Improved water vapor measurement using water vapor radiometers and the Global Positioning System

Water vapor radiometer (WVR) retrieval algorithms require a priori information on atmospheric conditions along the line of sight of the radiometer to derive opacities from observed brightness temperatures. The mean radiating temperature of the atmosphere (T{dollar}sb{lcub}rm mr{rcub}{dollar}) is utilized in these algorithms to relate WVR measurements to integrated water vapor. Current methods for specifying T{dollar}sb{lcub}rm mr{rcub}{dollar} rely on climatology, seasonal parameters, or information from nearby soundings to specify T{dollar}sb{lcub}rm mr{rcub}{dollar}. However, values of T{dollar}sb{lcub}rm mr{rcub}{dollar}, calculated from radiosonde data, vary not only according to site and season but also exhibit large fluctuations in response to local weather conditions. By utilizing output from numerical weather prediction models, T{dollar}sb{lcub}rm mr{rcub}{dollar} can be accurately prescribed for an arbitrary WVR site at a specific time.; Values of T{dollar}sb{lcub}rm mr{rcub}{dollar} obtained from current methods and this new approach are compared to those obtained from in situ soundings. The improvement of the T{dollar}sb{lcub}rm mr{rcub}{dollar} calculation using available model forecast data rather than climatological values yields a corresponding improvement of comparable magnitude in the retrieval of atmospheric opacity. Use of forecast model data relieves a WVR site of its dependency on local climatology or the necessity of a nearby sounding, allowing more accurate retrieval of observed conditions and increased flexibility in choosing site location. Furthermore, it is found that the calculation of precipitable water by means of atmospheric opacities does not require time dependent tuning parameters when model data are used.; The Global Positioning System (GPS) has been proposed as a source of meteorological information from which precipitable water values may be extracted from signal delays introduced by atmospheric water vapor. The utility of GPS derived precipitable water vapor measurements is demonstrated through the analysis of a mid-tropospheric cold front, or cold front aloft (CFA), which enhanced convection and resulted in severe thunderstorms during GPS-STORM. Surface based meteorological observations were insufficient for detecting and tracking the movement of the CFA, while vertical profiles obtained from the standard radiosonde observations lack the temporal and spatial resolution required to provide an adequate operational description of the feature which passed through the experimental domain midway between sounding times. Precipitable water measurements from GPS signals are shown to provide a valuable data source for operational forecasts as well as numerical weather prediction models.