A climatology of clear sky ultraviolet (UV)

An observing systems experiment to assess the impact of aircraft observations on analyses and forecasts from a numerical model is described. Parallel runs with and without aircraft observations show significant analysis differences in the upper troposphere over northern hemisphere oceans, the aircraft giving a better representation of jet strengths and less reliance on poorer quality observations. This positive impact extends to the shape and position of upper tropospheric features in forecasts from these analyses though the magnitude of the improvements is somewhat less.These results are in broad agreement with those from a similar experiment carried out at the European Centre for Medium Range Weather Forecasts. Some differences attributable to the response of the different analysis schemes used are discussed in the light of geostrophic adjustment theory and are studied further using model forecasts from perturbed initial conditions. These studies suggest that beneficial effects on forecasts can be achieved by distributing single-level data throughout several model levels and using wind observations to update the rotational component of the wind field.A simple linearized theoretical model supports these conclusions and is used to investigate the effects of the repeated insertion analysis procedure used in our experiments. This effectively suppresses the generation of high frequency waves but shows undesirable effects in the presence of a large mean advective velocity.

This paper gives an account of the global soundings system (GLOSS), which is the new method introduced at the Meteorological Office at Bracknell for the processing of global TOVS radiance data for assimilation into numerical weather prediction (NWP) models. The assimilating NWP models themselves provide the prior information necessary to infer temperature and humidity information from radiances. After a brief summary of the historical background, the GLOSS processing is described, noting in particular the differences from similar work elsewhere. Results are then presented from NWP impact studies of the assimilation of temperature profiles derived from satellite soundings. An advantage is demonstrated for the GLOSS temperature retrievals relative to the retrievals distributed by NESDIS and produced from the same radiance data. The advantage of GLOSS is clear and consistent in the extratropical regions of the northern hemisphere and, especially, of the southern hemisphere. In the tropics the results are more mixed. The paper concludes with a note of additional work required before operational implementation of GLOSS, and with an outline of other expected future developments.

The linearized two-dimensional equations of motion on an f plane arc solved for a localized perturbation using Local polar coordinates, and a general solution involving Bessel functions is found which can be fitted to any initial conditions. Three types of perturbation are used as examples; they show the formation of an external gravity wave which propagates outward leaving a geostrophically balanced residual flow as r + m. The divergent component of the initial wind field has no cffcct on this residual flow. Experiments in which the same perturbations were applied to all levels of a numerical model showed good agreement with the analytical solutions.Using an isentropic temperature profile as an example, the theory is extended to three-dimensional perturbations. As well as the gravity waves and geostrophic residual flow found in two dimensions, a third component of the solution is found, namely, an inertial oscillation originating in the deviation of the wind perturbation from its vertical mean. This oscillation continues indefinitely with the Coriolia frequency. Numerical model experiments show good agreement with the solutions in the early stages of adjustment though the inertial oscillation slowly decays through internal gravity wave generation. Finally, the analytical solutions are discussed in the light of the results of research on the design and performance of analysis schemes. No. 41, Met. 0. 979 Geostrophic adjustment. Rev. Geophys. Space Phys., 10,485-528 An investigation of the free oscillations of a simple current system. J . Meteorol., 2, 113-119 On the optimal specification of the initial state for deterministic forecasting. Mon. Weather Reu., 108, 1719-1735 Four-dimensional data assimilation and the slow manifold. ibid., 108, 85-99 Tables of integral transforms. McGraw-Hill Book Co. The 15-level weather prediction model. MeteoroI. Mag., 114, Some simple solutions for heat-induced tropical circulation. Q. J. R. Meteorol. SOC., 106, 447-462 Experiments in the four-dimensional assimilation of Nimbus 4 SIRS data. 1. Appl. Meteorol., 12, 425-436 Some simple analytical solutions to the problem of forced equatorial long waves. Q. J. R. Meteorol. Soc., 110,203-217 The statistical structure of short-range forecast errors as determined from radiosonde data. Part I: The wind field. Tellus, %A, 111-136 Nonlinear normal mode initialization and quasi-geostrophic theory. J. Atmos. Sci., 37, 958-968 222-226

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