Lower Tropospheric GPS Radio Occultation Bending Angle and Refractivity Errors in the Northeast Pacific

Over the subtropical eastern oceans, the strong subsidence in the free troposphere along with the cool sea surface temperature often results in a shallow stratocumulus cloud-topped atmospheric boundary layer (ABL). GPS radio occultation (RO) could provide high vertical resolution thermodynamic structure of the ABL. However, the sharp moisture gradient beneath the strong temperature inversion leads to large refractivity gradient and often causes ducting across the ABL top. The ducting results in systematically negative biases in the RO refractivity (i.e., N-bias) inside the ABL due to a non-unique Abel inversion problem. However, the RO bending angle errors hasn’t been thoroughly assessed yet, which could introduce additional errors in RO refractivity retrievals. Using 8-year (2006~2013) Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) RO soundings with respect to the collocated European Center for Medium-Range Weather Forecast (ECMWF) reanalysis interim (ERA-I), we firstly quantify the N-bias in the moist lower troposphere in the Northeast Pacific domain (160°W~110°W; 15°N~45°N), where the Marine Atmospheric Radiation Measurement (ARM) GPCI Investigation of Clouds (MAGIC) field campaign was carried out. The end-to-end simulation is then carried out to assess both the bending angle and refractivity biases in COSMIC RO soundings by comparing with the collocated radiosonde and ERA-I profiles. The negative N-bias in COSMIC soundings mainly lies below ~2 km, with major contribution from ducting. Moreover, significant negative bending angle biases are present below ~2 km, with the maximum as large as -­10% at about 0.8 km above surface. Such bending errors will introduce additional negative refractivity bias to the already large N-bias caused by ducting. Similar to the N-bias, the altitude of the peak negative bending angle bias follows the ABL height changes from near 1 km close to California coast to around 2 km near Hawaii. The significant bending angle biases in the lower troposphere could be resulting from the RO signal tracking and retrieval process which warrants further investigation.