Cloud Shading Effects on Characteristic Boundary-Layer Length Scales

Abstract

We studied the effects of shading by shallow cumulus (shallow Cu) and the subsequent effect of inducing heterogeneous conditions at the surface on boundary-layer characteristics. We placed special emphasis on quantifying the changes in the characteristic length and time scales associated with thermals, shallow Cu and induced thermal circulation structures. A series of systematic numerical experiments, inspired by Amazonian thermodynamic conditions, was performed using a large-eddy simulation model coupled to a land-surface model. We used four different experiments to disentangle the effects of shallow Cu on the surface and the response of clouds to these surface changes. The experiments include a 'clear case', 'transparent clouds', 'shading clouds' and a case with a prescribed uniform domain and reduced surface heat flux. We also performed a sensitivity study on the effect of introducing a weak background flow. Length and time scales were calculated using autocorrelation and two-dimensional spectral analysis, and we found that shading controlled by shallow Cu locally lowers surface temperatures and consequently reduces the sensible and latent heat fluxes, thus inducing spatial and temporal variability in these fluxes. The length scale of this surface heterogeneity is not sufficiently large to generate circulations that are superimposed on the boundary-layer scale, but the heterogeneity does disturb boundary-layer dynamics and generates a flow opposite to the normal thermal circulation. Besides this effect, shallow Cu shading reduces turbulent kinetic energy and lowers the convective velocity scale, thus reducing the mass flux. This hampers the thermal lifetime, resulting in a decrease in the shallow Cu residence time (from 11 to 7 min). This reduction in lifetime, combined with a decrease in mass flux, leads to smaller clouds. This is partially compensated for by a decrease in thermal cell size due to a reduction in turbulent kinetic energy. As a result, inter-cloud distance is reduced, leading to a larger population of smaller clouds, while maintaining cloud cover similar to the non-shading clouds experiment. Introducing a 1 m s −1 background wind speed increases the thermal size in the sub-cloud layer, but the diagnosed surface-cloud coupling, quantified by characteristic time and length scales, remains.

The role of 1-D and 3-D radiative heating in the organization of shallow cumulus convection and the formation of cloud streets

“…In this work, we will focus on the radiative impact, the most prominent effect of which being cloud shadows which modulate surface fluxes and consequently build up surface heterogeneities. These induced surface heterogeneities are the link between radiative transfer and buoyancy-driven convection (Lohou and Patton, 2014;Horn et al, 2015;Gronemeier et al, 2016). Our focus is therefore more on buoyancy-driven roll vortices in a linear shear environment (Asai, 1970) and less so on inflectionpoint instabilities.…”

“…This does not allow us to study the timescales on which radiation and dynamics may interact. Others investigated the influence of shading coupled to an interactive surface model (Vilà-Guerau de Arellano et al, 2014;Lohou and Patton, 2014;Horn et al, 2015). However, one particularly questionable issue with those studies was the application of 1-D radiative transfer solvers, which are known to introduce large spatial error in surface heating rates (O'Hirok and Gautier, 2005;Wapler and Mayer, 2008;Wissmeier et al, 2013;Jakub and Mayer, 2015).…”

Parameterizations for convective transport in various cloud-topped boundary layers

“…On the other hand, the continental ABL is affected by a diurnal cycle in the surface forcing. The large variation in surface forcing during the day drive the initiation of ShCu formation, therefore impacting the dynamical structures in the ABL (Horn et al, 2015). As this situation is harder to study and therefore less investigated, four continental campaigns are selected, ranging from the mid-latitudes to the tropics, to serve as inspiration for the LES numerical experiments: the Tropical Forest and Fire Emissions Experiment (TROFFEE; Karl et al, 2007), the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS; Jiang et al, 2008), the Small Cumulus Microphysics Study (SCMS; Neggers et al, 2003) and the Atmospheric Radiation Measurements (ARM; Brown et al, 2002).…”

“…As shown by Ouwersloot et al (2011), the chemical variability in clear sky conditions is affected by ABL dynamics, creating regions of high and low concentrations, thereby modifying the mean reactivity. Since ShCu impact the dynamical structures in the ABL (Horn et al, 2015), it will enhance this segregation of species (Kim et al, 2004). As below the ShCu, the concentrations of chemical species differ more from cloud-layer concentrations than the mean concentrations in the ABL .…”

“…During this process, the a cc increases fast, indicating that the threshold for active ShCu growth is overcome. At the end of the transition phase, cloud venting affects the sub-cloud layer structures by redistributing thermals (Horn et al, 2015). As this transport of energy out of the sub-cloud layer affects the thermal structures, the area fraction of forced clouds decreases due to a decrease in the amount of thermals that reach the cloud layer.…”