Assessment of Systematic Biases of Radiosonde Moisture Measurements using Global Positioning System Radio Occultation from COSMIC

Water vapor (WV) is one of the most important greenhouse gases in the atmosphere. Accurate and consistent water vapor measurements in the troposphere are critical for studying the water vapor feedback on clouds and hydrological cycles, which are still one of the largest uncertainties in understanding the global warming mechanism. Radiosondes have provided long-term and all-weather in situ operational measurements of atmospheric pressure, temperature, and humidity for decades and have been the backbone observation system for numerical weather prediction and climate monitoring. However, because radiosonde sensor characteristics can be affected by the changing environment, its measurement accuracy varies considerably in times and locations for different sensor types. All-weather water vapor profiles can also be obtained from Global Positioning System (GPS) radio occultation (RO) data. The high vertical resolution of RO derived humidity profile are from ~60 m near the surface to ~1.5 km at 40 km. Because this technique uses the time delay of the GPS radio signal, which is traceable to international standards of time, GPS RO technique, unlike radiosondes, shall provide consistent water vapor measurements among geographical locations. In this study, we compare specific humidity profiles derived from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) from August to November 2006 with those from different types of radiosonde and from ECMWF global analysis. Comparisons show that COSMIC specific humidity data agree well with ECMWF analysis over different regions of the world for both day and night times. On the contrary, evaluation against COSMIC specific humidity shows a distinct dry bias of Shang-E radiosonde (China) and obvious wet biases of VIZ-type (USA). No obvious specific humidity biases are found for MRZ (Russia) and MEISEI (Japan) radiosondes. These results demonstrate the usefulness of COSMIC water vapor for quantifying the dry/wet biases among different sensor types.