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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 Doernbrack, A Fix, G Ehret, E Holm
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Three extended airborne Differential Absorption Lidar (DIAL) sections of\ntropospheric water vapour across the tropical and sub-tropical Atlantic\nin March 2004 are compared to short-term forecasts of the European\nCentre for Medium Range Weather Forecasts (ECMWF). The humidity fields\nbetween 28 degrees S and 36 degrees N exhibit large inter air-mass\ngradients and reflect typical transport patterns of low- and\nmid-latitudes like convection (e. g. Hadley circulation), subsidence and\nbaroclinic development with stratospheric intrusion. These processes\nre-distribute water vapour vertically such that locations with\nextraordinary dry/moist air-masses are observed in the lower/upper\ntroposphere, respectively. The mixing ratios range over 3 orders of\nmagnitude. Backtrajectories are used to trace and characterize the\nobserved air-masses.\nOverall, the observed water vapour distributions are largely reproduced\nby the short-term forecasts at 0.25 degrees resolution (T799/L91), the\ncorrelation ranges from 0.69 to 0.92. Locally, large differences occur\ndue to comparably 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 and\nto shallow. Convective transport of humidity to the middle and upper\ntroposphere tends to be overestimated. Potential impacts arising from\ndata assimilation and model physics are considered. The matching of\nair-mass boundaries ( transport) is discussed with repect to scales and\nthe representativity of the 2-D sections for the 3-D humidity field. The\nnormalized bias of the model with respect to the observations is 6{%},\n11{%} and 0{%} (moist model biases) for the three along-flight sections,\nwhereby however the lowest levels are excluded.

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