Practical Soaring Weather Forecasting Colorado Performance Soaring Seminar 2012 Boulder, CO Dave Leonard Overview What do you need to know? What tools are available? How do you use the tools? Whats important? What the FAA wants you to know Big picture, bad weather
What the typical glider pilot wants to know Which day will be better this weekend? How good will today be? When will it start, when will it end, how strong, how tall? Any chance of really adverse weather (storms, winds, etc.)? You dont have to be a weather man How much weather knowledge is required? A little fundamental knowledge helps a lot Understanding how a system works makes predicting what it will do much easier
Practical experience helps even more Basic atmospheric info Basic useful facts about the atmosphere We worry mostly about the troposphere (< 35K ft) Pressure and temperature drop with altitude Air absorbs very little energy from the sun Air temperature is primarily driven by interaction with the earths surface Air is a poor conductor of heat The water vapor capacity of air is limited Water vapor changes to liquid when capacity is exceeded
This condensation releases heat energy Thermal energy and moisture are primarily transported by air motion Air moves in response to pressure gradients and density differences (buoyancy) How Thermals Start Thermals start when a parcel of air near the earths surface is warmer than the surrounding air The air is warmed by the surface, not directly by the sun The air parcel has no walls, so the pressure inside and out is the same The warmer air is less dense than the surrounding air making it buoyant
The warmer parcel of air moves up in response to the buoyancy force applied to it What happens in the thermal? The warm parcel is surrounded by air at a lower pressure as it rises The lack of walls means the pressure in the parcel drops to equal the surrounding air This results in an increase in volume of the parcel The parcel absorbs very little direct solar energy The combination causes the temperature of the parcel to drop The rate of temperature decrease with altitude is known as the Dry Adiabatic Lapse Rate = 9.8 deg C / km
What else happens? Water vapor content stays fairly constant But the pressure drop reduces the dewpoint temp About 2 deg C / km If the parcel rises high enough temperature drops to dewpoint
Moisture condenses and cloud forms Condensation releases heat into the parcel Increases buoyancy of the parcel Lapse rate drops as heat is now added as more vapor continues to condense out of the parcel These processes continue until surrounding air density is greater than the density in the parcel What does that have to do with anything? For thermal forecasting you need to know Temperature and dewpoint of air at the surface Temperature and dewpoint of the air at altitude
Measured or modeled If there are cumulus, cloudbase will be at ~(Surface Temp Dewpoint Temp) (deg C) * 400 ft Cumulus forecasting is most easily done graphically (or via computer) Thermal strength is highly correlated with thermal height and whether there are cumulus or not Empirical models do pretty well Temp vs Pressure Thermal
Forecast based on morning sounding Cu based around 14K ft Good chance of T-storms Moist Adiabatic Lapse Rate Dewpoint Temp vs
Pressure Cloudbase Constant Mixing Ratio Dry Adiabatic Lapse Rate Forecast Hi Temp 93 F Rolling together local soundings and bigger picture
To refine this forecast, need to take into account bigger picture Both to account for airmass changes during the day and terrain influenced air circulation patterns There are several good soaring forecast services that do this and more based on numerical weather model outputs XCSkies Dr Jack BLIP Maps Dr Jacks RASP BLIP Maps (run locally under license) Refining the forecast Big Picture Tools National surface and upper air charts
Looking for big changes coming Local NWS forecast picnic weather Professional guess at t-storms and winds Satellite images Looking for unforecast cloud, weather system position and movement Local AWOS / ASOS / ATIS reports A simple check against forecast for temp, dewpoint, wind NWS Forecast Discussion Professional assessment of whats going on, what about the forecast is worrisome, which model(s) are doing the best currently
DUAT or FSS (for the record) Understanding the Soaring Forecast Products NWS Soaring Forecast Based on morning sounding at DEN Sounding plots raw and various models BLIP Maps NAM, RUC, RASP, GFS Note local Front Range RASP is NAM based All 3 services produce similar products Different combinations of numerical models Different calculations of soaring parameters from the model outputs
Dr Jacks BLIP Maps and XCSkies forecast out a couple days ahead for picking your weekend day by Thursday or Friday Same products, but less forecast accuracy the farther ahead My morning pre-flight weather brief I have a set of links collected in a bookmark folder that I open all at once, then click through each tab
Unisys Weather - http://weather.unisys.com/ NWS 7 day forecast - http://forecast.weather.gov Kelly Airpark AWOS - http://www.nws.noaa.gov/obhistory/KMNH.html Dr Jack BLIP map - http://www.drjack.info/BLIP/univiewer.html NWS Forecast Discussion - http://www.nws.noaa.gov/view/prodsByState.php? state=co&prodtype=discussion Kevin Fords DEN Sounding - http://www.soarforecast.com/ti.cgi? SUBJECT=TI&Upperstation=DNR&Surfacestation=DEN&Forecasthigh=&MaxAltitude=25000 NWS Soaring Forecast -http://www.crh.noaa.gov/bou/include/showProduct.php? product=algbou&parentdir=routine FAA DUATS - https://www.duat.com/
Satellite Images - http://weather.rap.ucar.edu/satellite/ And of course the Colorado Front Range RASP Pulling it together Following is a walk through of the various types of BLIP map products for a mid summer soaring day to predict soaring conditions Included are some notes on how to interpret, correlate, and maybe understand the various products The trick is to review all the data thoroughly enough to get a good idea of what to expect
But not to spend all day looking at the data and miss a great flight Sample review of RASP forecast July 3, 2011 Start with Thermal Updraft Velocity Map for mid-afternoon Note strong lift Lots of small dead areas in the mountains
Early lift map shows late start in Boulder Valley Tow into the hills to get an early start Early start from Kelly likely Check same map later afternoon
Looking for extent of dead areas RASP normally shows earlier end of convection than actual Shows movement of dead areas off mountains Expect scattered OD Sfc Solar Radiation Shows extensive
cloud shadowing, mostly over the high ground, but pushing out over the plains Correlates with dead lift areas Check mid-afternoon sounding for Boulder Looking at upper winds
Steer storm movement and cirrus blowoff Note Temp and Dew Point lines merge at about 18K ft Cu wont go away when they die Wide inverted V indicates possible high based T-storms,
microbursts Cu where Potential > 0 Grey areas are forecast blue holes Shows high bases, dropping to the east Surface winds light E-NE over
plains, W-NW over northern mountains Winds are calm in blue hole south of Arkansas river Surface dewpoint map shows lots of moisture in the Arkansas and Platte river valleys Correlates with the
varied cloudbase Doesnt explain blue hole south of Arkansas river BL Up/Down Motion shows broken convergence line from border to border Blue areas indicate grid scale dumps,
likely virga showers CAPE map shows < 1000 J/Kg along Front Range Probably no heavy tstorms Microbursts, lightning, wind possible based on other maps Above about 1500 J/Kg can get ugly with
hale and heavy rain Farther east can go up to several thousand with really nasty results Forecast Summary for July 3, 2011 Looks like an interesting day High pressure building to the west Previous day was pretty punk
Forecast Strong, tall lift Good stretches of convergence Early starts possible Scattered to broken high based storms with significant dead areas by late afternoon Some risk of microbursts So how did this one work out? Several pilots flew from Boulder down to the NM border and back Pedja flew his first 1000 km OLC flight of the weekend from Boulder
Speeds were not outrageous due to detours around cells and dead areas and some weak climbs Several pilots had an interesting encounter with a high based t-storm cell moving across Boulder at landing time The RASP model hit it pretty well You really dont have to be a weather man Need to understand generally whats going on Need to know which models to trust at any given time Need to understand what the models are telling you
Need to recognize when the models are not helping and trust what you see out the window Great reference material Understanding the Sky Dennis Pagen Meteorology and Flight: Pilot's Guide to Weather - Tom Bradbury Meteorology for Glider Pilots - C.E. Wallington http://www.gliderpilot.org/Weather
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