Priority Program SPP 1167 of the DFG

Convective and Orographically Induced Precipitation Study Karlsruhe Institute of Technology

Analysis of the regional water cycle using GPS data and COSMO model simulations R. Sasse, G. Schädler, Ch. Kottmeier

Forschungszentrum Karlsruhe/Universität Karlsruhe

Reference: M. Baldauf, A Tool for Testing Conservation Properties in the COSMO-Model (LM), COSMO Newsletter No. 7Acknowledgement: This project is a contribution to the Helmholtz Research Network EOS and is a cooperation between Universität Karlsruhe and GFZ Potsdam.Contact: romi.sasse@imk.fzk.de

Summary and Outlook1. The correlation between GPS and COSMO IWV is not yet satisfying. More pointwise and also spatial comparisons have to be

performed in order to detect possible sources of this deficiency. Besides Near Real-Time GPS data post-processed data will be used. Comparisons for other COPS episodes with different weather situations are to be analysed. Precipitation data will be included.

2. The testing of the tool for the computation of balances has just started. The calculated values have to be checked to the fulfillment of the conservation equations. Other verfications can be done by changing the extent and position of the volume. Furthermore the turbulent fluxes have to be included in the balance calculation.

PurposeThe components of the atmospheric water cycle (water vapour, precipitation and evapotranspiration), which are relevant for the climate system, are insufficiently known. Experiments like COPS provide an ideal opportunity to analyse these components and their interactions by combining observations and model simulations on a regional scale, the focus of these studies being on the determination and analysis of balances. Besides the quantification of the components the influence of topography and land use as well as time-dependent factors like advection can be extracted. Balances are simulated with the COSMO model and compared with information from the COPS campaign, especially the GPS (Global Positioning System) data of the Integrated Water Vapour content (IWV). The aims of the work in this study include:

1. an analysis of the processes contributing to the water cycle2. establishing balances (water vapour, precipitation, evapotranspiration) e.g. for the COPS area and studying their temporal variation3. the usage of the results for the improvement of the model.

The study consists of work packages including comparisons between GPS and COSMO data on different spatial and temporal scales, the calculation of balances and analysis of the components of the regional water balance as well as their influencing processes primarily for episodes and afterwards for longer periods. In the following first results for IOP 8b (July 15, 2007) are presented.

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Fig. 2: Diurnal variation of IWV from July 15, 0:00 to July 16, 0:00

of the station 0401 (Freiburg)

Fig. 3: Scatter plot of hourly GPS and COSMO IWV(July 15, 0:00 to July 16, 0:00)

for every station

quantitative differences between GPS and COSMO IWV can be very large. To examine the agreement of GPS and COSMO, IWV values were plotted against each other (Fig. 3). The agreement is not satisfying yet (Pearson correlation coefficient = 0.62, slope = 1.05, intercept = 1.21). Furthermore it can be seen from the diurnal variations and the scatter plot that the COSMO water content is often higher than the GPS values. Further comparisons will be examined.

Comparison of GPS and COSMO (LM4.0) The GPS stations during the COPS campaign are shown in Fig. 1. The diurnal variations of the GPS and COSMO IWV were compared for 55 stations. As an example the results for the station 0401 are illustrated in Fig. 2. The diurnal variations of GPS and COSMO data exhibit a similar behaviour, but the

Fig. 1: Map of the GPS stations during COPS(http://eost.u-strasbg.fr/COPS/)

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Fig. 4: Illustration of a cuboidal volume in thesimulation area

(Baldauf, COSMO Newsletter No. 7)

Fig. 5: Diurnal variations of the advective surface fluxes of water vapour through the endwalls of a

volume over the COPS area from July 15, 0:00 to July 16, 0:00

(north = int_n, south = int_s, east = int_e, west = int_w)

Fig. 6: Horizontal wind vectors in a height of 10 mfor the Middle of Europe on July 15, 15:00.

The COPS area is marked with a red rectangle.

Calculation of balancesA tool for the computation of balances, written by M. Baldauf (DWD), has been implemented in COSMO (Fig. 4). The following conservation equations are used:

V

V

dAFdt

dQ

dVQ

:equation onConservati

:amount Total

Fig. 5 shows an example of the diurnal variations of the fluxes of water vapour. Positive fluxes indicate a flow out of the volume in agreement with the wind field shown in Fig. 6. Especially in the northwest of the COPS area (red rectangle) a strong flow to the north can be recognized. To account for processes like evapotranspiration and precipitation a sink/source term has to be included. This work is under way.