Mesoscale modelling of water vapour in the tropical UTLS: two case studies from the HIBISCUS campaignReport as inadecuate

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1 LPCE - Laboratoire de physique et chimie de l-environnement 2 GSMA - Groupe de spectrométrie moléculaire et atmosphérique 3 SA - Service d-aéronomie 4 Centro de Previsࢳ

Abstract : In this study, we evaluate the ability of the BRAMS mesoscale model compared to ECMWF global analysis to simulate the observed vertical variations of water vapour in the tropical upper troposphere and lower stratosphere UTLS. The observations are balloon-borne measurements of water vapour mixing ratio and temperature from micro-SDLA Tunable Diode Laser Spectrometer instrument. Data from two balloon flights performed during the 2004 HIBISCUS field campaign are used to compare with the mesoscale simulations and to ECMWF analysis. The mesoscale model performs significantly better than ECMWF analysis for water vapour in the upper troposphere and similarly or slightly worse for temperature. The improvement provided by the mesoscale model for water vapour comes mainly from i the enhanced vertical resolution in the UTLS 250 m for BRAMS and ~1 km for ECMWF model and ii the more detailed microphysical parameterization providing ice supersaturations as in the observations. The ECMWF vertical resolution ~1 km is too coarse to capture the observed fine scale vertical variations of water vapour in the UTLS. In near saturated or supersaturated layers, the mesoscale model relative humidity with respect to ice saturation is close to observations provided that the temperature profile is realistic. For temperature, ECMWF analysis gives good results partly thanks to data assimilation. The analysis of the mesoscale model results showed that in undersaturated layers, the water vapour profile depends mainly on the dynamics. In saturated-supersaturated layers, microphysical processes play an important role and have to be taken into account on top of the dynamical processes to understand the water vapour profiles. In the lower stratosphere, the ECMWF model and the BRAMS model give very similar water vapour profiles that are significantly dryer than micro-SDLA measurements. This similarity comes from the fact that BRAMS is initialised using ECMWF analysis and that no mesoscale process acts in the stratosphere leading to no modification of the BRAMS results with respect to ECMWF analysis.

Author: V. Marécal - G. Durry - K. Longo - S. Freitas - E. D. Rivière - M. Pirre -



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