Simulations of the Daytime Mountain Upslope Circulation as a Precursor to Cumulus Development

摘要

Observations and simulations using a dry version of a 2-D dynamic numerical model have shown that low-level convergence produced by mountain upslope flow during morning hours is likely to be an important trigger for setting off convective clouds. To further investigate the role of upslope circulations in initiating mountain cumuli, we have added moisture and warm-rain microphysics to the 2-D version of the numerical model that was used in the previous simulations. We have also added surface energy and moisture budget parameterizations to study the effects of the surface properties on the development of clouds. Runs were made with high and low values of surface heat flux (25 and 17 K cm/s) and high and low values of soil moisture (100% and 20% saturated). The runs with varying heat flux showed that the higher heat flux runs with lower heat flux generated weak circulations that produced clouds. Runs with varying soil moisture showed that higher soil moisture generated weaker circulations than the lower soil moisture cases. This is because when the soil moisture is higher, more of the incoming (net radiational) energy is partitioned into latent heat of evaporation and less into heat flux which is more efficient at driving the dynamic circulations. Considerable attention was paid to the initialization of the model runs. In order to develop a realistic nocturnal inversion layer by sunrise in the model, it was necessary to begin the model run before sundown on the previous evening. This allowed upper level winds to mix realistically down to the surface before nocturnal cooling commenced.