The BALTEX Bridge Campaign (BBC)
An integrated approach for a better understanding of clouds
Susanne Crewell, Clemens Simmer, Felix Ament, Ulrich Löhnert, Victor Venema
Meteorological Institute, University of Bonn, Bonn, Gemany
Arnout Feijt, Erik van Meijgaard, Andre van Lammeren, Hannelore Bloemink
Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands
Manfred Wendisch, Evelyn Jäkel, Sebastian Schmidt
Institute for Tropospheric Research, Leipzig, Germany
Marc Schröder, Jürgen Fischer
Institute for Space Sciences, Free University of Berlin, Germany
Ulrika Willen
Swedish Meteorological and Hydrological Institute (SMHI), Norrköping, Sweden
Markus Quante, Jens Meywerk, Dagmar Nagel
GKSS Reasearch Centre, Geesthacht, Germany
Thomas Trautmann
Meteorological Institute Leipzig, Germany
Andreas Macke
Institute for Marine Sciences, Kiel, Germany
Klaus Pfeilsticker, Thomas Scholl
Intitute for Environmental Physics, University of Heidelberg, Germany
1) Standard scientific abstract
The BALTEX BRIDGE campaign (BBC) was performed in response to the need for a better
understanding of cloud processes as these are the major contributors to the uncertainty in climate and
numerical weather forecast models due to their inhomogeneous structures and their complex interactions
with radiation. In order to tackle these problems we designed an integrated approach combining state-ofthe-art ground-based, aircraft and satellite measurements with accompanying atmospheric model
activities. The campaign took place in August/September 2001 in the Netherlands with a central facility
hosting advanced remote sensing instrumentation (Cabauw) surrounded by a regional network in a 100 by
100 km2 domain. Special focus was put on the observation of the cloud liquid water content as this is the
prognostic parameter used to treat clouds in atmospheric models and linking atmospheric dynamics to
radiation. The synergy of ground-based microwave radiometer, cloud radar, lidar and auxillary
measurements at Cabauw was used to derive the vertical structure of clouds in high temporal resolution.
The spatial characteristics of the cloud fields were investigated using satellite and aircraft data. Measures
for the cloud inhomogeneity were derived from ground-based oxygen-A-band spectroscopy and aircraft
imaging which provided important information for the validation of 3-D radiation transfer models. The
performance of four operational atmospheric models was evaluated using ground-based measurements and
revealed large discrepancies in the forecasted liquid water paths between the different models and
observations. The potential of these measurements to improve cloud parameterizations in atmospheric
models is demonstrated by investigating the vertical model resolution.
2) Abstract for the layman
As stated by the Intergovernmental Panel on Climate Change (IPCC), clouds are the single largest
uncertainty in determining the climate sensitivity to either natural or anthropogenic changes. Furthermore,
clouds dominate our perception of the weather and the poor forecast of cloud parameters in numerical
weather prediction (NWP) models is striking. In order to improve the treatment of clouds in climate and
NWP models the BALTEX BRIDGE campaign (BBC) was conducted in the Netherlands in
August/September 2001. The complex cloud processes involving scales from less than a micrometer
(condensation nuclei) to about thousand kilometers (frontal systems) required an integrated measurement
approach: 1) advanced remote sensing instruments were operated at the central facility in Cabauw to
derive the vertical cloud structure by employing new algorithm techniques, 2) a regional network of
stations within a 100 by 100 km2 domain to observe the solar radiation, cloud liquid water path, cloud base
temperature and height, 3) aircraft and tethered balloon measurements of cloud microphysical parameters
and solar radiation below, in and above the cloud and 4) satellite measurements to describe the spatial
structure of the cloud field. The measurement activities were accompanied by atmospheric modeling with
four operational climate and NWP models. A comparison of the measured cloud liquid water content with
the model forecasts revealed large discrepancies. Here we provide one example which shows how the
measurements can be used to improve the treatment of clouds in models.
3) Description of the proposed article
The BALTEX Bridge Campaign is the major field activity in the GEWEX (Global Energy and Water
cycle EXperiment) project BALTEX (BAMS, 82, 11, 2001). While water vapour has been the major
object of study within the ARM (BAMS, 84,2, 2003) project, BBC has focused on clouds and their
radiative effect. With more than 100 participating scientists BBC has been the major “cloud campaign” in
Europe. An integrated approach combining measurements from the ground, aircraft and satellite with
atmospheric modelling has been pursued. Special emphasis is placed on the liquid water content
(prognostic variable for clouds in atmospheric models) and the cloud structure which strongly affect the
radiation field.
The paper will include a description of the campaigns’ objectives, the scientific plan and setup, the time
table and an overview of the instrumentation used. Highlights of the measurement analysis will be shown
for a) cloud liquid water path (time series of remotely sensed quantities from the ground, satellite LWP
field and the combination of both) b) cloud liquid water profiles (retrieved using new types of algorithms
from the synergy of advanced remote sensing measurements), c) structure parameters of the cloud field
(derived from ground-based and aircraft high resolution spectroscopy), d) radiation measurements (aircraft
observations and accompanying radiative transfer calculations), and e) the comparison of cloud liquid over
the course of the whole campaign with operational atmospheric models. Finally, the further potential of
the measurements and implications for future measurement campaigns will be discussed.