Can GNSS help satellite measurements for thunderstorm nowcasting?

An accurate assessment of atmospheric water vapour is useful for the nowcasting of thunderstorms and heavy precipitation. Meteosat Third Generation (MTG) will offer new possibilities with the Flexible Combined Imager (FCI) channel at 0.91 µm for low level moisture and geostationary sounding using the Infra-Red Sounder (IRS), in addition to the "classic" water vapour imaging channels at 6.3 µm and 7.3 µm. Water vapour profiles offer better utility for nowcasting than integrated or single-level products. Profiles allow for the calculation of stability parameters such as Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN). They are regularly derived from radiosondes, satellite sounding, and numerical weather prediction (NWP). However, radiosonde measurements are expensive and sparse; satellite soundings have a limited vertical resolution and are relatively poor in the boundary layer; and forecasts have inherent uncertainties compared to measurements.

This study looks into the value of total precipitable water (TPW) measurements from ground-based Global Navigational Satellite Systems (GNSS) in combination with satellite soundings and images. Central Europe has a dense network of GNSS stations with hourly measurements. Although GNSS cannot measure profiles, TPW measurements (calculated from bending angles) are highly accurate. When the NWP forecast sounding disagrees with the satellite sounding and no radiosonde sounding is available, GNSS TPW may help to assess which profile is more consistent with TPW and may be more accurate for CAPE or CIN. In addition, the study looks into the added value of the reflectance at 0.91 µm. Note that this study is still in its early stages and any results presented are highly preliminary. While waiting for FCI and IRS measurements, OLCI and IASI are used as proxies.