Controls on Cloud Thermodynamic Phase in a Simulated Population of Deep Convective Clouds
Abstract
In this work, we investigate environmental and microphysical controls on cloud glaciation. We ran cloud resolving model (CRM) simulations in the Radiative-Convective-Equilibrium configuration and found that the theoretical arguments made by Korolev and Field (2008) nicely translate to the CRM scale. At low updraft speeds (< 2 m/s), glaciated clouds are found at temperatures as high as 255 K while stronger updrafts completely diminish the probability of finding a glaciated cloud above the homogenous freezing temperature (235 K). Theory dictates that the onset of the Wegener-Bergeron-Findeisen (WBF) process is controlled by whether the vapor depositional growth rate of ice exceeds the condensational growth of liquid. Since the relevant variables for this condition are updraft velocity, temperature, and the ice number concentration, we determined a critical ice number concentration (N*) that is needed to glaciate a cloud for a given temperature and updraft speed. We show that N* explains the distribution of cloud phase in our simulations; N* is on the order of 100 ice crystals per kg of air at low updrafts speeds but exceeds 10000 ice crystals per kg of air for faster ascents. N* is weakly dependent on temperature except above 260 K where the difference in the vapor pressures between ice and liquid exponentially increases with decreasing temperature. The correlation between the likeliness of glaciation and temperature can thus be mostly explained by the correlation between temperature and ice number concentration with a microphysical effect of slowly ascending parcels exhibiting anomolosly high ice number concentration playing a secondary - alebit important - role. The latter is due to faster updrafts having, on average, shorter residence times within the cold upper levels of the domain. Our hope is that these CRM results could be used to diagnose thermodynamic phase in cloud and convective schemes thus bridging a prevalent gap between theory and parametrization.
This work was performed under the auspices of the US DOE by Lawrence Livermore National Laboratory under contract No. DE-AC52-07NA27344 References: Korolev and Field, 2008. DOI: 10.1175/2007JAS2355.1- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A53R2952B
- Keywords:
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- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 3355 Regional modeling;
- ATMOSPHERIC PROCESSES;
- 3360 Remote sensing;
- ATMOSPHERIC PROCESSES;
- 1626 Global climate models;
- GLOBAL CHANGE