Evaluating the lower-tropospheric COSMIC GPS radio occultation sounding quality over the Arctic

Abstract

Lower-tropospheric moisture and temperature measurements are crucial for understanding weather prediction and climate change. Global Positioning System radio occultation (GPS RO) has been demonstrated as a high-quality observation technique with high vertical resolution and sub-kelvin temperature precision from the upper troposphere to the stratosphere. In the tropical lower troposphere, particularly the lowest 2ĝ€km, the quality of RO retrievals is known to be degraded and is a topic of active research. However, it is not clear whether similar problems exist at high latitudes, particularly over the Arctic, which is characterized by smooth ocean surface and often negligible moisture in the atmosphere. In this study, 3-year (2008-2010) GPS RO soundings from COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) over the Arctic (65-90°ĝ€N) show uniform spatial sampling with average penetration depth within 300ĝ€m above the ocean surface. Over 70ĝ€% of RO soundings penetrate deep into the lowest 300ĝ€m of the troposphere in all non-summer seasons. However, the fraction of such deeply penetrating profiles reduces to only about 50-60ĝ€% in summer, when near-surface moisture and its variation increase. Both structural and parametric uncertainties of GPS RO soundings were also analyzed. The structural uncertainty (due to different data processing approaches) is estimated to be within ĝ1/4 ĝ€0.07ĝ€% in refractivity, ĝ1/4 ĝ€0.72ĝ€K in temperature, and ĝ1/4 ĝ€0.05ĝ€gĝ€kgĝ'1 in specific humidity below 10ĝ€km, which is derived by comparing RO retrievals from two independent data processing centers. The parametric uncertainty (internal uncertainty of RO sounding) is quantified by comparing GPS RO with near-coincident radiosonde and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim profiles. A systematic negative bias up to ĝ1/4 ĝ€1ĝ€% in refractivity below 2ĝ€km is only seen in the summer, which confirms the moisture impact on GPS RO quality.