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In this study, the GPS radio occultation (RO) data from the Challenging Minisatellite Payload (CHAMP) and Satellite de Aplicaciones Cientificas-C (SAC-C) missions are assimilated. An updated version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) four-dimensional variational data assimilation system (4DVAR) is used to assess the impact of the GPS RO data on analyses and short-range forecasts over the Antarctic. The study was performed during the period of intense cyclonic activity in the Ross Sea, 9–19 December 2001. On average 66 GPS RO soundings were assimilated daily. For the assimilation over a single 12-h period, the impact of GPS RO data was only marginally positive or near neutral, and it varied markedly from one 12-h period to another. The large case-to-case variation was attributed to the low number of GPS RO soundings and a strong dependency of forecast impact on the location of the soundings relative to the rapidly developing cyclone. Despite the moderate general impact, noticeable reduction of temperature error in the upper troposphere and lower stratosphere was found, which demonstrates the value of GPS RO data in better characterizing the tropopause. Significant error reduction was also noted in geopotential height and wind fields in the stratosphere. Those improvements indicate that early detection of the upper-level precursors for storm development is a potential benefit of GPS RO data. When the assimilation period was extended to 48 h, a considerable positive impact of GPS RO data was found. All parameters that were investigated (i.e., temperature, pressure, and specific humidity) showed the positive impact throughout the entire model atmosphere for forecasts extending up to 5 days. The impact increased in proportion to the length of the assimilation period. Although the differences in the analyses as a result of GPS RO assimilation were relatively small initially, the subtle change and subsequent nonlinear growth led to noticeable forecast improvements at longer ranges. Consequently, the positive impact of GPS RO data was more evident in longer-range (e.g., greater than 2 days) forecasts. A correlation coefficient is introduced to quantify the linear relationship between the analysis errors without GPS RO assimilation and the analysis increments induced by GPS RO assimilation. This measure shows that the growth of GPS RO–induced modifications over time is related to the prominent error reduction observed in GPS RO experiments. The measure may also be useful for understanding how cycling analysis accumulates the positive impact of GPS RO data for an extended period of assimilation. [ABSTRACT FROM AUTHOR]

In s coordinates, a variable Φ[sube] (x, y, σ, t) whose horizontal gradient -ΔΦ[sube] is equal to the irrotational part of the horizontal pressure gradient force is referred to as an equivalent isobaric geopotential height. Its inner part can be derived from the solution of a Poisson equation with zero Dirichlet boundary value. Because -ΔΦ(x, y, σ, t) is also the irrotational part of the horizontal pressure gradient force in p coordinates, the equivalent geopotential Φ[sube] in σ coordinates can be used in the same way as the geopotential Φ(x, y, p, t) used in p coordinates. In the sea level pressure (SLP) analysis over Greenland, small but strong high pressure systems often occur due to extrapolation. These artificial systems can be removed if the equivalent geopotential Φ[sube] is used in synoptic analysis on a constant σ surface, for example, at &sigma = 0.995 level. The geostrophic relation between the equivalent geopotential and streamfunction at σ = 0.995 is approximately satisfied. Because weather systems over the Tibetan Plateau are very difficult to track using routine SLP, 850-hPa, and 700-hPa analyses, equivalent isobaric geopotential analysis in σ coordinates is especially useful over this area. An example of equivalent isobaric geopotential analysis at σ = 0.995 shows that a secondary high separated from a major anticyclone over the Tibetan Plateau when cold air affected the northeastern part of the plateau, but this secondary high is hardly resolved by the SLP analysis. The early stage of a low (or vortex), called a southwest vortex due to its origin in southwest China, over the eastern flank of the Tibetan Plateau is more clearly identified by equivalent isobaric geopotential analysis at σ = 0.825 and 0.735 than by routine isobaric analysis at the 850- and 700-hPa levels. Anomalous high and low systems in the SLP analysis over the Tibetan Plateau... [ABSTRACT FROM AUTHOR]

In comparison to the Tatsumi's spectral method, the harmonic-Fourier spectral method has two major advan-tages. 1) The semi-implicit scheme is quite efficient because the solutions of the Poisson and Helmholtz equations are readily derived. 2) The lateral boundary value problem of a limited-area model is easily solved. These advantages are the same as those of the spherical harmonics used in global models if the singularity at the pole points for a globe is considered to be the counterpart of the lateral boundary condition for a limited region. If a limited-area model is nested in a global model, the prediction of the limited-area model at each time step is the sum of the inner part and the harmonic part predictions. The inner part prediction is solved by the double sine series from the inner part equations for the limited-area model. The harmonic part prediction is derived from the prediction of the global model. An external wind lateral boundary method is proposed based on the basic property of the wind separation in a limited region. The boundary values of a limited-area model in this method are not given at the closed boundary line, but always given by harmonic functions defined throughout the limited domain. The harmonic functions added to the inner parts at each time step represent the effects of the lateral boundary values on the prediction of the limited-area model, and they do not cause any discontinuity near the boundary. [ABSTRACT FROM AUTHOR]