Global 3D Features of Error Variances of GPS Radio Occultation and Radiosonde Observations

Xu Xu,Xiaolei Zou

doi:10.3390/rs13010001

Abstract

Global Positioning System (GPS) radio occultation (RO) and radiosonde (RS) observations are two major types of observations assimilated in numerical weather prediction (NWP) systems. Observation error variances are required input that determines the weightings given to observations in data assimilation. This study estimates the error variances of global GPS RO refractivity and bending angle and RS temperature and humidity observations at 521 selected RS stations using the three-cornered hat method with additional ERA-Interim reanalysis and Global Forecast System forecast data available from 1 January 2016 to 31 August 2019. The global distributions, of both RO and RS observation error variances, are analyzed in terms of vertical and latitudinal variations. Error variances of RO refractivity and bending angle and RS specific humidity in the lower troposphere, such as at 850 hPa (3.5 km impact height for the bending angle), all increase with decreasing latitude. The error variances of RO refractivity and bending angle and RS specific humidity can reach about 30 N-unit2, 3 × 10−6 rad2, and 2 (g kg−1)2, respectively. There is also a good symmetry of the error variances of both RO refractivity and bending angle with respect to the equator between the Northern and Southern Hemispheres at all vertical levels. In this study, we provide the mean error variances of refractivity and bending angle in every 5°-latitude band between the equator and 60°N, as well as every interval of 10 hPa pressure or 0.2 km impact height. The RS temperature error variance distribution differs from those of refractivity, bending angle, and humidity, which, at low latitudes, are smaller (less than 1 K2) than those in the midlatitudes (more than 3 K2). In the midlatitudes, the RS temperature error variances in North America are larger than those in East Asia and Europe, which may arise from different radiosonde types among the above three regions.

Highlights

Global Positioning System (GPS) radio occultation (RO) and radiosonde (RS) observations are both important in initializing numerical weather models, conducting global and regional reanalysis, and calibrating and validating satellite data
This quality control (QC) step is a biweight check on the variables participating in the error variance estimation, aimed at removing outliers of the collocated observations
We provide the values of the RO bending angle error variances in every 5◦ latitudinal band and 0.2 km impact height interval between 0◦ and 60◦ N (Table A2 in the Appendix ??)

Summary

Introduction

Global Positioning System (GPS) radio occultation (RO) and radiosonde (RS) observations are both important in initializing numerical weather models, conducting global and regional reanalysis, and calibrating and validating satellite data. RS soundings are in situ measurements made under all-weather conditions with a long history, often considered a standard for comparisons with satellite observations and reanalysis data [1,2,3]. RS stations are distributed more densely in East Asia, Europe, and North America than elsewhere. Satellite missions, such as the Constellation Observing System for Meteorology, Ionosphere, and Climate-1, -2 [5,6,7], provide nearly evenly and densely distributed RO profiles over the world, making up for the lack of conventional observations over the ocean

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Conclusion