Analysis of vertical wave number spectrum of atmospheric gravity waves in the stratosphere using COSMIC GPS radio occultation data

T Tsuda,H Hayashi,X Lin

doi:10.5194/amt-4-1627-2011

Abstract

Abstract. GPS radio occultation (RO) is characterized by high accuracy and excellent height resolution, which has great advantages in analyzing atmospheric structures including small-scale vertical fluctuations. The vertical resolution of the geometrical optics (GO) method in the stratosphere is about 1.5 km due to Fresnel radius limitations, but full spectrum inversion (FSI) can provide superior resolutions. We applied FSI to COSMIC GPS-RO profiles from ground level up to 30 km altitude, although basic retrieval at UCAR/CDAAC sets the sewing height from GO to FSI below the tropopause. We validated FSI temperature profiles with routine high-resolution radiosonde data in Malaysia and North America collected within 400 km and about 30 min of the GPS RO events. The average discrepancy at 10–30 km altitude was less than 0.5 K, and the bias was equivalent with the GO results. Using the FSI results, we analyzed the vertical wave number spectrum of normalized temperature fluctuations in the stratosphere at 20–30 km altitude, which exhibits good consistency with the model spectra of saturated gravity waves. We investigated the white noise floor that tends to appear at high wave numbers, and the substantial vertical resolution of the FSI method was estimated as about 100–200 m in the lower stratosphere. We also examined a criterion for the upper limit of the FSI profiles, beyond which bending angle perturbations due to system noises, etc., could exceed atmospheric excess phase fluctuations. We found that the FSI profiles can be used up to about 28 km in studies of temperature fluctuations with vertical wave lengths as short as 0.5 km.

Highlights

The Global Positioning System (GPS) radio occultation (GPS-RO) technique is an active limb sounding observation of the Earth’s atmosphere using a GPS receiver onboard a low Earth orbit (LEO) satellite
Because of its advantages in vertical resolution, accuracy, and self-calibration, the GPS RO technique is promising for areas such as the atmospheric and ionospheric sciences, global and mesoscale weather forecasting, climate change monitoring, and boundary layer research
Because of limitations due to the Fresnel radius for geometrical optics (GO), its vertical resolution is limited to about 1.5 km in the lower stratosphere, while full spectrum inversion (FSI) is not affected by such limitations

Summary

Introduction

The Global Positioning System (GPS) radio occultation (GPS-RO) technique is an active limb sounding observation of the Earth’s atmosphere using a GPS receiver onboard a low Earth orbit (LEO) satellite. Tsuda et al.: Analysis of vertical wave number spectrum of atmospheric gravity waves atmosphere Such a good height resolution, comparable to a ground-based radiosonde, has not been attained with other conventional satellite techniques. GPS RO profiles normally appear as a projection of many gravity waves with different frequency, vertical and horizontal wavenumbers. Statistical characteristics of such perturbations can be described well by analyzing a vertical wavenumber spectrum. It is noteworthy, that the horizontal resolution of GPS RO is about 300 km, being common to a limb scanning method GPS RO can resolve most of the vertical wave number spectrum with wavelengths ranging from a few hundred meters to about 10 km, which is very unique compared to other satellite instruments

Retrieval of GPS RO profiles

Validation of GPS RO temperature profiles with radiosondes

Vertical wave number spectrum

Upper limit of the FSI profiles

Findings

Concluding remarks