A methodology for the validation of temperature profiles from hyperspectral infrared sounders using GPS radio occultation: Experience with AIRS and COSMIC

Abstract

This paper presents a methodology for the validation of vertical temperature profile retrievals from infrared and microwave sounders by intercomparison with Global Positioning System (GPS) radio occultation (RO) profiles matched in time and space. GPS RO provides an estimate of atmospheric temperature with a traceable path to absolute standards and well-characterized structural uncertainty. While the radiosonde network is a longstanding validation reference for sounder temperature profile retrievals, GPS RO has the advantage of homogeneous global coverage that is unbiased toward land or ocean and has the potential of high absolute accuracy in the upper troposphere to middle stratosphere where little water vapor is present. The polar orbiting sounders utilize cross-track scanning in highly repeatable Sun-synchronous orbits, while GPS RO is characterized by pseudorandom sampling in time and space. A comparison approach is described which minimizes sampling uncertainties. The impact of including the horizontal averaging inherent in the RO measurements on the statistical validation is shown. In order to demonstrate the method, bias and RMS vertical profiles have been created for global latitude zones for a range of time intervals using NASA Atmospheric Infrared Sounder (AIRS) Level 2 products compared to dry temperature measurements made by the U.S./Taiwan Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) network. The bias and RMS error profiles are shown to depend strongly on the vertical averaging applied to the difference profiles but are relatively insensitive to horizontal and temporal mismatch.

Accuracy and precision of polar lower stratospheric temperatures from reanalyses evaluated from A-Train CALIOP and MLS, COSMIC GPS RO, and the equilibrium thermodynamics of supercooled ternary solutions and ice clouds

“…We have not implemented the line of sight corrections (Feltz et al, 2014b, a) that are possible when comparing against gridded temperature fields. On a case-by-case basis, the correction can be > 1 K, depending on the orientation of the GPS ray path with respect to temperature gradients, but the ray path averaging method has been shown to reduce the bias (Feltz et al, 2014b, a).…”

“…On a case-by-case basis, the correction can be > 1 K, depending on the orientation of the GPS ray path with respect to temperature gradients, but the ray path averaging method has been shown to reduce the bias (Feltz et al, 2014b, a). For larger sample sizes, the biases are reduced because the positive/negative temperature gradients are averaged away even for a simple closest matching pair method (Feltz et al, 2014b, a).…”

Application of GPS radio occultation to the assessment of temperature profile retrievals from microwave and infrared sounders

“…To obtain the various GPS RO and sounder temperature profile matchup data sets used in this study, the matchup methodology described in Feltz et al (2014) is employed. This matchup methodology has a 1-hour matchup time criterion and calculates a "ray-path" sounder profile that accounts for the GPS RO profile geometry and theoretical spatial resolution.…”

“…The bias and rms statistics are computed for the total number of samples within each zone without area weighting. Details of the sensitivity of this method to changes in the matchup criteria can be found in Feltz et al (2014).…”

“…Previous studies have suggested that GPS RO could be used to assess sounder product accuracies and brightness temperature observations (Yunck et al, 2009;Ho et al, 2009;von Engeln et al, 2010;Weng et al, 2013;Feltz et al, 2014Feltz et al, , 2013Zou et al, 2014). The active GPS RO measurements have higher vertical resolution than the passive near nadir sounders (Kursinski et al, 1997;Staten and Reichler, 2009;Yunck et al, 2009).…”