Abstract
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 2 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° N) show uniform spatial sampling with average penetration depth within 300 m above the ocean surface. Over 70 % of RO soundings penetrate deep into the lowest 300 m of the troposphere in all non-summer seasons. However, the fraction of such deeply penetrating profiles reduces to only about 50–60 % 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, ∼ 0.72 K in temperature, and ∼ 0.05 g kg−1 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 European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim profiles. A systematic negative bias up to ∼ 1 % in refractivity below 2 km is only seen in the summer, which confirms the moisture impact on GPS RO quality.
Highlights
Over the Arctic, the surface temperature has increased twice as much as the global average rate in the past 100 years (Bernstein et al, 2007; Chae et al, 2015; Najafi et al, 2015), indicated by the decline of sea ice cover
Ture and its variation in summer, even though relatively small compared to the tropics, can lead to significant Global Positioning System radio occultation (GPS radio occultation (RO)) signal-to-noise ratio (SNR) and excess Doppler variations, which could complicate the Global Positioning System (GPS) RO signal tracking and lead to early sounding termination before reaching the surface
The parametric uncertainty is quantified by comparing RO with the near-coincident radiosonde and the ERA-I reanalysis
Summary
Over the Arctic, the surface temperature has increased twice as much as the global average rate in the past 100 years (Bernstein et al, 2007; Chae et al, 2015; Najafi et al, 2015), indicated by the decline of sea ice cover. The change of the Arctic climate (e.g., temperature) along with the decline of sea ice cover is expected to affect the global climate (Vihma, 2014). The lower troposphere is one of the most critical components of the Arctic climate system, which has been intensely investigated by various observations (e.g., in situ balloon sounding; ground-based, airborne, and satellite remote sensing). Traditional radiosonde balloon soundings have long been the most reliable for sensing the atmospheric properties (e.g., temperature, pressure, and humidity) with high vertical resolution (Pelliccia et al, 2011) and widely used to calibrate and validate the satellite-borne retrievals (John and Buehler, 2005; Kuo et al, 2005).
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