Factors Limiting Convective Cloud-Top Height at the ARM Nauru Island Climate Research Facility

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 midtroposphere. Despite this importance they are not necessarily simulated very well in global climate models. Surface remote sensing observations and soundings from the Atmospheric Radiation Measurement (ARM) climate research facility at Nauru Island are coupled with a simple parcel model in order to address the following questions about these cloud types: 1) Which environmental factors play a role in determining the depth of tropical convective clouds? 2) What environmental parameters are related to entrainment rate in cumulus congestus clouds? The results presented herein suggest that at Nauru Island a drying of the midtroposphere is more likely to be responsible for limiting congestus cloud-top heights than is a stabilizing of the freezing level. It is also found that low-level CAPE and the RH profile account for the largest portion of the variance in cumulus congestus entrainment rates, consistent with the idea that entrainment rate depends on the buoyant production of turbulent kinetic energy. If the analysis is limited to cases where there is a sounding during the hour preceding the cumulus congestus observations, it is found that the low-level CAPE accounts for 85% of the total variance in entrainment rate.