Download Assimilation of high resolution dial water vapor data into the MM5

Bauer, Wulfmeyer, Grzeschik, Behrendt, Vandenberge, Browell, Ismail, and Ferrare
ASSIMILATION OF HIGH_RESOLUTION DIAL WATER VAPOR DATA
INTO THE MM5 MODELING SYSTEM: COMPARISON OF THE
PERFORMANCE OF DIFFERENT ASSIMILATION METHODS
H.-S. Bauer1, V. Wulfmeyer1, M. Grzeschik1, A. Behrendt1, F. Vandenberghe2, E. W. Browell3, S.
Ismail3, and R. A. Ferrare3
1 Institute of Physics and Meteorology, University of Hohenheim, [email protected]
2 National Center for Atmospheric Research, Research Application Program, Boulder, CO
3 Atmospheric Science Division, NASA Langley Research Center, Hampton, VA
The distribution and intensity of precipitation is the most important parameter in today’s weather
forecasts. At the same time its prediction is very difficult since the development of precipitation occurs at
the end of a long chain of complicated processes which are only crudely represented even in highresolution numerical weather prediction (NWP) models. Another reason for the bad performance of
quantitative precipitation forecasts is that the 4-d distribution of water vapor used for model initialization
is based on the coarse resolution radiosonde network and therefore not very accurate. This introduces
errors into the water vapor initial field which accumulate to large errors in the predicted precipitation
patterns. This is especially problematic for rapidly developing, severe, and small-scale precipitation
events.
Lidar systems are admittedly capable of closing gaps in the existing observational network, since they are
able to observe parameters like temperature, wind, and water vapor with high spatial and temporal
resolution as well as accuracy. LASE is an extensively characterized airborne DIAL system which
provides not only high resolution water vapor profiles but is also capable of accurately specifying the
errors of the system.
In this paper high-resolution water vapor profiles from the NASA LASE DIAL system are assimilated
into the NCAR/PennState MM5 modeling system using different assimilation methods, namely 3DVAR,
4DVAR, and FDDA. As case study a convective situation during the IHOP_2002 field campaign was
selected. On the 24th of May 2004 a strong moisture transport from the Gulf of Mexico, an approaching
cold front from the north and an eastward moving dryline build a classical situation for the development
of severe thunderstorms in the southern Great Plains.
Results of experiments with the 4DVAR system are very promising. The assimilation of LASE water
vapor profiles during a 3-hr assimilation window significantly modifies the water vapor as well as
temperature and wind fields even long after the assimilation has been switched off. Promising
improvements are also found in the precipitation patterns which corresponds much better to radar
observations as compared to the control simulation without assimilated LASE water vapor profiles.
Detailed comparisons of experiments using the different assimilation methods as well as quantitative
comparisons with independent observations collected during IHOP_2002 should be presented at the
conference.
WWRP International Symposium on Nowcasting and Very Short range Forecasting