Assessments of cloud liquid water contributions to GPS radio occultation refractivity using measurements from COSMIC and CloudSat

Global cloud parameters including cloud liquid water, cloud base height, cloud top height, and cloud type are observed from the cloud profiling radar onboard CloudSat. Global Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) data during a 3 year period from 2007 to 2009 are collocated in space and time with CloudSat data. The collocated data set is then classified into seven groups: one clear‐sky condition and six different cloud types with liquid water content (LWC) measurements. For each group, atmospheric refractivity, temperature, and water vapor derived from COSMIC GPS ROs are compared with those of the European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses. It is found that the COSMIC GPS RO refractivity observations are systematically greater than the refractivity calculated from ECMWF analyses, which is to be referred as a positive N bias in clouds. The fractional N bias is as high as 1.2% depending on cloud types. Using CloudSat LWC, it is demonstrated that LWC can contribute 0.8% of the total refractivity by individual clouds and 0.16% of the positive N bias. The 0.16% positive N bias is comparable in magnitude to the retrieval uncertainty quantified by the mean difference (Δ N O‐R ) between the observed refractivity with LWC contribution subtracted and the refractivity calculated using GPS retrieved profiles of temperature, pressure, and humidity. The values of Δ N O‐R increase linearly with LWC as anticipated theoretically. Positive N biases are spatially correlated with positive water vapor biases, negative temperature biases, and large liquid water contents.