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

Abstract

Abstract. The formation of shallow cumulus cloud streets was historically attributed primarily to dynamics. Here, we focus on the interaction between radiatively induced surface heterogeneities and the resulting patterns in the flow. Our results suggest that solar radiative heating has the potential to organize clouds perpendicular to the sun's incidence angle.To quantify the extent of organization, we performed a high-resolution large-eddy simulation (LES) parameter study. We varied the horizontal wind speed, the surface heat capacity, the solar zenith and azimuth angles, and radiative transfer parameterizations (1-D and 3-D). As a quantitative measure we introduce a simple algorithm that provides a scalar quantity for the degree of organization and the alignment. We find that, even in the absence of a horizontal wind, 3-D radiative transfer produces cloud streets perpendicular to the sun's incident direction, whereas the 1-D approximation or constant surface fluxes produce randomly positioned circular clouds. Our reasoning for the enhancement or reduction of organization is the geometric position of the cloud's shadow and its corresponding surface fluxes. Furthermore, when increasing horizontal wind speeds to 5 or 10 m s −1 , we observe the development of dynamically induced cloud streets. If, in addition, solar radiation illuminates the surface beneath the cloud, i.e., when the sun is positioned orthogonally to the mean wind field and the solar zenith angle is larger than 20 • , the cloud-radiative feedback has the potential to significantly enhance the tendency to organize in cloud streets. In contrast, in the case of the 1-D approximation (or overhead sun), the tendency to organize is weaker or even prohibited because the shadow is cast directly beneath the cloud. In a land-surface-type situation, we find the organization of convection happening on a timescale of half an hour. The radiative feedback, which creates surface heterogeneities, is generally diminished for large surface heat capacities. We therefore expect radiative feedbacks to be strongest over land surfaces and weaker over the ocean. Given the results of this study we expect that simulations including shallow cumulus convection will have difficulties producing cloud streets if they employ 1-D radiative transfer solvers or may need unrealistically high wind speeds to excite cloud street organization.

The Impact of Surface Heterogeneities and Land‐Atmosphere Interactions on Shallow Clouds Over ARM SGP Site

“…AVE_SFC_RADSW also produces larger clouds and less count of clouds than the control. This is consistent with results from previous studies with more idealized setups (e.g., Gronemeier et al, ; Horn et al, ; Jakub & Mayer, ). When the shift distance increases to more than 3 km, further change in cloud size and cloud count becomes small in Figure , reminiscent of what we see in the cloud LWP response in Figure .…”

“…For a 3‐km shift (which corresponds roughly to a 60° solar incidence angle for 20060808), the cloud LWP response (to removing cloud shading‐induced heterogeneities) becomes negative and larger in magnitude than the (positive) response with no shift for both 20060808 and 20070514 in d03. These results are consistent with findings from recent studies by Gronemeier et al () and Jakub and Mayer () and adds more complexity to the land‐atmosphere coupling in the presence of shallow clouds.…”

“…Two new studies by Gronemeier et al () and Jakub and Mayer () considered 3‐D radiative transfer and showed that larger zenith angles tend to promote larger horizontal structures in shallow clouds, in contrast to the small sensitivity to solar zenith angle found in the idealized study of Schumann et al (). LES studies that considered idealized static land surface heterogeneities (e.g., checkboard patterns) in addition to cloud shading (e.g., Gronemeier et al, ; Rieck et al, ) also suggested dependence of cloud structure and development on the characteristics of these static heterogeneities.…”

Substantial Reductions in Cloud Cover and Moisture Transport by Dynamic Plant Responses

“…For instance, Jakub and Mayer (2017) show that the three-dimensional radiative effects on the surface leads to the organization of cumulus clouds streets, while a three-dimensional incanopy radiative transfer can improve modeled photosynthesis levels (Kobayashi et al, 2012). Although this also has not been accounted for in regional and global weather models, it is known to influence in-canopy processes that ultimately affect the overlying atmosphere.…”

A Path‐Tracing Monte Carlo Library for 3‐D Radiative Transfer in Highly Resolved Cloudy Atmospheres

“…While many components of the Earth system interact with radiation, clouds play a key role because of their strong impact (globally cooling the Earth; Ramanathan et al, ), their high frequency of occurrence (Rossow & Dueas, ), and their inherent complexity in both space and time (Davis et al, ). Radiation and its interactions with clouds are involved in various atmospheric applications at a large range of scales: from the Earth's energy balance and cycle relevant to numerical weather predictions (Hogan et al, ) and climate studies (Cess et al, ; Dufresne & Bony, ) to the inhomogeneous heating and cooling rates that modify dynamics and cloud processes at small scales (Jakub & Mayer, ; Klinger et al, , ), and to the retrieval of atmospheric state and properties from radiative quantities such as photon path statistics, spectrally resolved radiances, and polarized reflectances (Cornet et al, ), observed by both active and passive remote sensors.…”