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The formation of shallow cumulus cloud streets was historically\udattributed primarily to dynamics.  Here, we focus on the interaction\udbetween radiatively induced surface heterogeneities and the\udresulting patterns in the flow.  Our results suggest that solar\udradiative heating has the potential to organize clouds perpendicular\udto the sun's incidence angle.\ud  \udTo quantify the extent of organization, we performed a high-resolution large-eddy\udsimulation (LES) parameter study.  We varied the horizontal wind\udspeed, the surface heat capacity, the solar zenith and azimuth\udangles, and radiative transfer parameterizations (1-D and\ud3-D).  As a quantitative measure we introduce a simple algorithm\udthat provides a scalar quantity for the degree of organization and\udthe alignment.  We find that, even in the absence of a horizontal\udwind, 3-D radiative transfer produces cloud streets perpendicular to\udthe sun's incident direction, whereas the 1-D approximation or\udconstant surface fluxes produce randomly positioned circular\udclouds.  Our reasoning for the enhancement or reduction of\udorganization is the geometric position of the cloud's shadow and its\udcorresponding surface fluxes.  Furthermore, when increasing\udhorizontal wind speeds to 5 or 10 m s<sup>−1</sup>, we observe\udthe development of dynamically induced cloud streets.  If, in\udaddition, solar radiation illuminates the surface beneath the cloud,\udi.e., when the sun is positioned orthogonally to the mean wind field\udand the solar zenith angle is larger than 20°, the\udcloud-radiative feedback has the potential to significantly enhance\udthe tendency to organize in cloud streets.  In contrast, in the case\udof the 1-D approximation (or overhead sun), the tendency to organize\udis weaker or even prohibited because the shadow is cast directly\udbeneath the cloud. In a land-surface-type situation, we find the\udorganization of convection happening on a timescale of half an hour.\udThe radiative feedback, which creates surface heterogeneities, is\udgenerally diminished for large surface heat capacities.  We\udtherefore expect radiative feedbacks to be strongest over land\udsurfaces and weaker over the ocean. Given the results of this study\udwe expect that simulations including shallow cumulus convection will\udhave difficulties producing cloud streets if they employ 1-D\udradiative transfer solvers or may need unrealistically high wind\udspeeds to excite cloud street organization

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