INTERACTIONS BETWEEN TOPOGRAPHIC AIR-FLOW AND CLOUD PRECIPITATION DEVELOPMENT DURING THE PASSAGE OF A WINTER STORM IN ARIZONA

A case study showing comparisons between observations and numerical si mulations of the passage of a winter storm over complex terrain is pre sented. The interactions between the mesoscale and cloud environments and the microphysical and dynamical processes are addressed using both observations and numerical simulations. A three-dimensional, time-dep endent, nested grid model was used to conduct numerical simulations of the three-dimensional airflow and cloud evolution over the Mogollon R im and adjacent terrain in Arizona. The modeling results indicated tha t the flow patterns and cloud liquid water (CLW) were closely linked t o the topography. To a large extent, gravity waves excited by the flow over the mountains determine the distribution of clouds and precipita tion. The waves extend through deep layers of the atmosphere with subs tantial updrafts and downdrafts, at times exceeding 5 m s-1. The simul ated vertical velocities and horizontal wavelengths of about 20 km wer e in good agreement with the aircraft observations. The CLW regions as sociated with the waves extended through much deeper layers of the atm osphere and in quantities a factor of 2 larger than those associated w ith the forced ascent over the ridges. The CLW associated with waves m ay provide an additional source for precipitation development not prev iously considered in cloud seeding experiments. In addition, synoptic- scale flow patterns over the area change from one storm system to the next and even during one storm system. Consequently. both the winds an d the evolution of clouds over the area are highly space and time depe ndent.