An Elliptical Approach to Comparing Radio Occultation and Radiosonde Observations

An Elliptical Approach to Comparing Radio Occultation and Radiosonde Observations

Shay Gilpin (1, 2), Therese Rieckh (2), Rick Anthes (2), and Gang Lu (3)

(1)  Significant Opportunities in Atmospheric Research and Science (SOARS), Boulder, CO

(2)  COSMIC Program Office, University Corporation for Atmospheric Research, Boulder, CO

(3)  High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO

Radio occultation (RO) is a relatively new method of obtaining atmospheric soundings, producing vertical profiles of bending angles and refractivity in the stratosphere and troposphere. From the bending angles and refractivities, both temperature and water vapor pressure can be derived and used in various meteorological and climatological applications, including assimilation in numerical weather prediction models. Comparing RO with other types of atmospheric observations, such as radiosondes, is crucial in understanding the properties and quality of RO observations. Conducting these comparisons can be difficult for two reasons: first, RO is a unique type of observation, consisting of a weighted average of atmospheric properties within a thin cylindrical volume of atmosphere (~200km in the horizontal and ~500m in the vertical). Second, RO and other observations are rarely taken at the exact same time or location, introducing sampling errors resulting from atmospheric variability that can overshadow fundamental differences in the RO and observations under comparison.

Previous studies have compared radiosonde observations with RO observations from a certain time window within circles of a given radius (ranging between 100-300km) centered at the location of the radiosonde (Kuo et al., 2005; Xu et al., 2009; He et al., 2009; Sun et al., 2010; and Wang et al., 2013). This study investigates whether comparisons between RO and radiosondes or models within ellipses with semi-major axis along the direction of wind flow, rather than circles, will minimize sampling errors. It is hypothesized that the refractivity gradient tends to be perpendicular to the local wind direction, resulting in less variation of refractivity along the direction of flow and more variation perpendicular to the flow.

The hypothesis was first tested using the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA Interim) data over the Tropical West Pacific. Model data were used first due to their high density of data points and ability to sample at exactly the same times, eliminating errors associated with sampling time differences. The ERA Interim refractivity field showed a strong correlation between refractivity and the direction of wind flow, especially in regions of high wind speeds, suggesting that comparisons along the wind flow would be an optimal method to minimize sampling errors.

ERA Interim model refractivites within an ellipse parallel to wind flow, perpendicular to wind flow, and two circles were compared with a reference ERA Interim point at the center (simulating a radiosonde location) with the expectation that the parallel ellipse will have a smaller sampling error than the other three. The semi-major axis of the ellipse and radius of the larger circle was 6° in latitude (666km), and the radius of the smaller circle was 2.6° in latitude (~300km). Comparisons within each ellipse and circle were done twice a day (00:00UTC and 12:00UTC) at six different pressure levels and two different locations for January-February 2007.

Statistical analysis over the two-month period showed lower root mean square (RMS) differences in refractivity for the parallel ellipse by a factor of two to three compared to the perpendicular ellipse and larger circle at all six pressure levels and at both locations. This supports the hypothesis that, on average, refractivity varies less along the direction of wind flow rather than across. The smaller circle, which contained roughly the same number of ERA Interim data points as the ellipses, had RMS differences in refractivity similar to the parallel ellipse and at certain pressure levels slightly lower RMS values than for the parallel ellipse.

We compared ERA Interim refractivities with actual RO observations in ellipses parallel and perpendicular to the flow, as well as the two circles, over the same two-month time period. As seen with the model data, RMS differences between ERA Interim and RO refractivities were significantly less within the parallel ellipse, along the direction of wind flow, compared to perpendicular to the flow or within the larger circle.

Comparisons between RO observations and the Guam radiosonde observations within the two ellipses and two circles are being conducted at the time of the abstract submission (3 August 2016). Results of the radiosonde to RO comparison as well as an overall summary of the effectiveness of this new method will be presented at the conference.

References:

He, Wenying et al., 2009: Assessment of radiosonde temperature measurements in the

upper troposphere and lower stratosphere using COSMIC radio occultation data.

Geophys. Res. Letters, 36. L17807, doi:10.1029/2009GL038712.

Kuo, Y.-H. et al. 2005: Comparison of GPS radio occultation soundings with

radiosondes. Geophy. Res. Letters, 32, L05817, doi:10.1079/2004GL021443.

Sun, Bomin et al., 2010: Comparing radiosonde and COSMIC atmospheric profile data to

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