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William C. Conant
,
V. Ramanathan
,
Francisco P. J. Valero
, and
Jens Meywerk

Abstract

Measurements of downward surface solar radiation (global radiation) and albedo taken during the Central Equatorial Pacific Experiment (CEPEX) are used to obtain baseline estimates for two quantities concerning the radiation budget of the tropical oceans: 1) surface absorption of solar radiation in the central equatorial Pacific under cloud-free conditions, and 2) the corresponding absorption by the atmosphere. These values are then compared to two state-of-the-art radiative transfer models to determine if the models are accurately partitioning solar absorption between the atmosphere and the ocean.

The paper develops an independent approach to obtain a clear-sky signal from 10-s resolution surface pyranometer data that is in excellent agreement with upper envelope methods. Over a diurnal average, the ocean absorbs 70.9% ± 1.3% of the solar radiation incident at the top of the atmosphere (TOA). The data, measured from ship and low-flying aircraft platforms, also yield the zenith angle dependence of the surface absorption. The clear-sky data are representative of dry regions east of the date line during March 1993.

Likewise, a combination of tropopause albedo measurements from the ER-2 aircraft and Earth Radiation Budget Experiment (ERBE) clear-sky TOA albedos are used to find the absorption of solar radiation by the atmosphere (integrated from the surface to the TOA). Clear-sky TOA albedo is computed from the ER-2 tropopause measurements using a radiative transfer model and measurements of stratospheric aerosol and ozone. The computed TOA albedos agree with ERBE at about 6% for overhead sun. The diurnal average fractional atmospheric column absorption is 20.2% ± 1.6%.

Two multispectral radiation models agree to within 5 W m−2 of the observed daily average clear-sky oceanic solar absorption when the atmospheric profile is constrained by measurements and the observed TOA albedo is used as a boundary condition.

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S. Crewell
,
H. Bloemink
,
A. Feijt
,
S. G. García
,
D. Jolivet
,
O. A. Krasnov
,
A. van Lammeren
,
U. Löhnert
,
E. van Meijgaard
,
J. Meywerk
,
M. Quante
,
K. Pfeilsticker
,
S. Schmidt
,
T. Scholl
,
C. Simmer
,
M. Schröder
,
T. Trautmann
,
V. Venema
,
M. Wendisch
, and
U. Willén

Clouds cause uncertainties in the determination of climate sensitivity to either natural or anthropogenic changes. Furthermore, clouds dominate our perception of the weather, and the relatively poor forecast of cloud and precipitation parameters in numerical weather prediction (NWP) models is striking. In order to improve modeling and forecasting of clouds in climate and NWP models the BALTEX BRIDGE Campaign (BBC) was conducted in the Netherlands in August/September 2001 as a contribution to the main field experiment of the Baltic Sea Experiment (BALTEX) from April 1999 to March 2001 (BRIDGE). The complex cloud processes, which involve spatial scales from less than 1 mm (condensation nuclei) to 1000 km (frontal systems) require an integrated measurement approach. Advanced remote sensing instruments were operated at the central facility in Cabauw, Netherlands, to derive the vertical cloud structure. A regional network of stations was operated within a 100 km × 100 km domain to observe solar radiation, cloud liquid water path, cloud-base temperature, and height. Aircraft and tethered balloon measurements were used to measure cloud microphysical parameters and solar radiation below, in, and above the cloud. Satellite measurements complemented the cloud observations by providing the spatial structure from above. In order to better understand the effect of cloud inhomogeneities on the radiation field, three-dimensional radiative transfer modeling was closely linked to the measurement activities. To evaluate the performance of dynamic atmospheric models for the cloudy atmosphere four operational climate and NWP models were compared to the observations. As a first outcome of BBC we demonstrate that increased vertical resolution can improve the representation of clouds in these models.

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E. Raschke
,
J. Meywerk
,
K. Warrach
,
U. Andrea
,
S. Bergström
,
F. Beyrich
,
F. Bosveld
,
K. Bumke
,
C. Fortelius
,
L. P. Graham
,
S.-E. Gryning
,
S. Halldin
,
L. Hasse
,
M. Heikinheimo
,
H.-J. Isemer
,
D. Jacob
,
I. Jauja
,
K.-G. Karlsson
,
S. Keevallik
,
J. Koistinen
,
A. van Lammeren
,
U. Lass
,
J. Launianen
,
A. Lehmann
,
B. Liljebladh
,
M. Lobmeyr
,
W. Matthäus
,
T. Mengelkamp
,
D. B. Michelson
,
J. Napiórkowski
,
A. Omstedt
,
J. Piechura
,
B. Rockel
,
F. Rubel
,
E. Ruprecht
,
A.-S. Smedman
, and
A. Stigebrandt

The Baltic Sea Experiment (BALTEX) is one of the five continental-scale experiments of the Global Energy and Water Cycle Experiment (GEWEX). More than 50 research groups from 14 European countries are participating in this project to measure and model the energy and water cycle over the large drainage basin of the Baltic Sea in northern Europe. BALTEX aims to provide a better understanding of the processes of the climate system and to improve and to validate the water cycle in regional numerical models for weather forecasting and climate studies. A major effort is undertaken to couple interactively the atmosphere with the vegetated continental surfaces and the Baltic Sea including its sea ice. The intensive observational and modeling phase BRIDGE, which is a contribution to the Coordinated Enhanced Observing Period of GEWEX, will provide enhanced datasets for the period October 1999–February 2002 to validate numerical models and satellite products. Major achievements have been obtained in an improved understanding of related exchange processes. For the first time an interactive atmosphere–ocean–land surface model for the Baltic Sea was tested. This paper reports on major activities and some results.

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