Richard A. Anthes

GPS meteorology: Remote sensing of atmospheric water vapor using the global positioning system

We present a new approach to remote sensing of water vapor based on the global positioning system (GPS). Geodesists and geophysicists have devised methods for estimating the extent to which signals propagating from GPS satellites to ground-based GPS receivers are delayed by atmospheric water vapor. This delay is parameterized in terms of a time-varying zenith wet delay (ZWD) which is retrieved by stochastic filtering of the GPS data. Given surface temperature and pressure readings at the GPS receiver, the retrieved ZWD can be transformed with very little additional uncertainty into an estimate of the integrated water vapor (IWV) overlying that receiver. Networks of continuously operating GPS receivers are being constructed by geodesists, geophysicists, government and military agencies, and others in order to implement a wide range of positioning capabilities. These emerging GPS networks offer the possibility of observing the horizontal distribution of IWV or, equivalently, precipitable water with unprecedented coverage and a temporal resolution of the order of 10 min. These measurements could be utilized in operational weather forecasting and in fundamental research into atmospheric storm systems, the hydrologic cycle, atmospheric chemistry, and global climate change. Specially designed, dense GPS networks could be used to sense the vertical distribution of water vapor in their immediate vicinity. Data from ground-based GPS networks could be analyzed in concert with observations of GPS satellite occultations by GPS receivers in low Earth orbit to characterize the atmosphere at planetary scale.

A High-Resolution Model of the Planetary Boundary Layer—Sensitivity Tests and Comparisons with SESAME-79 Data

Abstract A high-resolution, one-dimensional, moist planetary boundary layer (PBL) model is developed following Blackadar, and verified using the 10 April 1979 SESAME data set. The model consists of two modules to predict the time-dependent behavior of the PBL under various surface characteristics. Under stable conditions, turbulent fluxes are related to a local Richardson number. In contrast, under conditions of free convection, the exchange of heat, moisture and momentum occurs through mixing between convective elements originating at the surface and environmental air in the PBL. Sensitivity tests showed that the daytime PBL structure is most sensitive to moisture availability, roughness length, albedo and thermal capacity, in that order. It is less sensitive in the nighttime to the above parameters. The wind profile is extremely sensitive to the specified geostrophic wind profile at all times. Simulations over both dry and moist terrain indicate that both the free convection (daytime) and the stable (no...

GPS meteorology: mapping zenith wet delays onto precipitable water

Abstract Emerging networks of Global Positioning System (GPS) receivers can be used in the remote sensing of atmospheric water vapor. The time-varying zenith wet delay observed at each GPS receiver in a network can be transformed into an estimate of the precipitable water overlying that receiver. This transformation is achieved by multiplying the zenith wet delay by a factor whose magnitude is a function of certain constants related to the refractivity of moist air and of the weighted mean temperature of the atmosphere. The mean temperature varies in space and time and must be estimated a priori in order to transform an observed zenith wet delay into an estimate of precipitable water. We show that the relative error introduced during this transformation closely approximates the relative error in the predicted mean temperature. Numerical weather models can be used to predict the mean temperature with an rms relative error of less than 1%.

THE COSMIC/FORMOSAT-3 MISSION : Early Results

The radio occultation (RO) technique, which makes use of radio signals transmitted by the global positioning system (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere with high precision, accuracy, and vertical resolution in all weather and over both land and ocean. On 15 April 2006, the joint Taiwan-U.S. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)/Formosa Satellite Mission 3 (COSMIC/FORMOSAT-3, hereafter COSMIC) mission, a constellation of six microsatellites, was launched into a 512-km orbit. After launch the satellites were gradually deployed to their final orbits at 800 km, a process that took about 17 months. During the early weeks of the deployment, the satellites were spaced closely, offering a unique opportunity to verify the high precision of RO measurements. As of September 2007, COSMIC is providing about 2000 RO soundings per day to support the research and operational communities. COSMIC RO dat...

A Cumulus Parameterization Scheme Utilizing a One-Dimensional Cloud Model

Abstract A method for parameterizing the effects of deep cumulus clouds on the larger scale thermodynamic and moisture fields in numerical models is proposed. Rigorous derivations of the effect of cumulus clouds on their environment are derived for two definitions of the large-scale averaged variables. In the first, the classical Reynolds averaging method is used and the averaged variables vary continuously over the domain. In the second method, which has been popular in the derivation of cumulus parameterization schemes, the averages are defined by dividing an incremental area of the domain (usually the mesh aim) into a region occupied by convection and the remainder of the region which is free of convection. In this method, the large-scale averages assume discrete values over each incremental area. The differences between the large-scale equations that result from these two methods and some possible difficulties that may be encountered when the averaging interval approaches the aim of the convective clo...

Analysis and validation of GPS/MET data in the neutral atmosphere

The Global Positioning System/Meteorology ( GPS/MET) Program was established in 1993 by the University Corporation for Atmospheric Research ( UCAR) to demonstrate active limb sounding of the Earths atmosphere using the radio occultation technique. The demonstration system observes occulted GPS satellite signals received by a low Earth orbiting ( LEO) satellite, MicroLab-1, launched April 3,1995. The system can profile ionospheric electron density and neutral atmospheric properties. Neutral atmospheric refractivity, density, pressure, and temperature are derived at altitudes where the amount of water vapor is low. At lower altitudes, vertical profiles of density, pressure, and water vapor pressure can be derived from the GPS/MET refractivity profiles if temperature data from an independent source are available. This paper describes the GPS/MET data analysis procedures and validates GPS/MET data with statistics and illustrative case studies. We compare more than 1200 GPS/MET neutral atmosphere soundings to correlative data from operational global weather analyses, radiosondes, and the GOES, TOVS, UARS/MLS and HALOE orbiting atmospheric sensors. Even though many GPS/MET soundings currently fail to penetrate the lowest 5 km of the troposphere in the presence of significant water vapor, our results demonstrate 1°C mean temperature agreement with the best correlative data sets between 1 and 40 km. This and the fact that GPS/MET observations are all-weather and self-calibrating suggests that radio occultation technology has the potential to make a strong contribution to a global observing system supporting weather prediction and weather and climate research.

GPS Sounding of the Atmosphere from Low Earth Orbit: Preliminary Results

Abstract This paper provides an overview of the methodology of and describes preliminary results from an experiment called GPS/MET (Global Positioning System/Meteorology), in which temperature soundings are obtained from a low Earth-orbiting satellite using the radio occultation technique. Launched into a circular orbit of about 750-km altitude and 70° inclination on 3 April 1995, a small research satellite, MicroLab 1, carried a laptop-sized radio receiver. Each time this receiver rises and sets relative to the 24 operational GPS satellites, the GPS radio waves transect successive layers of the atmosphere and are bent (refracted) by the atmosphere before they reach the receiver, causing a delay in the dual-frequency carrier phase observations sensed by the receiver. During this occultation, GPS limb sounding measurements are obtained from which vertical profiles of atmospheric refractivity can be computed. The refractivity is a function of pressure, temperature, and water vapor and thus provides informat...

Development of Hydrodynamic Models Suitable for Air Pollution and Other Mesometerological Studies

Abstract We describe the development of a general, predictive, hydrostatic meteorological model. The model is three-dimensional and is suitable for a wide variety of problems, ranging from the synoptic scale to the small end of the mesoscale. The model contains provisions for variable terrain, a moisture cycle, sensible heat addition at the earths interface, and high- and low-resolution boundary layer physics. This paper presents the mathematical and numerical formulation used in the various options of the model. First we write the basic equations on a Lambert conformal projection. Then we describe the horizontal and vertical grid structure, the finite-difference equations, and the energetics of the three-dimensional model and its two-dimensional analog. We consider the role of the lateral boundary conditions for limited area forecasts, with emphasis on their effect on the mean motion over the domain. Two options for including the frictional and diabatic effects at the earths surface are presented. Thes...

Enhancement of Convective Precipitation by Mesoscale Variations in Vegetative Covering in Semiarid Regions

Abstract It is hypothesized that planting bands of vegetation with widths of the order of 50–100 km in semiarid regions could, under favorable large-scale atmospheric conditions, result in increases of convective precipitation. These increases, which could be greater than those associated with the uniform vegetating of large areas, would occur through three major mechanisms. The first would be the modification of the environment to a state more conducive to the formation of moist convection through an increase of low-level moist static energy. This increase would be associated with a decrease in albedo, an increase in net radiation, and an increase in evapotranspiration. The second important mechanism would be the generation of mesoscale (horizontal scale of 20–200 km) circulations associated with the surface inhomogeneities created on this scale by the vegetation. The third mechanism would be the increase of atmospheric water vapor through decreased runoff and increased evaporation. A number of observati...

The Initialization of Numerical Models by a Dynamic-Initialization Technique

Abstract A dynamic-initialization technique is tested with three models of fluid flow. In this technique data are assimilated through the inclusion of terms in the forecast equations which force the model atmosphere toward the observations. Results indicate that accurate, dynamically balanced mass and momentum fields can be obtained from unbalanced, inexact first guesses. In midlatitudes for horizontal scales less than 2000 km, observations of the wind are more important than observations of mass in producing a successful initialization for the systems presented here. There is also evidence that forcing the model winds toward the divergent wind component may slow the initialization process.