Mesoscale modelling of water vapour in the tropical UTLS: two case studies from the HIBISCUS campaign

[摘要] In this study, we evaluate the ability of the BRAMS (Brazilian RegionalAtmospheric Modeling System) mesoscale model compared to ECMWF globalanalysis to simulate the observed vertical variations of water vapour in thetropical upper troposphere and lower stratosphere (UTLS). The observationsare balloon-borne measurements of water vapour mixing ratio and temperaturefrom micro-SDLA (Tunable Diode Laser Spectrometer) instrument. Data from twoballoon flights performed during the 2004 HIBISCUS field campaign are usedto compare with the mesoscale simulations and to the ECMWF analysis.

The observations exhibit fine scale vertical structures of water vapour of afew hundred meters height. The ECMWF vertical resolution (~1 km) is toocoarse to capture these vertical structures in the UTLS. With a verticalresolution similar to ECMWF, the mesoscale model performs better than ECMWFanalysis for water vapour in the upper troposphere and similarly or slightlyworse for temperature. The BRAMS model with 250 m vertical resolution isable to capture more of the observed fine scale vertical variations of watervapour compared to runs with a coarser vertical resolution. This is mainlyrelated to: (i) the enhanced vertical resolution in the UTLS and (ii) to themore detailed microphysical parameterization providing ice supersaturationsas in the observations. In near saturated or supersaturated layers, themesoscale model predicted relative humidity with respect to ice saturationis close to observations provided that the temperature profile is realistic.For temperature, the ECMWF analysis gives good results partly attributed todata assimilation. The analysis of the mesoscale model results showed thatthe vertical variations of the water vapour profile depends on the dynamicsin unsaturated layer while the microphysical processes play a major role insaturated/supersaturated layers.

In the lower stratosphere, the ECMWF model and the BRAMS model give verysimilar water vapour profiles that are significantly drier than micro-SDLAmeasurements. This similarity comes from the fact that BRAMS is initialisedusing ECMWF analysis and that no mesoscale process acts in the stratosphereleading to no modification of the BRAMS results with respect to ECMWFanalysis.

[学科分类] 大气科学