Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
1680-7316
1680-7324
7
19
2007
10.5194/acp-7-5033-2007
http://www.atmos-chem-phys.net/7/5033/2007/
http://www.atmos-chem-phys.net/7/5033/2007/acp-7-5033-2007.html
http://www.atmos-chem-phys.net/7/5033/2007/acp-7-5033-2007.pdf
5033
5042
2007-10-01
Evaluation of ECMWF water vapour fields by airborne differential absorption lidar measurements: a case study between Brazil and Europe
H. Flentje
harald.flentje@dwd.de
A. Dörnbrack
A. Fix
G. Ehret
E. Hólm
Deutscher Wetterdienst, Hohenpeißenberg, Germany
DLR Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
European Centre For Medium Range Weather Forecasts, Reading, UK
Three extended airborne Differential Absorption Lidar (DIAL) sections of
tropospheric water vapour across the tropical and sub-tropical Atlantic in
March 2004 are compared to short-term forecasts of the European Centre for
Medium Range Weather Forecasts (ECMWF). The humidity fields between 28° S
and 36° N exhibit large inter air-mass gradients and reflect typical
transport patterns of low- and mid-latitudes like convection (e.g. Hadley
circulation), subsidence and baroclinic development with stratospheric
intrusion. These processes re-distribute water vapour vertically such that
locations with extraordinary dry/moist air-masses are observed in the
lower/upper troposphere, respectively. The mixing ratios range over 3 orders
of magnitude. Back-trajectories are used to trace and characterize the
observed air-masses.
<br><br>
Overall, the observed water vapour distributions are largely reproduced by
the short-term forecasts at 0.25° resolution (T799/L91), the correlation
ranges from 0.69 to 0.92. Locally, large differences occur due to comparably
small spatial shifts in presence of strong gradients. Systematic deviations
are found associated with specific atmospheric domains. The planetary
boundary layer in the forecast is too moist and to shallow. Convective
transport of humidity to the middle and upper troposphere tends to be
overestimated. Potential impacts arising from data assimilation and model
physics are considered. The matching of air-mass boundaries (transport) is
discussed with repect to scales and the representativity of the 2-D sections
for the 3-D humidity field. The normalized bias of the model with respect to
the observations is 6%, 11% and 0% (moist model biases) for the
three along-flight sections, whereby however the lowest levels are excluded.
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