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 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.) Related to the vertical structure of atmosphere

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

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

Dust Devil in Arizona

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

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

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

Vertical structure of atmosphere The following three parameters can be used to anticipate most of the hazards in a forecast vertical sounding: (Analysis Tool:

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ALTPRESDIRSPDTEMPDEWP LAYERMECHTHERMCLOUD ftmbC°ktCC ft (MSL)TURB? LAYERLCL (ft) TURB TURB TURB TURB TURB TURB TURB TURB TURB TURB TURB TURB 12, CLOUD CLOUD

Dry thermals Convectively unstable layer

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 ( Spreadsheet tool is available on the ERAU Department of Meteorology website ( Questions?