ESTIMATING WET DELAYS USING NUMERICAL WEATHER ANALYSES AND PREDICTIONS

Zenith wet delay (ZWD) is a key parameter in analyses of microwave pro pagation delays caused by atmospheric water vapor. This paper explores the possible use of numerical weather analyses and numerical weather forecasts to estimate the ZWD in nearly real time or, more likely, aft er the fact. The predicted precipitable water (PW) field is transforme d into a predicted ZWD field using the relationship ZWD = PW/Pi, where Pi is a function of various physical constants and a weighted mean te mperature of the atmosphere, T-m, that is also predicted using the num erical weather analysis or forecast. We use the + 12-hour forecasts of the Nested Grid Model to predict ZWD and test these predictions again st values derived from independent radiosonde observations. By perform ing these comparisons at more than 100 North American radiosonde stati ons over a 6-month period we find that forecast errors tend to increas e as the magnitude of ZWD increases. This provides a simple framework for characterizing the maximum level of error associated with a ZWD es timates obtained by interpolating in time between analytical ''nowcast s'' and numerical weather predictions. We develop a preliminary approa ch to estimating the ZWD forecast errors at any time between zero and 12 hours from model initialization.