Calculation of zenith delays from meteorological data comparison of NWP model, radiosonde and GPS delays

Abstract

We discuss aspects of how to derive precise estimates of zenith total delays (ZTD's) of the neutral atmosphere (its non-ionized constituents) from meteorological data in order to validate ZTD's derived from ground based GPS (Global Positioning System) observations. The goal is that a later stage GPS ZTD's may conversely be used in numerical weather prediction (NWP) model validation and forecasting. We determine ZTD by numerical integration over modeled or measured profiles of the atmosphere. Doing so, we find it is necessary to discriminate between geopotential and geometric heights. A term is added for the delay arising above the known atmospheric profile. Results obtained for the dry delay by numerical integration and by the Saastamoinen compare well, offsets being of sub millimeter scale only. The offsets we attribute to the time variations of the atmospheric temperature and humidity profile not being accounted for in the Saastamoinen formula. It is found that conversion of the dewpoint temperatures appearing in radiosonde (RS) reports to relative humidities may constitute a problem, and give an estimate of the maximum error associated with that. The GPS sites are in general not co-located with the RS sites nor with the grid-points of the model fields. Before deriving and comparing ZTD's it is necessary to correct for such positional offsets. We find that the correction for vertical offsets between GPS and RS sites or model orography is ambiguous, which can introduce errors.

Using our ZTD calculation algorithms we compare ZTD's based on data from the first one and half year of the MAGIC project. The results are: