Impacts of Ice Clouds on GPS RO Measurements

Mathematical solutions accounting for the effects of liquid and ice clouds on the propagation of the GPS radio signals are first derived. The percentage contribution IWC to the total refractivity increases linearly with the amount of IWC at a rate of 0.6 per g m-3. Measurements of coincident profiles of ice water content (IWC) from CloudSat in deep convection during 2007 through 2010 are then used for estimating the ice-scattering effects on GPS RO measurements from COSMIC. The percentage contribution of IWC to the total refractivity from CloudSat measurements is consistent with theoretical model, reaching about 0.6% at 1 g m-3 IWC. The GPS RO refractivity observations in deep convective clouds are found to be systematically greater than the refractivity calculated from the ECMWF analysis. The fractional N-bias (GPS minus ECMWF) can be as high as 1.8% within deep convective clouds. Compared with ECMWF analysis, the GPS RO retrievals have a negative temperature biases and a positive water vapor bias, which is consistent with a positive bias in refractivity. The relative humidity calculated from GPS retrievals is usually as high as 80-90% right above the zero degree temperature level,in deep convection, and is about 15-30% higher than the ECMWF analysis. The majority of the data points in deep convection is located at the negative side of temperature differences and positive side of relative humidity differences between GPS RO retrievals and ECMWF analysis.