Spatial and temporal variability of clouds and precipitation over Germany: multiscale simulations across the &amp;quot;gray zone&amp;quot;

C Barthlott,C Hoose

doi:10.5194/acp-15-12361-2015

Abstract

Abstract. This paper assesses the resolution dependance of clouds and precipitation over Germany by numerical simulations with the COnsortium for Small-scale MOdeling (COSMO) model. Six intensive observation periods of the HOPE (HD(CP)2 Observational Prototype Experiment) measurement campaign conducted in spring 2013 and 1 summer day of the same year are simulated. By means of a series of grid-refinement resolution tests (horizontal grid spacing 2.8, 1 km, 500, and 250 m), the applicability of the COSMO model to represent real weather events in the gray zone, i.e., the scale ranging between the mesoscale limit (no turbulence resolved) and the large-eddy simulation limit (energy-containing turbulence resolved), is tested. To the authors' knowledge, this paper presents the first non-idealized COSMO simulations in the peer-reviewed literature at the 250–500 m scale. It is found that the kinetic energy spectra derived from model output show the expected −5/3 slope, as well as a dependency on model resolution, and that the effective resolution lies between 6 and 7 times the nominal resolution. Although the representation of a number of processes is enhanced with resolution (e.g., boundary-layer thermals, low-level convergence zones, gravity waves), their influence on the temporal evolution of precipitation is rather weak. However, rain intensities vary with resolution, leading to differences in the total rain amount of up to +48 %. Furthermore, the location of rain is similar for the springtime cases with moderate and strong synoptic forcing, whereas significant differences are obtained for the summertime case with air mass convection. Domain-averaged liquid water paths and cloud condensate profiles are used to analyze the temporal and spatial variability of the simulated clouds. Finally, probability density functions of convection-related parameters are analyzed to investigate their dependance on model resolution and their impact on cloud formation and subsequent precipitation.

Highlights

The quantitative forecast of precipitation and clouds is still a challenge for state-of-the-art numerical models on both short-range weather time scales and climate time scales
The focus of this paper lies on the impact of a higher grid spacing on cloud and precipitation development and on the variability of convection and cloud-related parameters and how this variability changes with model resolution
Through a series of grid-refinement resolution tests, the applicability of the COnsortium for Small-scale MOdeling (COSMO) model in the gray zone and its large-eddy simulation capability were tested with horizontal grid spacings from 2.8 km down to 250 m for seven real cases over Germany

Summary

Introduction

The quantitative forecast of precipitation and clouds is still a challenge for state-of-the-art numerical models on both short-range weather time scales and climate time scales. A large part of the inaccuracy results from the difficulties of the models to initiate cloud formation and convective processes at the right place and time (e.g., Barthlott et al, 2011). The successful simulation of convection initiation over land, which is strongly forced from the surface, depends on having a reasonable representation of boundary-layer processes and the development of shallow cumulus convection (e.g., Petch et al, 2002). Some of the boundary-layer circulations, such as convective rolls, drylines, gust fronts, orographic circulations, and circulations resulting from mesoscale surface heterogeneities (i.e., land use, soil moisture), are precursors of cloud formation and convective development (Jorgensen and Weckwerth, 2003)

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