Modeling of aggregation and riming in orographic clouds over the northern Colorado Rockies

Abstract

This thesis addresses problems arising in the modeling of aggregation and riming in mesoscale cloud models. Two case studies are used to test these portions of the microphysical parameterization in the Colorado State University Regional Atmospheric Modeling System. The cases modeled were taken from the Colorado Orographic Seeding Experiments. Each case was dominated by a particular precipitation formation mechanism . This allowed for the separate testing of the riming and aggregation parameterizations . The portions of the storms modeled were documented in great detail with ground and aircraft observations. The modeling portion of this study involved control simulations as well as several experiments geared to correct deficiencies identified using the controls . The main conclusions are as follows . The model has difficulty predicting the presence of cloud water in areas observed in the field . The vertical distribution of the concentration of ice crystals predicted contradicts observed distributions. Also, the amount of graupel predicted by the model did not mirror observed precipitation characteristics. From the experiments conducted suggestions can be .made for improvements to the model's microphysical parameterization . The graupel density currently used should be lowered . The value of 0 . 2 g/cm3 seemed to work best for the case examined . Also, the addition of the dependency of crystal concentration on air density brought predicted concentrations closer to those observed but had no effect on the vertical distribution problem. An exploratory sensitivity test showed that the introduction of longwave radiation into the model should be further investigated.