Observations of fair-weather cumuli over land: Dynamical factors controlling cloud size and cover

Abstract

Comprehensive observations of shallow convection at the Atmospheric Radiation Measurement Southern Great Plains site are carefully analyzed to study the macrophysical and dynamical properties of active and forced cumuli separately and investigate their relationship to the subcloud layer turbulent structure. Clearly, active clouds possess stronger dynamics and greater horizontal extent than their forced counterpart. As previously reported, upper level stability and relative humidity do control the predominance of active clouds. While cloud cover remains difficult to associate to mixed‐layer parameters (small correlation coefficients), mixed‐layer top vertical velocity skewness, and coherent updraft fraction most significantly correlate to cumulus cloud cover and especially the portion attributed to active clouds; both of which are not currently considered in shallow cloudiness parameterizations. This study also points to several factors that continue to limit our ability to adequately sample shallow cumuli and suggests that forward models will be necessary to bridge observations and model outputs.

The Implications for Radiative Cloud Forcing via the Link Between Shallow Convection and Planetary Boundary Layer Mixing

“…The feedback of shallow convection on low‐level clouds is mainly caused by the heat and moisture transfer between the boundary layer and the free atmosphere (e.g., Browning et al, ; Zhang & Klein, ). However, with an average spatial scale of 0.5–1.5 km (e.g., Lamer & Kollias, ), shallow convective clouds are not resolved in the typical grid resolution of weather forecasting and climate models, so parameterization has to be used to capture the impact of these shallow convective clouds using the grid‐scale meteorological state (Han & Pan, ; Hong et al, ). The adequate modulation of the shallow convection scheme plays an important role in forming and dissipating low‐level clouds in the tropics and subtropics (Szoeke et al, ).…”

Estimation of cloud fraction profile in shallow convection using a scanning cloud radar

“…Despite their low cloud fraction, shallow cumuli also have an important role in modulating shortwave radiative forcing [ Berg et al ., ]. However, with an average spatial scale of 0.5–1.5 km [e.g., Lamer and Kollias , ], shallow cumuli are not resolved in weather forecast and climate models, which rely on parameterizations of the subgrid‐scale properties to capture the impact of these clouds on the grid‐scale meteorological state. The cloud fraction profile (CFP) is a key property of interest because the fractional cloudiness of a gridbox affects the radiative transfer [e.g., Albrecht , ; Larson et al ., ] and the CFP affects the vertical cumulus mass flux [e.g., de Roode and Bretherton , ; van Stratum et al ., ].…”

“…The simulated cloud structures and properties are typically evaluated using observations. At the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility's Southern Great Plains (SGP) site, several long‐term studies of shallow cumuli have been conducted by using aircraft and ground‐based observations [ Berg and Kassianov , ; Vogelmann et al ., ; Chandra et al ., ; Zhang and Klein , ; Lamer and Kollias , ]. Zenith profiling cloud radar and lidar measurements traditionally have been used to provide CFP estimates [e.g., Hogan et al ., ; Kollias et al ., ; Rémillard et al ., ].…”

“…The typical maximum range of a SCR is 20 km, and thus, following equation , the SCR sensitivity drops by 26 dB at 20 km compared to its sensitivity at 1 km. Since shallow cumuli over land typically have low reflectivities, falling below Z min ( r ) [e.g., Lamer and Kollias , ], the strong drop in the SCR sensitivity with range creates the illusion of a cloudier atmosphere closer to the radar location. These issues likely have a greater impact when detecting shallow cumuli compared to deeper, mesoscale precipitation systems or stratocumulus cloud decks due to the large spatial heterogeneity and relatively low liquid water content of shallow cumuli [e.g., Warner , ; Blyth , ].…”

Cloud characteristics, thermodynamic controls and radiative impacts during the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) experiment

“…There are several limitations when designating cloud boundaries and hourly CF observations from vertically pointing cloud radars beyond the capabilities of single radar platforms or ARSCL methods (e.g., Lamer and Kollias, 2015;Oue et al, 2016). The primary limitation among these is that the WACR experiences attenuation in rain that manifests as erroneously low or missing cloud-top boundaries (e.g., Feng et al, 2009Feng et al, , 2014.…”