Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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7
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2007
10.5194/acpd-7-4405-2007
http://www.atmos-chem-phys-discuss.net/7/4405/2007/
http://www.atmos-chem-phys-discuss.net/7/4405/2007/acpd-7-4405-2007.html
http://www.atmos-chem-phys-discuss.net/7/4405/2007/acpd-7-4405-2007.pdf
4405
4425
2007-03-30
Evaluation of ECMWF water vapour analyses by airborne differential absorption lidar measurements: a case study between Brasil 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
Airborne Differential Absorption Lidar (DIAL) observations of tropospheric
water vapour over Brazil and between Brazil and south Europe in March 2004
are compared to 1-hourly short-range forecasts of the European Centre for
Medium Range Weather Forecasts (ECMWF). On three along-flight sections
across the tropical and sub-tropical Atlantic between 28° S and 37° N
humidity fields are observed which represent typical low latitude
conditions. H<sub>2</sub>O mixing ratios vary between q≈0.01–0.1 g/kg
in the upper troposphere (UT), in subsiding air layers and a stratospheric
intrusion. They reach up to 0.5 g/kg at UT levels inside the Intertropical
Convergence Zone (ITCZ) and exceed 10 g/kg at lower levels.
Back-trajectories reveal that the humidity fields are largely determined by
transport.
<br><br>
The observed water vapour distributions are properly reproduced by 1-hourly
ECMWF Integrated Forecasting System (IFS) short-range forecasts at T799/L91
spectral resolution. As transport largely determines the water vapour
fields, the IFS skill is to a large extend based on a good representation of
the dynamics. The mean relative bias accounts to few percent (0%, 3%
and 6% for the three sections) being about or even below the accuracy of
the DIAL measurements of 5%. The larger deviations between analyses and
observations on small scales are due to relative spatial shifts of features
with large gradients. The correlation is quite high, ranging between 0.71
and 0.88. Over sea the analyses tend to underestimate the PBL height. At
mid-levels near deep convection the mid-troposphere tends to be analyzed too
humid indicating shortcomings in the convection parameterization. Humid
tendencies are also found in the upper troposphere, particularly in tropical
regions.
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