
avoid the introduction of numerical noise the topography must be
averaged over several grid intervals, so the net result is that the
model topography only resolves features which are over 40 km in width
whereas our coastal mountains are about 40 km wide so the resolution
is marginal at best. The second point is that the models use
"envelope" topography, which means that instead of employing the
average elevation over each 20x20 km area the maximum elevation is
used. This has advantages for predicting air flow, since winds
channeled by the Sierra mountains, for example, are affected by the
maximum elevation of that mountain ridge, not by its 20x20km average
elevation. However, this choice leads to difficulties when
trying to forecast the atmospheric structure near the surface, upon
which a TI calculation is dependent. (In fact, when the MAPS
model forecasts its own surface temperatures it uses a different grid,
one more representative of the average elevation.) If the
naif attempts to forecast Hollister conditions by simply using the
model grid point closest to Hollister's coordinates, he will obtain a
sounding having a "surface" of over 1000 ft, considerably above
Hollister's actual surface elevation of 230 ftMSL (and even above it's
traffic pattern!). The error in surface elevation is large
relative to the average TI=-4 height, which is around
3000 ftMSL. and is not an appropriate surface elevation to predict TI
heights at Hollister. This result is a combined consequence of the
envelope topography and the coarse horizontal resolution which greatly
broadens the Diablo mt range, as seen on the
20km MAPS
topography. One might obtain a lower elevation, 660 ftMSL,
by choosing the next grid point towards the ocean, but the ocean
influence is then exaggerated. For the present, I have decided
that it is best to forecast for the coordinates of Oakland
airport, which is located roughly the same distance from the ocean as
Hollister and also surrounded by higher terrain, since the model
surface elevation there is 540-590 ft (depending upon the
model) and surface temperature predictions made for the Hollister
elevation are then more appropriate. The difference in location
between Hollister and OAK parallels the ocean and this horizontal
shift is considered to have less effect on TI predictions than other
factors. Also, from the scientific point of view a comparison
between forecasts and observations both made for OAK is a more
meaningful test of forecast accuracy.
Because models have biases, they are often
better at forecasting changes than absolute values. Hence the TI predictions
in the model forecast sections include changes from the model TI
prediction for the current morning (when an observed sounding is available
for comparison)..
I hope that that recognition of the various
model limitations does not make the user overly pessimistic concerning
the model forecasts. I believe that they can be of value, but on
the other hand the user should not expect too much or accept them blindly
for the models are not magical. The models tend to provide large-scale
"broad brush" differences while smoothing out the small-scale variations
which also exist in the atmosphere. The uncertainties resulting from
model limitations are one reason why the TIP provides several forecasts
rather than a single one. I do think you will find the forecasts
useful, but they are subject to error. At some later time I intend
to do a historical analysis evaluating the accuracy of the forecasts and
look for model biases; once such biases are known they can be used to better
evaluate future TI predictions.
Because the surface elevations of the sundry
observed and forecast soundings differ, a method of surface elevation
adjustment has been incorporated into the model. The compensation
is not perfect, requiring some assumptions when the assumed surface lies
below the bottom of a sounding, but is better than nothing. The resulting
TI adjustments lessen the difference between TI heights obtained for the
morning observed sounding and those obtained from the model analyses for
that time, but are relatively small for the Hollister predictions,
To make comparisons between "tomorrow" and "day after tomorrow" TI
predictions more meaningful, since NWS Tmax predictions are being used
for the previous days but only WxC Tmax predictions are available for
the "day after tomorrow", I am adjusting the "day after
tomorrow" Tmax by assuming that the difference between the NWS and WxC
Tmaxs will be the same on the "day after tomorrow" as it was
"tomorrow". A "wxc" identifier (rather than "WxC") indicates that
this adjustment has been applied to the WxC Tmax.
Further information on the TI analysis methodology
can be found at
Kevin Ford's
TI calculation description, since the TI calculation is based upon
the Ford subroutine. The Ford calculation has been altered to allow
TI calculation at a surface elevation which differs from that of the sounding,
but this requires some assumptions about the atmospheric structure when
the assumed surface elevation lies below the bottom of the sounding.
The program was originally developed to obtain
TI analyses of observed morning soundings from
Kevin
Ford's TI calculation website, but that site's data sources were unreliable
so the program was converted to instead gather the sounding data itself
and compute the TI internally, using a modified version of the Ford routine.
This also allows TI calculations to be made from forecast and archived
sounding data, all data except for the MAPS soundings now being obtained
from the
Storm Chaser's
Weather Machine. If any sounding data is missing when the program
is first run, the program will continue to try to gather data for 20 minutes
before sending an email with analyses of the available data. If temperature
data is not immediately obtained from either the NWS or WxC site, the program
continues to query that site for 30 minutes prior to abandoning its attempts.
If temperature data is not available for either site, then an error email
is sent. Analysis of 8 months of historical sounding data has shown
the actual computations of TI, W*, etc. to be robust, so long as valid
data is obtained from the web. However, the program depends upon
a chain of events (available data, sites running, etc.) and that chain
may break on any given day. In particular the program relies upon
the formats from all web sites remaining unchanged. If the program
"breaks" and errors occur, I will try to fix the problem or make the program
smarter. The program is a combination of Unix and Perl scripts, using
the "Curl" open source software to interact with the web, running on an
SGI computer.
The TIP treatment
of cloud influences is weak, due to (1) the complex dependence of
soaring upon clouds, since clouds aid soaring by increasing thermal
updraft strength through the release of buoyancy aloft but also limit
soaring since a cloud base can prevent pilots from ascending to the top
of a thermal (2) clouds formation is difficult to predict, due to a
cloud's sensitive dependence upon humidity which is inherently
difficult to predict using numerical models.
Any opinions on whether the reference TI criteria
should be changed, or adjusted for soaring locations not directly above
Hollister, would be of interest if based upon actual gliding experience
at Hollister. Also, I will attempt to answer any email queries containing
questions not covered in the description above. I should warn such
inquirers, however, that I am a research meteorologist, not a forecaster,
and moreover my specialty is the atmospheric boundary layer where thermals
develop, so while I will try to give short answers I will not make complex
meteorological problems simplistic. My decision not to give a single
number supposedly representing predicted thermalling conditions at Hollister
is an example of that mindset. I can be reached at
Or you can read about what I do for a living at the
DrJack home
page kindly provided by Webbnet.
HISTORICAL AVERAGES
AND VARIATION AT HOLLISTER
To establish a baseline providing perspective
on the variation to be expected for TI forecasts at Hollister, I performed
a historical TI analysis using the observed temperature maxima and Oakland
sounding data that I have been archiving since April of this year.
This analysis is given below for those interested. I also have the
TI results for individual days, should anyone have an interest in a particular
day over this period.
ANALYSIS OF HOLLISTER DATA FOR 2000
Average of indicated number of days in each month
followed by daily variation (±standard deviation)
[Uses OAK sounding and observed Tmax at Hollister Maze MS]
MONTH DAYS Tmax TI=-3.6F TI=0 W SI
----- ---- --------- ---------- ---------- -------- --------
Apr 7 76.0 ±7.3 4885 ±1338 5724 ±1545 545 ±101 356 ±158
May 30 75.7 ±9.9 3837 ±1594 5009 ±2212 465 ±138 329 ±274
Jun 24 78.4 ±9.4 2681 ±1244 3227 ±1632 360 ±120 255 ± 85
Jul 21 75.5 ±6.6 2649 ± 663 2966 ± 807 360 ± 72 156 ±113
Aug 30 81.3 ±7.3 2395 ± 724 2805 ±1115 363 ± 90 192 ± 75
Sep 18 81.8 ±9.0 3425 ±2220 4310 ±2368 426 ±175 292 ±225
Oct 27 71.7 ±6.6 2717 ± 980 4030 ±1528 395 ± 95 285 ±146
Nov 15 63.3 ±6.1 2756 ± 973 4073 ±1442 378 ± 89 235 ±165
ALL 172 76.0 ±9.5 3008 ±1422 3847 ±1871 400 ±124 256 ±184
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