Observation of the gravity waves from GPS/MET radio occultation data

Abstract

We show that the amplitude of the Global Positioning System (GPS) signals in the radio occultation (RO) experiments is sensitive to the atmospheric wave structures. Earlier the phase of the GPS occultation signals have been used for statistical investigation of the gravity waves (GW) activity in the height interval 10–40 km on a global scale. Analysis of the RO amplitude data revealed wave clusters (quasi-regular structures) with the vertical size of about 10 km and interior vertical period ∼0.8–2 km in the tropopause and lower stratosphere. The amplitude RO data may be utilized to determine the temperature vertical profiles and its vertical gradient in the upper troposphere and stratosphere. In the considered RO events variations of the vertical temperature gradient dT(h)/dh corresponding to the amplitude clusters are in the range from −9 K/km to 6 K/km with vertical scales ∼1–3 km. We show that these variations can be linked to the GW propagation in the atmosphere. We use the polarization and dispersion relationships and Hilbert transform to find the 1-D GW image in the atmosphere by analyzing the vertical temperature gradient d T( h)/d h. The GW image consists of the phase and amplitude of the GW as functions of height. The GW amplitude is non-uniformly distributed with main contribution associated with the tropopause and the secondary maximums in height interval 18–35 km. Using our method we find vertical profiles of the horizontal wind perturbations and their vertical gradient associated with the GW influence. The horizontal wind perturbations are changing in the interval v∼2–12 m/s with vertical gradients d v/d h∼4–25 m s −1 km −1) in the tropopause area and v∼ 3–9 m/s, dv/dh ∼ 2–15 m/(s km) in the stratosphere for the considered events. For one RO event we compared the estimated values of the horizontal wind perturbations with aero-logical data and found fairly good agreement. The height dependence of the GW vertical wavelength was inferred through the differentiation of the GW phase. Analysis of this dependence using the dispersion relationship for the GW gives estimation of the GW intrinsic phase speed. For the considered events the magnitude of the intrinsic phase speed changes between 1.5–15 m/s at heights 10–40 km. We conclude that the amplitude of the GPS occultation signals contain important information about the GW propagation in the atmosphere on a global scale.