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What is CREW?
Eumetsat's CREW (Cloud Retrieval Evaluation Workshop) is a research activity to evaluate the strengths and weaknesses of the most important algorithms that retrieve cloud property from passive imager instruments onboard both polar and geostationary satellites (SEVIRI, AVHRR, and MODIS). The CREW working group members operate the most advanced cloud retrieval algorithms. Their level-2 data products are inter-compared and validated against observations from the A-train satellite constellation (CALIPSO, CLOUDSAT, and AMSR). The results of the inter-comparison are discussed at the Eumetsat Cloud Retrieval Intercomparioson Workshops. The first CREW took place in Norrköping, Sweden from 17.-19. May 2006, and the second CREW in Locarno, Switzerland in February 2009.
The third CREW meeting will be held in Madison, USA from 15.-17. November 2011.
Why? – The science
Clouds cover about 70% of the Earth’s surface. They appears in various forms as marine stratocumulus, deep convective clouds in the tropics, frontal systems and many more. On the one hand cloud clouds have a global extent. On the other hand cloud formation is based on micro physics. Cloud droplets and ice crystal sizes are usually in the micro meter range. Temperature, humidity, aerosols and turbulence are critical parameters for cloud growth. The clouds may be described by their macro physical properties (like cloud phase, water content, cloud top and base height) or their optical properties (like cloud optical thickness and effective radius). Cloud influence in several ways atmospheric physics. Clouds play a dominant role in the hydrological cycle of our planet. By condensations water vapor is removed from the air. Droplets and ice crystals are transported within clouds by convection, are horizontally advected, and precipitation is formed. The phase change of water release latent energy altering small-scale convective or turbulent circulations and the vertical heating profile. Furthermore clouds modify the radiative transfer through the atmosphere. Cloud reflects a part of the sunlight, this effect cools the earth. They also reduce the amount of thermal radiation emitted to space thus warming the earth. The net effect depend on the cloud properties. For low and middle clouds the cooling effects dominates, while high clouds tend to have a warming effect. Due to their large impact on the energy and water cycle, cloud have to be well presented in weather and climate models. In order to validate this models accurate observations are needed. Where ground based measurements provide information of high accuracy at certain places, satellite observations provide continuous observation of the atmospheric state over the whole globe being indispensable for the validation of global models.
How? - The approach
Currently 13 groups provide data sets of retrieved cloud physical properties, i.e., Cloud mask, cloud fraction, cloud top temperature, cloud top pressure, cloud top height, cloud optical depth, effective radius, cloud water path, cloud type, and convective signature. The data is inter-compared for 4 days, that were most suitable for validation purposes. We cooperate with the GEWEX - Cloud Assessment project that rather compares climatologies, whereas CREW is intended for investigating the details of the retrieval algorithms.
Who? - The members
Funded by
Organised by
List of Participants
For whom? – The users
Clouds probably will alter their coverage and physical properties in a changing climate. Due to the complexity of cloud microphysical processes especially with aerosols and their radiative interactions cloud representation in global circulation models (GCM) remains a challenging task. Using different techniques several GCM groups obtained widely varying results for the climate feedback of clouds, where even the sign remains unsure. This uncertainty is cited as one of the key factors explaining the spread in model simulations of future climate for a given emission scenario. A long history of cloud observations now runs parallel to that of model development. The comparison of observation and model results at daily, seasonal and inter-annual scale therefor is a necessary way to constrain models.
Apart from the validation of climate models, satellite observations of clouds provide the data base for a variety of applications, too. They are directly used for the short term monitoring and forecasts of irradiance for solar energy. Cloud observations are used to determine the irradiance in the ultra violet wavelength range having an effect on human health, the biological environment, and photochemical reactions. Finally the retrieved cloud properties are used to judge convective conditions in the atmosphere for aviation and estimations of the precipitation.
