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
Purpose
The 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 cycle
2. establishing balances (water vapour, precipitation, evapotranspiration) e.g. for the COPS area and studying their temporal variation
3. 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.
4,0E+07
int_n
int_s
int_e
int_w
Q    dV
2,0E+07
V
1,0E+07
Conservation equation :
0,0E+00
0
5
10
15
20
-1,0E+07
-2,0E+07
35
COSMO
COSMO IWV [kg/m²]
30
25
20
15
30
25
20
15
10
10
5
5
0
0
0
2
4
6
8
10
12
14
16
18
20
Hour
Fig. 2: Diurnal variation of IWV
from July 15, 0:00 to July 16, 0:00
of the station 0401 (Freiburg)
22
24
0
5
10
15
20
25
30
35
40
GPS IWV [kg/m²]
Fig. 3: Scatter plot of
hourly GPS and COSMO IWV
(July 15, 0:00 to July 16, 0:00)
for every station
dQ
  F dA
dt V
Hour
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)
GPS
35
Calculation of balances
A tool for the computation
of balances, written by M.
Baldauf (DWD), has been
implemented in COSMO
(Fig. 4). The following
conservation
equations
are used:
Total amount :
3,0E+07
40
40
IWV [kg/m²]
Fig. 4: Illustration of a cuboidal volume in the
simulation area
(Baldauf, COSMO Newsletter No. 7)
Flux of specfic humidity [kg/s]
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 Fig. 1: Map of the GPS stations during COPS
(http://eost.u-strasbg.fr/COPS/)
similar behaviour, but the
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.
Fig. 6: Horizontal wind vectors in a height of 10 m
for the Middle of Europe on July 15, 15:00.
The COPS area is marked with a red rectangle.
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.
Summary and Outlook
1.
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.
Reference: M. Baldauf, A Tool for Testing Conservation Properties in the COSMO-Model (LM), COSMO Newsletter No. 7
Acknowledgement: This project is a contribution to the Helmholtz Research Network EOS and is a cooperation between Universität Karlsruhe and GFZ Potsdam.
Contact: [email protected]