Tuesday, 31 July 2001: 11:30 AM
Numerical simulations of a landfalling cold front observed during COAST: Rapid evolution and responsible mechanisms
This paper examines the mechanisms for the rapid development and evolution of a landfalling cold front along the mountainous northern California coast on 1 December 1995 of the COAST field experiment. This event was simulated down to 3-km horizontal resolution using The Penn State/NCAR mesoscale model (MM5V2) and compared with the airborne Doppler observations collected by the NOAA P-3. The MM5 simulations help illustrate the complex thermal, wind, and precipitation structures in the coastal zone. Upstream flow blocking by the steep coastal terrain led to the development of a mesoscale pressure ridge and a prefrontal barrier jet exceeding 25 m/s. Because of the irregular coast-line and highly three-dimensional terrain, the barrier jet structure was not uniform along the coast. Rather, prefrontal southerly flow resulted in relatively weak flow downwind of the major Capes and there were localized down-gradient accelerations adjacent to regions of higher topography. Terrain blocking resulted in a surface frontal orientation that curved outward from the coast and a 30-40 km tipped forward frontal slope between the surface and 850 mb.
The MM5 was used to investigate some of the reasons for the rapid intensification of the frontal temperature gradient and precipitation in the coastal zone. During this event the large-scale vertical motions increased in an environment favorable for moist convection, and a simulation without coastal topography illustrated rapid development of coastal precipitation even in the absence of terrain blocking. The coastal topography helped to further enhance and collapse the frontal rainband and temperature gradients through enhanced deformation frontogenesis associated with the prefrontal barrier jet. Diabatic effects from precipitation were also shown to be important in organizing the precipitation in the coastal zone and enhancing the frontal temperature gradient.