The Great Plains Dryline

1. The Great Plains Dryline Type 1 - Diurnally-Forced Dryline Type 2 - Dynamically-Forced Dryline

2. Type 1 - Diurnally Forced Dryline • Air on upwind (upslope) side of dry line is represented by a deep mixed layer • Air west of the dryline has broken the inversion and so upper level dry and warm air from the EML has mixed down • The inversion breaks on its western extremity first because the inversion is closest to the surface there • As the daytime surface mixed layer deepens to the inversion throughout the day, the inversion break (dryline position) works its way eastward • West of the dryline, mixing of the deep EML with the shallow moist layer produces : • Gusty westerly surface winds • Very low humidities • Warm temperatures

3. Type 1 - Diurnally Forced Dryline (Cont'd.) • Air east (downslope) of the dryline is relatively moist and cool, although there tends to be only a small virtual temperature contrast across the dryline • Shallow moist boundary layer • Source of moisture includes • Evapotranspiration • Advection from east (Gulf) • Moist boundary layer reaches depth of the inversion at the point of the dryline

4. Type 1 - Daily Evolution • Late morning and afternoon: • Boundary layer grows below nocturnal inversion • dryline eats its way eastward • Evening: • Nocturnal inversion forms • Moisture trapped under inversion • Early morning • Plants become active and transpire raising moisture under inversion • Slope flow commences enhancing inversion

6. Type 2 - Dynamically Forced Dryline • Response to mountain wave drag on the tropospheric jet stream • Integrates mesoscale downslope flow surges of momentum and dry air into a coherant streamwise front • Results in meso-beta and meso-gamma scale gravity wave-induced bulges of dry air, momentum and nonconserved isentropic potential vorticity • Severe convection focused in meso-gamma scale gravity wave-induced bulge locations