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MS Defense

Department of Atmospheric Science

Fri., Nov. 30, 1:00 pm, EN6085

Evaluating and Improving Parameterization of Ice Fall Velocity in Convective Clouds: Using the NCAR CAM-SCM with TWP-ICE and MC3E Data

Lin Lin

University of Wyoming

Abstract

Accurately representing convective ice-phase particle fall velocity in large-scale models is of great importance for current and future climate projection because of its strong impact on convective precipitation and anvil clouds. In this study, two new ice-phase particle fall velocity parameterizations were implemented and tested in the convective cloud microphysics scheme using the National Center for Atmospheric Research (NCAR) Community Atmospheric Model (CAM) version 5.3 in the single column mode (SCM). One terminal velocity parameterization is based on the terminal velocity-diameter (vt-D) power law relationship with the pre-factor and exponential factor varying with temperature, ice water content, and pressure derived from in-situ measurements. The other one is physically based on the tank experiment considering both the particle and air properties. Simulated cloud properties with SCM were evaluated against observations from the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement Program’s Tropical Warm Pool-International Cloud Experiment (TWP-ICE) and Midlatitude Continental Convective Cloud Experiment (MC3E) and from satellites. CAM5 with the default terminal velocity parameterization overestimates the frozen water content (FWC, ice plus snow) by a factor of 3-4 and underestimates the liquid water content (LWC, cloud liquid plus rain) by 25%. With the new terminal velocity parameterizations, CAM5 simulated FWC profiles are in better agreement with observations, with 40-50% reduction in snow at mid troposphere, and LWC closer to observations. Hydrometeor mass flux analyses indicate that vertical variation of convective ice particles is dominated by convective transport while that of snow particles is by sedimentation. Our results suggest that new mass-weighted terminal velocity for snow particles increases by a factor of 2 at lower altitudes compared with the default parameterization. However, the fall speed of snow particles is still not sufficiently large to redistribute the cloud condensate vertically due to the incapability of treating the graupel. Further investigations are required to examine the ice particle size distribution and to represent the precipitating-particle processes and detrainment in the model.

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