An Observational Analysis of Cumulus Congestus at Nauru

Jensen, Michael	Brookhaven National Laboratory
Del Genio, Anthony	NASA/Goddard Institute for Space Studies

Cumulus congestus clouds, with moderate shortwave albedos and cloud-top temperatures near freezing, occur fairly often in the tropics. These clouds may play an important role in the evolution of the Madden-Julian oscillation and the regulation of relative humidity in the mid-troposphere. Despite this importance that are not necessarily simulated very well in global climate models. We use ARM TWP observations from Nauru to address some important research questions about these cloud types: 1) Which environmental factors play a role in determining the depth of tropical convective clouds? and 2) What environmental parameters are related to entrainment rate in cumulus congestus clouds? We use the ARSCL data product to define cumulus congestus cases for the entire Nauru dataset (76 cases with soundings in the previous 6 hr). We then use a simple parcel model with the nearest preceding sounding to estimate the entrainment rate (assuming constant entrainment rate with height) needed to get the observed cloud-top height. Assuming this entrainment profile is correct we then substitute the temperature profile from a sounding with weak stability near the freezing level and the relative humidity profile from a sounding with a moist mid-troposhpere in order to test the sensitivity of the soundings level of neutral buoyancy to the environmental thermodynamic profile. Our results show that the level of neutral buoyancy is more sensitive to the mid-level humidity compared to the stability at the freezing level. For the cumulus congestus cases the entrainment rates are approximately constant for cloud-top hegths up to the mid-troposphere, then decreasing entrainment rates are required to get higher cloud-tops. The entrainment rate tends to increase with increasing CAPE and increasing mid-level RH.

This poster will be displayed at the ARM Science Team Meeting.