A 2-channel microwave radiometer (left) and a 12-channel microwave radiometer profiler (right) are part of a larger collection of instruments deployed at the ARM Mobile Facility site in Heselbach, Germany, in 2007.

Microwave radiometers (MWRs) are instruments used to measure emissions of water vapor and liquid water molecules in the atmosphere at specific microwave frequencies. Different MWRs are used to measure various frequencies, but the accuracy of all their retrievals is somewhat suspect, particularly when clouds are “thin”—less than 100 gm 2. In June 2007, a 6-month field campaign began at the ARM Mobile Facility site in Heselbach, Germany, to help scientists to unravel the different types of uncertainties inherent in the various radiometer measurements retrievals. Called the Cloud Microwave Validation Experiment, scientists will compare measurements from numerous MWRs deployed by both the ARM Mobile Facility and European collaborators taking part in the Convective and Orographic Precipitation Study (COPS). Their goal is to conduct intercomparisons between the instruments and to assess the data from multiple instruments for possible explanations as to the unresolved microwave uncertainties.

From the tropics to the Arctic, 50% or more of the clouds have liquid water paths below the 100gm 2 limit. Because the Earth’s radiative energy balance is particularly sensitive to small changes in the cloud liquid water path, small uncertainties in their optical properties can easily affect changes in the local radiative energy balance. For the climate research and modeling community, resolving such uncertainties would have a huge impact on related science issues, such as cloud radiative impacts and aerosol indirect effects.

In support of COPS, the ARM Mobile Facility instrument suite includes a variety of MWRs, including a 2-channel MWR, a 12-channel MWR profiler, and a new 90/150 GHz MWR. Collocated at the ARM Mobile Facility site are a European 90/150 GHz MWR and a 14-channel profiling MWR. In addition, water vapor and temperature-profiling information from frequent radiosonde launches, plus retrievals from a water vapor differential absorption lidar and temperature profiles from a Raman lidar will be available from the COPS deployment. Observations from this collection of instruments will produce an excellent database to address the unresolved microwave uncertainties.

The ARM Mobile Facility is hosting several guest instruments during COPS to support related but separate field campaigns. See the ACRF field campaign website for more information.