Comparison of cloud liquid water retrieved from an airborne millimetre wave radiometer with SSM/I and in situ cloud measurements

The validation of retrievals of the liquid water path (LWP) from microwave radiometers (e.g. SMMR, SSM/I) has proven difficult due to the scarcity of adequate ground truth. However validation is essential to allow proper use of this data in numerical weather prediction and climate studies. In the paper the authors compare three independent measures of LWP. The SSM/I has been widely used to retrieve LWP using statistical algorithms and in the paper the authors also use an iterative physical approach. Finer resolution LWP retrievals can be made by flying an airborne microwave radiometer just above cloud top but coincident with the SSM/I overpass. Data from the Microwave Airborne Radiometer Scanning System (MARSS) on the UK meteorological Office C-130 aircraft has been used and the retrieval of LWP is from an iterative physical scheme. This aircraft is also equipped with a hot wire Johnson-Williams probe for direct in situ measurements of cloud liquid content. The cloud liquid water content can also be estimated by integrating the dropsize distributions measured by the PMS FSSP and 2DC dropsize counters. By profiling through the cloud the LWP can be calculated. Despite the difference of scales of the different measurements which make direct comparison difficult excellent relative and reasonable absolute agreement has been achieved, especially using a physical iterative approach. Pollution tracks from ships can significantly modify the microphysics of stratocumulus. The sensitivity of MARSS LWP retrievals to changes in the microphysics has been tested using observations from the MAST experiment