Orographic and convective gravity waves above the Alps and Andes Mountains during GPS radio occultation events – a case study

摘要

Gravity waves (GWs) and convective systems play a fundamental role inatmospheric circulation, weather, and climate. Two usual main sources of GWsare orographic effects triggering mountain waves and convective activity. Inaddition, GW generation by fronts and geostrophic adjustment must also beconsidered. The utility of Global Positioning System (GPS) radio occultation(RO) observations for the detection of convective systems is tested. Acollocation database between RO events and convective systems oversubtropical to midlatitude mountain regions close to the Alps and Andes isbuilt. From the observation of large-amplitude GW structures in theabsence of jets and fronts, subsets of RO profiles are sampled. Arepresentative case study among those considered at each region is selectedand analyzed. The case studies are investigated using mesoscale Weather Research and Forecasting (WRF)simulations, ERA-Interim reanalysis data, and measured RO temperatureprofiles. The absence of fronts or jets during both case studies revealssimilar relevant GW features (main parameters, generation, and propagation).Orographic and convective activity generates the observed GWs. Mountain wavesabove the Alps reach higher altitudes than close to the Andes. In the Andescase, a critical layer prevents the propagation of GW packets up tostratospheric heights. The case studies are selected also because theyillustrate how the observational window for GW observations through ROprofiles admits a misleading interpretation of structures at differentaltitude ranges. From recent results, the distortion introduced in themeasured atmospheric vertical wavelengths by one of the RO events isdiscussed as an illustration. In the analysis, both the elevation angle ofthe sounding path (line of tangent points) and the gravity wave aspect ratioestimated from the simulations and the line of sight are taken into account.In both case studies, a considerable distortion, over- and underestimation ofthe vertical wavelengths measured by RO, may be expected.