Evaluating the Lower Tropospheric COSMIC GPS Radio Occultation Sounding Quality over the Arctic
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Abstract. Lower tropospheric moisture and temperature measurements are crucial for understanding weather predication and climate change. Global Positioning System radio occultation (GPS RO) has been demonstrated as a high-quality observation with high-vertical-resolution and sub-Kelvin temperature precision from the upper troposphere to the stratosphere. In the tropical lower troposphere, particularly the lowest 2km, 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 in the high latitudes, particularly over the Artic, which is characterized by smooth ocean surface and often negligible moisture in the atmosphere. In this study, three-year (20082010) GPS RO soundings from COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) over the Arctic (65N-90N) show uniform spatial sampling with average penetration depth within 300m above the ocean surface. Over 70% soundings penetrate below 300m in all non-summer seasons but only about 5060% 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 0.07% in refractivity, 0.72K in temperature and 0.029g/kg in specific humidity below 10 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 ECMWF ERA-Interim profiles. A systematic negative bias up to ~1% in refractivity below 2km is only seen in the summer, which confirms the moisture impact on GPS RO quality.
