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Evaluation of ECMWF water vapour fields by airborne differential absorption lidar measurements: a case study between Brazil and Europe

by H. Flentje, A. Dörnbrack, A. Fix, G. Ehret, E. Hólm
Atmospheric Chemistry and Physics ()

Abstract

Three extended airborne Differential Absorption Lidar (DIAL) sections\nof tropospheric water vapour across the tropical and sub-tropical\nAtlantic in March 2004 are compared to short-term forecasts of the\nEuropean Centre for Medium Range Weather Forecasts (ECMWF). The humidity\nfields between 28� S and 36� N exhibit large inter air-mass gradients\nand reflect typical transport patterns of low- and mid-latitudes\nlike convection (e.g. Hadley circulation), subsidence and baroclinic\ndevelopment with stratospheric intrusion. These processes re-distribute\nwater vapour vertically such that locations with extraordinary dry/moist\nair-masses are observed in the lower/upper troposphere, respectively.\nThe mixing ratios range over 3 orders of magnitude. Back-trajectories\nare used to trace and characterize the observed air-masses.\n\nOverall, the observed water vapour distributions are largely reproduced\nby the short-term forecasts at 0.25� resolution (T799/L91), the correlation\nranges from 0.69 to 0.92. Locally, large differences occur due to\ncomparably small spatial shifts in presence of strong gradients.\nSystematic deviations are found associated with specific atmospheric\ndomains. The planetary boundary layer in the forecast is too moist\nand to shallow. Convective transport of humidity to the middle and\nupper troposphere tends to be overestimated. Potential impacts arising\nfrom data assimilation and model physics are considered. The matching\nof air-mass boundaries (transport) is discussed with repect to scales\nand the representativity of the 2-D sections for the 3-D humidity\nfield. The normalized bias of the model with respect to the observations\nis 6%, 11% and 0% (moist model biases) for the three along-flight\nsections, whereby however the lowest levels are excluded.

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