Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Analysis of the Role of Atmospheric Cyclones in the Moisture Transport
from the Atlantic Ocean to Europe and European Precipitation
Irina Rudeva(1), Sergey Gulev(1), Olga Zolina(1) and Eberhard Ruprecht(2)
1 P.P.Shirshov Institute of Oceanology, RAS, 36 Nakhimovsky av., 117218 Moscow, Russia; rudeva@sail.msk.ru
2 Institut für Meereskunde, Düsternbrooker Weg 20, D-24105 Kiel, Germany
1. Introduction
We analyse in this study the structure of mesoscale and
synoptic-scale precipitation fields associated with midlatitudinal
cyclones over the North Atlantic and Western
Europe, using observational data, available form the routine
merchant ship observations over the ocean and from rain
gauges over the continent. The main objective is to design
composites of precipitation fields, co-located with the major
structural elements of the propagating cyclones during their
life cycle. Of a particular interest is to link the cyclone
characteristics with the intensity of rainfall and to
quantitatively describe the projections of atmospheric
moisture transport over the Atlantic Ocean onto European
precipiation variability.
2. Data and preprocessing
Cyclone tracks and characteristics of the cyclone life cycle
(intensity, deepening rate, life time, propagation velocity)
were obtained from the tracking of 6-hourly sea level
pressure fields from NCEP/NCAR Reanalysis for the period
from 1948 to 2002 (Gulev et al. 2001, Zolina and Gulev
2002). Tracking was carried out by the numerical scheme,
searching the consequent locations of cyclone centers and
accounting for the cyclone propagation through the
dynamical interpolation. Besides the locations of the cyclone
centers and central pressure, we also analysed the vorticity,
pressure gradient and the other parameters which allow for
the co-location of the cyclone structure with cloud and
precipitation fields.
Over the mid-latitudinal North Atlnatic precipitation
estimates were derived from the Voluntary Observing Ship
(VOS) data using indirect information reported by VOS
(past and present weather code, humidity, cloudiness). The
VOS data were taken from the latest updates of the I-
COADS (International Comprehensive Ocean-Atmosphere
Data Set (Woodruff et al. 1998). Retrievals of the
precipitation intensity from the weather code and
meteorological observations were done according to the
experimental relationships. Sampling density of the VOS
observations over the North Atlantic Ocean for selected
years and time periods allows for quite reliable description
of synoptic-scale to meso-scale features of precipitation in
the propagating synoptic transients. Figure 1 shows an
example of VOS data points distribution for 06:00 UTC
26.03.94 overplotted with sea level (SLP) field for this
moment.
Over European continent precipitation estimates were taken
from the long-term station rain gauge observations, available
from different collections of daily and higher resolution
precipitation data sets. These collections consist of the
KNMI European Climate Assessment data set, NCDC
collection of station observations of precipitation and
precipitation data from Russian Metoffice (Zolina et al.
2004). The best sampling density over the continent is
available for the period starting from 1994 onwards.
Transport of moisture by mean flow and atmospheric
cyclones has been quantified using the method developed
by Ruprecht et al. (2002) on the basis of reanalyses data.
3. Results overview
Ensembles of precipitation composites were performed for
different typical cyclone life cycles over the midlatitudinal
ocean (for the well sampled cases and periods)
and over the continent. Composites were designed by
performing statistical analysis over precipitation and cloud
cover fields for similar cyclones in different regions of the
North Atlantic and over western Europe. In order to
achieve an effective co-location the composites have been
performed for the different stages of the cyclone
development over the North Atlantic mid latitudes. Of a
special importance on this stage was the definition and
quantification of the cyclones are and its association with
the local pressure gradients, vorticity and parameters of
the cyclone life cycle.
The composites for the precipitation derived from the
VOS data were then compared with those derived from the
reanalyses in order to quantify to what extent the NWP
(numerical weather prediction) products can adequately
represent the mesoscale precipitation structure. We show
in particular, that the relative comparability is higher over
the oceans rather than over the continental Europe. On the
other hand, it is still unclear whether this can be attributed
to better skills of reanalyses to represent the precipitation
structure over the ocean, or to a better quality of in-situ
data over the continents.
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Figure 1. Spatial distribution of the VOS
observations over the North Atlantic and the
corresponding SLP field for 06:00 UTC 26.03.94.
Finally, we analysed interannual variability of the cyclone
life cycle and associated precipitation patterns over the
North Atlantic and Europe. This variability is considered
in a view of the leading climate modes in the Atlantic-
European sector (North Atlantic Oscillation and others). In
particular, we tried to link changes in the Atlantic midlatitudinal
precipitation and the cyclone water vapour
content over the North Atlantic with the statistical