Benefit of convection permitting climate model sim

Authors: G Fosser S Khodayar P Berg

Publish Date: 2014/08/02

Volume: 44, Issue: 1-2, Pages: 45-60

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Abstract

A major source of uncertainty in regional climate model RCM simulations arises from the parameterisation of subgrid scale convection With increasing model resolution approaching the socalled convection permitting scale it is possible to switch off most of the convection parameterisations A set of simulations using COSMOCLM model has been carried out at different resolutions in order to investigate possible improvements and limitations resulting from increased horizontal resolution For our analysis 30 years were simulated in a triple nesting setup with 50 7 and 28 km resolutions with ERA40 reanalysis data at the lateral boundaries of the coarsest nest The investigation area covers the state of BadenWürttemberg in southwestern Germany which is a region known for abundant orographically induced convective precipitation A very dense network of high temporal resolution rain gauges is used for evaluation of the model simulations The purpose of this study is to examine the differences between the 7 and 28 km resolutions in the representation of precipitation at subdaily timescales and the atmospheric conditions leading to convection Our results show that the highest resolution of RCM simulations significantly improves the representation of both hourly intensity distribution and diurnal cycle of precipitation In addition at convection permitting scale the atmospheric fields related to convective precipitation show a better agreement with each other The results imply that higher spatial resolution partially improves the representation of the precipitation field which must be the way forward for regional climate modellingPrecipitation is one of the most crucial variables to assess in a changing climate However it is also one of the most difficult to simulate Although daily precipitation statistics improve with increasing spatial resolution Boberg et al 2009 2010 Berg et al 2013 problems remain with the subdaily statistics such as the diurnal cycle Dai and Trenberth 2004 Dai 2006 Recent investigations have shown the importance of higher temporal resolution in precipitation statistics Haerter et al 2010 Berg et al 2013 especially for assessments of convective type precipitation Lenderink and van Meijgaard 2008 Haerter and Berg 2009A high temporal resolution of precipitation statistics is especially important in summer when precipitation over land is often convective Dai and Trenberth 2004 Dai 2006 In regional climate models RCMs convective precipitation usually occurs too early in the day and the amplitude of its diurnal cycle is usually overestimated Brockhaus et al 2008 Yang and Slingo 2001 Dai and Trenberth 2004 Dai 2006 The correct representation of convective precipitation is a very complicated task in modelling because it involves complex interactions between the surface the boundary layer and the free troposphere Khodayar et al 2013 Furthermore vertical profiles of temperature and humidity define the convective available potential energy CAPE Moncrieff and Miller 1976 thereby affecting the strength of the convective systems and the associated precipitation intensities eg Emanuel 1994 Since convection acts on a scale of only a few kilometres it is necessary to use parameterisations for typical RCM simulations with grid resolutions of 10–100 km According to Weisman et al 1997 4 km spatial resolution may be sufficient for nonhydrostatic models to explicitly represent convective systems Previous studies eg Hohenegger et al 2008 Brockhaus et al 2008 Bechtold et al 2004 recognised the parameterisation of convection as a major source of uncertainties and errors in simulating the diurnal cycle of precipitationIncreasing the spatial resolution towards convection permitting scales provides the possibility to switch off a major part of the convective parameterisations In addition a higher resolution model allows a better representation of orography and surface fields which are crucial for the initiation of convection in complex terrain Hohenegger et al 2008 Copious literature proves the added value of this spatial resolution in the representation of the precipitation field especially in cases of moist convection and/or in regions with strong orography Langhans et al 2013 Mass et al 2002 Miura et al 2007 Grell et al 2000 Richard et al 2007 Lean et al 2008 Schwartz et al 2009 Weusthoff et al 2010 Baldauf et al 2011 Hohenegger et al 2008 Prein et al 2013 Most of these studies are related to numerical weather prediction NWP or limited to few summers The computational cost is high for running longterm highresolution simulations and there is furthermore a shortage of suitable observational datasets available for evaluationThe goal of this paper is to investigate the possible improvements and limitations in the representation of convective precipitation resulting from increased horizontal resolution in climate model simulations For this purpose we compare the performance of two climatological simulations of 30 years each with the COSMOCLM COnsortium for Small scale Modelling modelin CLimate Model here abbreviated CLM one at 7 km CLM7 horizontal resolution with parameterisation of convection and one at 28 km resolution CLM28 with most convective parameterisations turned off such that no subgrid scale precipitation is calculated A high temporal resolution and spatially dense station observational data set is used for validation Since the main difference between the two simulations in terms of model setup resides in the convection scheme we explore the differences between the two resolutions in the representation of the conditions leading to convection in detail In this context the instability of the atmosphere vertical profiles of temperature and humidity as well as cloud cover radiation budget and triggering mechanisms are investigated In Sect 2 we present the investigation area and its characteristics and in Sect 3 we describe the model configuration for the different simulations as well as the observational data used for validation purpose An overview of the methods is provided in Sect 4 In Sect 5 we discuss the results on the representation of precipitation and the atmospheric conditions leading to convection and we end with conclusions in Sect 6The panels show a the simulation domain for each step of the nesting strategy namely 50 km domain red 7 km blue 28 km black b the topographical features of the simulation domain at 28 km solid box located in BadenWürttemberg in southwestern Germany The dashed box indicates the investigation area common to all simulations The blue crosses indicate the locations of the precipitation gaugesCOSMO is a nonhydrostatic local area model originally created by the German Weather Service DWD for weather predictions Steppeler et al 2003 and later adapted to perform climatological simulations by the COSMOCLM community Böhm et al 2006 Using ERA40 reanalysis data Uppala et al 2005 as driving data for the simulations a triple nesting procedure in rotated coordinates was employed to reach the final high resolution Fig 1a The coarsest nest at 044° resolution circa 50 km comprises large parts of Europe 118 × 112 grid points The next finer nest here referred to as CLM7 at 00625° circa 7 km covers all of Germany and the near surroundings 165 × 200 grid points and the finest nest here referred to as CLM28 used here is at 0025° circa 28 km and concentrates on the state of BadenWürttemberg 140 × 116 grid points similarly to Prein et al 2013 in southwestern Germany Fig 1 All simulations use 40 vertical levels and nine soil layers and are carried out for the period 1968–1999 with the first 3 years considered as spinup time and therefore not included in the analysis The present study focuses on the added value of resolving convection in the representation of atmospheric conditions leading to precipitation In this context the coarser nest at 50 km resolution see Berg et al 2013 is not included in the current analysisThe 7 km resolution is in a spatial range where larger convective processes can be resolved while smaller ones need a parameterisation thus there could be an overlap between resolved and parameterised convection Therefore in addition to CLM28 and CLM7 a shorter run 1991–1999 including 1 year spinup time called CLM7 conv at 00625° was performed over the same simulation domain as CLM7 but without most of the convection parameterisations The setup used in this study CLM version 48 employs a Runge–Kutta timestepping scheme and a deltatwostream radiation scheme according to Ritter and Geleyn 1992 called every hour for CLM7 and CLM7 conv and every 15 min for CLM28 Convective mass flux is parameterised in CLM7 according to the Tiedtke 1989 scheme with a moisture convergence CLM28 and CLM7 conv utilise this parameterisation only for shallow convection thus no precipitation is produced for the subgrid Vertical turbulent diffusion uses prognostic turbulent kinetic energy TKE Raschendorfer 2001 including effects from subgridscale condensation and from thermal circulations The Kessler scheme Kessler 1969 is used for the calculation of grid scale clouds and precipitation and it considers the following hydrometeor species water vapour cloud droplets rain snow cloud ice and for CLM28 additionally graupel More details of CLM can be found in Doms et al 2011

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