Anticipating Aviation Weather Hazards in the Southwest Dr. Curtis N. James Department of Meteorology Prescott, Arizona

Overview What are some general characteristics of the climate of the Southwest? How is aviation affected by this climate? How can the aviation weather hazards be anticipated and avoided? Why is an understanding of the vertical structure of the atmosphere necessary?

General Climate of the Southwest Located in a latitude belt (~30°) where air generally sinks and warms (usually clear & dry; 300+ flying days / year) Rugged terrain (clouds/precip usually more frequent over windward slopes w/ lee rain shadowing & waves) Continental climate, isolated from oceans by terrain (generally dry air w/ high temperature variability) Prevailing surface wind generally southwesterly (except where terrain generates local winds) Prevailing wind aloft westerly in cold season, southerly in summer (associated with the SW monsoon) In warm season, deep convective layer near the ground

Aviation Hazards of the Southwest Related to the vertical structure of atmosphere Deep convective boundary layer Low-level turbulence and dust devils Thunderstorms (esp. July – September) Downbursts (especially dry microbursts) Hail, lightning, turbulence near thunderstorms Mountain waves / shears & lee turbulence Other (icing, low clouds, IMC, LLWS, etc.)

Deep convective boundary layer (more stable air above) 20,000’ MSL or more thermal dust devil Hot, dry, unstable air

Convective boundary layer (Prescott, AZ) Fall 2000—Photo by Joe Aldrich

Dust Devil in Arizona www.nasa.gov www.nasa.gov

Mountain waves ACSL clouds Strongest wind speed Roll cloud Cap cloud Cloudy, cooler, possible fog & precip Lee waves Mountain Dust may be visible Clear, warm, dry & windy

Mountain wave clouds (PRC) 2000—Photo by Ben Small

Lenticular clouds (near Denver) 2000—Photo by Josh Richmeier

Dry microbursts When precipitation falls through unsaturated air, evaporative cooling may produce dry microbursts Result in very hazardous shear conditions Visual clue: fallstreaks or virga (fall streaks that don’t reach the ground) Flight path of plane 45 kt downburst 45 kt headwind 45 kt tailwind

Downburst (Phoenix, AZ) July 2003—Photo by Phillip Zygmunt

Downburst (Prescott Valley, AZ) 1999—Photo by Jacob Neider

ERAU Aircraft #N519ER 08 June 2003 Virga KPRC 081953Z 19008G15KT 160V220 10SM CLR 29/01 A2999 RMK AO2 PK WND 13027/1921 SLP060 VIRGA N-E TCU W-SE T02890006 KPRC 082053Z 25011G17KT 10SM CLR 32/M01 A2998 RMK AO2 SLP052 T03221011 56010 KPRC 082153Z 25013G18KT 10SM CLR 32/M01 A2996 RMK AO2 SLP048 ACFT MISHAP T03171006

ERAU Aircraft #N518ER 29 November 2003 Kingman, AZ Stable air over less stable air, increasing wind speed with height Downslope wind?

Vertical structure of atmosphere The following three parameters can be used to anticipate most of the hazards in a forecast vertical sounding: (Analysis Tool: http://meteo.pr.erau.edu/links.php)

(http://rucsoundings.noaa.gov/gifs/)

ALT PRES DIR SPD TEMP DEWP   LAYER MECH THERM CLOUD ft mb C° kt C ft (MSL) TURB? LCL (ft) 4895 845 144 22 24 4.3 4895 - 4950 TURB 4950 843 143 23 4950 - 5068 5068 840 25 22.6 4.2 5068 - 5246 5246 835 145 26 4.1 5246 - 5479 5479 828 27 21.3 4 5479 - 5836 5836 817 147 20.2 3.7 5836 - 6197 6197 807 148 19.1 3.5 6197 - 6558 6558 797 150 18 3.3 6558 - 6925 6925 786 151 17 3.1 6925 - 7296 7296 776 153 15.9 2.8 7296 - 7667 7667 766 156 14.8 2.5 7667 - 8044 8044 755 158 13.7 2.2 8044 - 8425 8425 745 162 12.6 1.8 8425 - 8809 8809 735 167 11.6 1.3 8809 - 9199 9199 724 173 21 10.6 0.6 9199 - 9593 9593 714 189 9.6 -0.4 9593 - 9990 9990 704 213 8.8 -2.6 9990 - 10141 12,775  24330 400 259 42 -25.6 -28.2 24330 - 24888 24888 390 254 43 -27 -29.2 24888 - 30561

Convectively unstable layer Dry thermals

WSR-88D Radar Images NM/Holloman AFB

Summary The Southwest has a fascinating climate, with a number of aviation hazards Many hazards may be anticipated using a vertical profile of the atmosphere Suggest analyze forecast sounding prior to flight (http://rucsoundings.noaa.gov/gifs/) Spreadsheet tool is available on the ERAU Department of Meteorology website (http://meteo.pr.erau.edu/links.php) Questions?