Ronald McPherson, Eugenia Kalnay, Stephen Lord
National Center for Environmental Prediction, National Weather Service
Operational numerical weather prediction (NWP) applies the laws of physics, which govern the behavior of atmosphere, to the practical problem of weather prediction. In mathematical terms NWP is an initial value problem, in that the physical laws are used to calculate the temporal evolution of the physical state of the atmosphere, from an estimate of the initial atmospheric conditions.
Determining this “initial state” of the atmosphere is one of the three central problems in operational NWP. It requires observations of wind, temperature, pressure and humidity through the depth of the atmosphere, plus observations of some characteristics of the earth's surface such as snow cover, wetness, vegetation, and sea-surface temperature. These observations are presently obtained by a mixture of observing techniques that have evolved in a largely unplanned manner over the last five or six decades. For forecast projections longer than three or four days the complete global atmosphere must be sampled, and this has led to a considerable emphasis on space-based remote sensing techniques. The second section of this essay describes briefly the current observing system.
A second requirement for determining the initial state of the atmosphere is a system for assimilating disparate observations from this mixture of observing systems into a coherent, dynamically consistent, digital description of the atmosphere. Originally concerned merely with spatial interpolation of radiosonde data to grid of regularly-spaced points, modern four-dimensional data assimilation (4DDA) systems now seek to blend observations of many quantities from observing systems with widely differing error characteristics, with a highly accurate background (or “first guess”) estimate of the state of the atmosphere. Importantly, modern 4DDA systems are capable of ingesting observed quantities such as radiances or radar backscatter cross-sections rather than converting these quantities to more familiar meteorological variables such as temperature, wind, etc. The third section of this paper discusses characteristic of 4DDA systems that are relevant for the use of GPS/MET data.
From time to time, new observing technologies appear, offering either new data (to fill gaps), or better data (more accurate), or cheaper data. Several such possibilities are now, or soon will be, available. Governments that operate the existing, composite observing system are under enormous financial pressure to reduce the costs of observing the atmosphere. Therefore, the U.S. has recently undertaken a systematic redesign of the North American Observing System (NAOS), with the intent of better observing at less cost. Several new technologies will be considered in this redesign effort, which will last for several years. One of those new technologies, using radio occultation techniques in connection with the Global Positioning System, is the subject of this essay. The last section of this paper addresses the potential usefulness of atmospheric refractivity inferred from these techniques in operational numerical weather prediction.
Vertical profile observations of the mass field (i.e., temperature) are obtained from two principal sources: balloon-borne radiosondes flown twice daily from about 600 stations world-wide, and from passive radiometric measurements from satellite platforms. The former are quite accurate, with standard errors of 0.5 - 0.8C, have excellent vertical resolution, and have for many years been the backbone of the global observing system. On the other hand, radiosonde stations are mostly located on northern hemisphere continents, which provides very uneven spatial coverage, and are expensive to operate. Satellite temperature observations are less accurate with standard errors of 2C, and have poorer vertical resolution, but offer excellent spatial coverage. Current satellite systems are also extremely expensive.
Wind profiles are available from radiosondes, from ground-based radar wind profilers, Doppler weather-surveillance radars, and increasingly from wide-bodied jet aircraft on ascent and descent near airports. Single-level wind observations are obtained from aircraft, and by tracking cloud and moisture patterns in geostationary