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

Abstract

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.

Aerosol Effects on Cumulus Congestus Population over the Tropical Pacific: A Cloud-Resolving Modeling Study

“…The cloud columns with surface radar reflectivity higher than 5 dBZ and with the 0 dBZ (cloud top) heights between 3 and 5 km are defined as congestus. The threshold of 5 km for the cloud top is lower than most of the previous studies (e.g., Jensen and Del Genio 2006); however, when we raise this upper limit, the results tend to overestimate the congestus population by mislabeling some of the dissipating stratiform area as congestus, especially during the quasi-steady state later in the simulation. In the current definition, the differences between the two methods are primarily at the beginning of the simulation, where congestus clouds are mislabeled as deep convection when their cloud top heights surpass 5 km.…”

“…Previous studies used different criteria to define congestus, mainly depending on the observational instruments used (e.g., Johnson et al 1999, Masunaga et al 2005, Jensen and Del Genio 2006, Luo et al 2009). Nevertheless, there are two main physical characteristics of cumulus congestus: 1) the convection is deep enough to rise above the trade inversion and produces significant surface rain (as opposed to the shallow trade wind cumuli) and 2) there are little or no ice phase particles in the convection (as opposed to the deep cumulonimbus).…”

On the role of aerosols, humidity, and vertical wind shear in the transition of shallow-to-deep convection at the Green Ocean Amazon 2014/5 site

“…Shallow and congestus convection transports moisture from the atmospheric boundary layer (BL) to the lower and middle troposphere, thus allowing for the development of deep convection (Zhuang et al, 2017;Del Genio and Wu, 2010;Jensen and Del Genio, 2006). However, many previous studies illustrate difficulties in representing the shallow-to-deep evolution in models (Del Genio and Wu, 2010;Waite and Khouider, 2010).…”

Reproducibility of precipitation distribution over the tropical oceans in CMIP5 multi-climate models compared to CMIP3

“…1), where only light precipitation associated with shallow convection is observed despite the relatively high SST. Many recent observation based studies as well as model studies suggest importance of environmental humidity air in dynamical suppression of deep convection (Sherwood 1999;Jensen and Del Genio 2006;Wang et al 2007;Holloway and Neelin 2009). For example, Takayabu et al (2010) explained that since the mid-to-lower troposphere is very dry over large-scale subsidence regions, entrainment of dry environmental air to a convective parcel effectively reduces the parcel's buoyancy suppressing deep convection.…”