Zenith Delay Data as a Constraint on the State of the Atmosphere above the Greenland Ice Sheet

Abstract:

The Greenland GPS Network (GNET) consists of ~ 50 GPS stations surrounding the margins of the Greenland Ice Sheet (GrIS). It is now well established that assimilating GPS delay parameters - most commonly zenith total delay (ZTD) estimates - into a numerical weather model (NWM) improves its characterization of water vapor distribution in all three spatial dimensions plus time, and thus improves forecasts of precipitation, and often of other meteorological variables, including cloud cover and surface temperature. Given that NWMs are now used to study the climatology of the GriS, including changes in surface mass balance over time, we argue that assimilating ZTD data from GNET into NWMs using 4DVAR, or similar assimilation techniques, will improve the utility of these models for climatology. The constraint imposed by ZTD observations is somewhat different in character in polar settings than at low or mid latitudes. In southern Greenland, close to sea level, the temporal variability of ZTD is dominated by the variability of total-column integrated water vapor, or precipitable water, especially in summer. But in the high interior of the GrIs, particularly in winter, the temporal variability of ZTD is dominated by changes in surface pressure. We will use GNET and radiosonde observations to illustrate these and some related ideas.