Abstract
Abstract. The global positioning system (GPS) radio occultation (RO) method is a relatively new technique for taking atmospheric measurements for use in both weather and climate studies. As such, this technique needs to be evaluated for all parts of the globe. Here, we present an extensive evaluation of the performance of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) GPS RO observations of the Southern Ocean boundary layer. The two COSMIC products used here are the "wetPrf" product, which is based on 1-D variational analysis with European Centre for Medium-Range Weather Forecasts (ECMWF), and the "atmPrf" product, which contains the raw measurements from COSMIC. A direct comparison of temporally and spatially co-located COSMIC profiles and high resolution radiosonde profiles from Macquarie Island (54.62° S, 158.85° E) highlights weaknesses in the ability of both COSMIC products to identify the boundary layer structure, as identified by break points in the refractivity profile. In terms of reproducing the temperature and moisture profile in the lowest 2.5 km, the "wetPrf" COSMIC product does not perform as well as an analysis product from the ECMWF. A further statistical analysis is performed on a large number of COSMIC profiles in a region surrounding Macquarie Island. This indicates that, statistically, COSMIC performs well at capturing the heights of main and secondary break points. However, the frequency of break points detected is lower than the radiosonde profiles suggest, but this could be simply due to the long horizontal averaging in the COSMIC measurements. There is also a weak seasonal cycle in the boundary layer height similar to that observed in the radiosonde data, providing some confidence in the ability of COSMIC to detect an important boundary layer variable.
Highlights
The structure and dynamics of the atmospheric boundary layer (ABL) directly impact the weather, through the transport of scalars such as water vapour, and the climate, most obviously through their role in the formation and dissipation of clouds
Given that clouds over the Southern Ocean is responsible for large biases in modelled net radiation (Trenberth and Fasullo, 2010), it is important to understand the fundamental processes at work in this region, which is dominated by the presence of boundary layer cloud year round (Huang et al, 2012a)
The aim of this study is to evaluate the performance of the global positioning system (GPS) radio occultation (RO) technique of measuring boundary layer height, and other significant inversions, as well as the ability of the “wetPrf” COSMIC product to reproduce the temperature and moisture profiles within the boundary layer
Summary
The structure and dynamics of the atmospheric boundary layer (ABL) directly impact the weather, through the transport of scalars such as water vapour, and the climate, most obviously through their role in the formation and dissipation of clouds. Another distinctive feature of the Southern Ocean is high wind speeds and wind shear (Hande et al, 2012b; Xiaojun, 2004), producing some of the largest observed wave heights on the globe (Vinoth and Young, 2011) These conditions lead to the possibility of significant sea spray being injected into the boundary layer. The aim of this study is to evaluate the performance of the GPS RO technique of measuring boundary layer height, and other significant inversions, as well as the ability of the “wetPrf” COSMIC product to reproduce the temperature and moisture profiles within the boundary layer An evaluation of this COSMIC data product is needed in this region because the structure of the boundary layer is unlike that of the tropical and subtropical high-pressure region, where the break point method for determining the ABL height has been shown to perform well
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