Evaluation of Water Vapour Assimilation in the Tropical Upper Troposphere and Lower Stratosphere by a Chemical Transport Model
Abstract. The present analysis deals with one of the most debated aspect of the studies on the Upper Troposphere/Lower Stratosphere (UTLS), namely the budget of the water vapour (H2O) at the tropical tropopause. Within the French project "Multiscale water budget in the upper troposphere and lower stratosphere in the TROpics" (TRO-pico), a global-scale analysis has been set up based on space-borne observations, model and assimilation techniques. The MOCAGE-VALENTINA assimilation tool has been used to assimilate the Aura Microwave Limb Sounder (MLS) version 3.3 H2O measurements within the 316–5 hPa hPa range from August 2011 to March 2013 with an assimilation window of 1 hour. Diagnostics are developed to assess the quality of the assimilated H2O fields depending on several parameters: model error, observation minus analysis and forecast. Comparison with an independent source of H2O measurements in the UTLS based on the spaceborne Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) observations and with meteorological ARPEGE analyses are also shown. Sensitivity studies of the analyzed fields have been performed by: 1) considering periods when no MLS measurements are available and 2) using another MLS version 4.2 H2O data. The studies have been performed within 3 different spaces in time and space coincidences with MLS and MIPAS observations and with the model outputs and at 3 different levels: 121 hPa (upper troposphere), 100 hPa (tropopause), and 68 hPa (lower stratosphere) in January and February 2012. In the MLS space, the analyses behave consistently with the MLS observations from the upper troposphere to the lower stratosphere. In the model space, the analyses are wetter than the “true” atmosphere as represented by ARPEGE and MLS in the upper troposphere (121 hPa) and around the tropopause (100 hPa), but consistent with MLS and MIPAS in the lower stratosphere (68 hPa). In the MIPAS space, the sensitivity and the vertical resolution of the MIPAS data set at 121 and 100 hPa prevent to assess the behaviour of the analyses at 121 and 100 hPa particularly over intense convective areas as the Southern American, the African and the Maritime continents but, in the lower stratosphere (68 hPa), the analyses are very consistent with MIPAS. Sensitivity studies show the great improvement on the H2O analyses in the tropical UTLS when assimilating spaceborne measurements of better quality particularly over the convective areas.