Fourth Study Conference on BALTEX Scala Cinema Gudhjem

- 9 -
Precipitation Type Statistics in the Baltic Region Derived from Three Years
of BALTEX Radar Data Centre (BRDC) Data
Ralf Bennartz 1,2 and Andi Walther 2
1 University of Wisconsin, Atmospheric and Oceanic Sciences, Madison, Wisconsin, US, bennartz@aos.wisc.edu
2 Free University of Berlin, Inst. For Space Sciences, Berlin, Germany
1. Abstract
A method to classify precipitation events based on their
spatial extend and texture has been developed. The method
allows to distinguish large-scale precipitation features
typically associated with frontal systems from more smallscale
features which are usually found in isolated
convective cells. We use the Baltic Radar Data Centre
(Michelson et al., 2000) composite images to classify
precipitation events based on their temporal and spatial
characteristics. The questions we wish to address are:
− How much precipitation falls directly associated with
frontal overpasses?
− What is the annual cycle and interannual variability of
the frontal precipitation events?
− Can we use the diagnostic tools to validate regional
climate models for the current state and thus assign
error margins to climate change scenarios?
We present the classification method and results for the
Baltic area for a three year period starting January 2000.
2. Introduction
Previous radar work has mainly emphasized the distinction
between different types of cloud microphysical processes
that trigger precipitation. These classifications typically
assign a precipitation event either the classes stratiform or
convective based on the main cloud microphysical processes
that drive the precipitation event ( e.g. Steiner et al. (1995),
Biggerstaff and Listemaa (2000) or Anagnostou and
Kummerow (1997). While these classifications are of great
importance to understand the day to day variability of radar
events and also to understand the variability in the relation
between rain rate and radar reflectivity, they clearly can not
serve to answer the above questions.
The classification approach we take is therefore not strictly
microphysical but distinguishes between different types of
precipitation based on the large scale characteristics of radar
composites. Such characteristics include the horizontal
extend of individual precipitating system, their spatial
homogeneity, as well as their temporal variation. We show
that this classification allows to distinguish between
precipitation events associated with fronts and precipitation
events that are triggered by convection.
In the subsequent considerations we will therefore
distinguish between frontal precipitation and convective
precipitation events. Both the terms frontal and convective
are hereafter used to describe the weather situation that
triggers the precipitation event and not the microphysical
processes that lead to precipitation formation. It might well
be that within a frontal precipitation event precipitation is
formed via strong updrafts and hence is convective in a
microphysical sense (embedded convection). Similarly,
even the most intensive convectively driven precipitation
events usually consist of parts where precipitation
generation is driven by stratiform processes (e. g. stratiform
tail). It is therefore important to note that we use the terms
frontal and convective in a synoptic sense.
3. Results
Figure 1 shows the amount of precipitation events not
associated with frontal passages for the years 2000 until
2002. In the summer month up to 90 % of the
precipitation events are due to isolated convective events.
A strong interannual variability can also be observed. In
2000 a single maximum of convective precipitation occurs
in July and August. In the years 2001 and 2002 two
maxima can be identified, one in May and one in July,
August. Frontal passages prevail in various periods in the
fall an winter season.
Figure 1. Percentage of convective precipitation in the
BRDC-area for as a function of local time for the period
January 2000 until December 2002.
References
Anagnostou, N. E. and Kummerow, C,1997: Stratiform
and convective classification of rainfall using SSM/I
85-Ghz brightness temperature observations, Journal
of Atmospheric and Oceanic Technology, 14, 570-
575.
Biggerstaff, M.I. and Listemaa, S., 2000: An improved for
convective/ stratiform echo classification using radar
reflectivity. Journal of Applied Meteorology, 39,
2129-2150.
Michelson, D. and Andersson, T. and Koistinen, J. and
Collier, C. and Riedl, J. and Szturc, J. and Gjertsen, U.
and Nielsen, A. and Overgaard, S., 2000: BALTEX
Radar DATA Centre Products and their
Methodologies, SMHI-Report 90, SMHI, SE-60176
Norrköping, Sweden.
Steiner, M. and Houze Jr. , R. A. and Yuter, S., 1997:
Climatological characterizations of three-dimensional
storm structure from operational radar and rain gauge
data. Journal of Applied Meteorology, 34, 1978-2007.