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PhD Defense

Department of Atmospheric Science

Thurs., June 21, 3:00 pm, EN6085

Vertical Air Motion and Phase Partitioning in Convective Clouds: Observing and Modelling Studies

Jing Yang

University of Wyoming

Abstract

Convective clouds are important to global energy balance and water cycle because they dynamically couple the planetary boundary layer to the free troposphere through vertical transport of heat, moisture, and mass. Vertical air motion and ice generation control the life cycle and precipitation efficiency in convective clouds, however, they are still not well understood. This study aims to improve our understandings of vertical air motion and phase partitioning in convective clouds using aircraft measurements and model simulations, and to evaluate and improve parameterizations of ice generation in numerical models.
There are two main parts in this study, in the first part, the vertical velocity and air mass flux in isolated convective clouds are statistically analyzed using airborne measurements from three field campaigns: High-Plains Cumulus (HiCu) conducted over the mid-latitude High Plains, COnvective Precipitation Experiment (COPE) conducted in a mid-latitude coastal area, and Ice in Clouds Experiment-Tropical (ICE-T) conducted over a tropical ocean. The results show small-scale updrafts and downdrafts (< 500 m in diameter) are frequently observed in the three field campaigns, and they make important contributions to the total air mass flux. The probability density functions (PDFs) of vertical velocity and air mass flux are exponentially distributed, and the updrafts generally strengthen with height. Relatively strong updrafts (> 20 m s-1) were observed in COPE and ICE-T, and the observed downdrafts are stronger in HiCu and COPE than in ICE-T.
In the second part particle size distributions (PSDs) and liquid-ice partitioning in tropical maritime convective clouds are studied using observations from ICE-T and numerical models. The airborne measurements show the liquid fraction between 0 C and -15 C decreases by a factor of about 3 in developing convective clouds, and a factor of 2 in mature clouds. In dissipating clouds, ice dominates in all temperature ranges. The airborne observations are used to evaluate the simulations using a parcel model and WRF with spectral bin microphysics (SBM). The results show there are two main differences between the modelled and observed results: 1) the modelled ice PSDs in strong updrafts between -7 °C and -10 °C are much broader than the observation. To interpret this difference, the freezing times of supercooled drops are calculated. The results indicate drop freezing is not instantaneous, and it is longer for large drops than for small drops. This offers a feasible explanation for the temperature-dependent ice size evolution in strong updrafts. 2) The modelled ice concentrations are significantly underestimated. Based on previous laboratory experiments and constraints from aircraft measurements, drop freezing-splinter (FS) and droplet collisional freezing (CF) mechanisms are parameterized and tested using a parcel model and WRF. With FS and CF implemented, the modelled PSDs and liquid-ice mass partitioning are more consistent with aircraft observations, suggesting FS and CF mechanisms are potentially important to the ice production in convective clouds.

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