Abstract
A correct representation of the coupling between convection and circulation constitutes a prerequisite for a correct representation of precipitation at all scales. In this study, the coupling between convection and a sea breeze is investigated across three main resolutions: large-eddy resolution where convection is fully explicit, convection-permitting resolution where convection is partly explicit and coarse resolution where convection is parameterised. The considered models are the UCLA-LES, COSMO and ICON. Despite the use of prescribed surface fluxes, comparison of the simulations reveals that typical biases associated with a misrepresentation of convection at convection-permitting and coarser resolutions significantly alter the characteristics of the sea breeze. The coarse-resolution simulations integrated without convective parameterisation and the convection-permitting simulations simulate a too slow propagation of the breeze front as compared to the large-eddy simulations. From the various factors affecting the propagation, a delayed onset and intensification of cold pools primarily explains the differences. This is a direct consequence of a delayed development of convection when the grid spacing is coarsened. Scaling the time the sea breeze reaches the centre of the land patch by the time precipitation exceeds 2 mm day−1, used as a measure for significant evaporation, yields a collapse of the simulations onto a simple linear relationship although subtle differences remain due to the use of different turbulence and microphysical schemes. Turning on the convection scheme significantly disrupts the propagation of the sea breeze due to a misrepresented timing (too early triggering) and magnitude (too strong precipitation evaporation in one of the tested convection schemes) of the convective processes.
Highlights
The distribution of convective precipitation, being at local, meso- or larger scales, is determined by the thermodynamical structure of the atmosphere and its interactions with dynamical processes
The choice of a threshold of 2 mm day(1 is motivated by the results of sections 3 and 4 which have shown that differences in the onset time of cold pools primarily explain differences in to collide (Tcol) among the experiments using explicit convection
This study investigated the coupling between convection and a sea breeze at three main resolutions: large-eddy resolution where convection is fully explicit, convectionpermitting resolution where convection is partly explicit and coarse resolution where convection is parameterised
Summary
The distribution of convective precipitation, being at local, meso- or larger scales, is determined by the thermodynamical structure of the atmosphere and its interactions with dynamical processes. Convective clouds can develop randomly as turbulent plumes rise through the boundary layer, their development is often linked to the existence of circulations. Such circulations can emerge from horizontal gradients in heating rates, either as a result of convection itself The wellknown tight interplay between convection and circulation is a critical factor hampering a correct representation of precipitation at all scales, but such scale interactions are. The situation of a sea breeze interacting with convection is a natural laboratory to investigate issues related to convectionÁcirculation coupling and their representation across resolutions. On the one hand and as compared to a situation without a background pre-existing circulation, sea breezes alter the timing, amount and location of precipitation The development of clouds and precipitation modifies the sea breeze characteristics (Berg and Oerlemans, 1985; Segal et al, 1986; Nicholls et al, 1991) and especially leads to a faster propagation of License (http://creativecommons.org/licenses/by/4.0/), allowing third parties to copy and redistribute the material in any medium or format and to remix, transform, and build upon the material for any purpose, even commercially, provided the original work is properly cited and states its license
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