AROSA

Starting point / motivation

Austria depends significantly on high quality, highly resolved weather forecasts, especially due to its complex orography, manifold landscapes and special meteorologically induced natural hazards in the alpine area and its important economic branches agriculture and tourism, which are strongly impacted by weather.

The success of these forecasts is determined by a precise definition of the current state of the 3D atmosphere with highly resolved measurements due to the nonlinear nature of atmospheric processes.

Contents and goals

Radio occultation methods investigate the bending of a radio signal on its way through the atmosphere by measuring the Doppler shift between a global navigation satellite system (GNSS) and a low earth orbit satellite (LEO) and their precise positions. The bending and refraction of the signal depend on atmospheric properties like ionisation of the upper atmosphere and moisture and temperature in lower levels.

So, these properties can be indirectly estimated by the bending with a high vertical resolution on the meter scale in the upper levels, where conventional observations (aircraft and radio soundings) are relatively scarce. The observation number of public financed probes dropped down recently by aging and breakdown of the LEO satellites, while on contrary a huge number of recently commercially launched and maintained satellites of the Spire Inc increased the amount of radio occultation data drastically.

Methods

In addition to atmospheric monitoring, the occultation method can be used for the initialisation of numerical weather prediction models, as it was already shown for some global models (Arpège, GME, ECMWF-IFS), but also limited area models (WRF).

Especially, in the latter case with higher model resolutions the definition of the observation operator simulating the measured parameter is rather crucial to succeed. Within the scope of this project, the new occultation measurements of Spire Inc will be assimilated for the first time into the numerical weather prediction system of ZAMG named AROME over Austria. 

To achieve this aim, data pre-processing is necessary (derivation of the bending angle, quality check by passive assimilation and first guess departure checks). For the time being, a 2D observation operator for bending angle is available in the AROME code, which was developed for coarser resolutions.

Expected results

Within the project, it will be investigated, how it can be improved and adapted to higher resolutions and which steps would be necessary to reach this goal. The possible impact of the new observations on the model performance will be estimated by case studies and a longer test period using intercomparison to a reference run without radio occultation assimilation.

Finally the potential of an operational application of the data within the AROME system will be envisaged. The impact on the forecast in data poor regions outside Europe will be estimated by stronger thinning of the other observations.

Coordinator

ZAMG - Central Institute for Meteorology and Geodynamics

Project partner

Universität Graz - Wegener Center für Klima und Globalen Wande

ZAMG - Central Institute for Meteorology and Geodynamics
Dr. Stefan Schneider
Hohe Warte 38
A-1190 Vienna