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C. P. R. Saunders

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R. W. Saunders

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For this study, a sampled dataset from the original METEOSAT images was taken over a seven-month period for areas with a variety of different surface types and geographical locations. This was the initial BI dataset requested for the International Satellite Cloud Climatology Project (ISCCP). Cloud amounts were derived from these raw radiances from a scheme which utilized all three METEOSAT channels. For the daylight slots the visible channel was used to discriminate between low cloud and surface. In addition, a “spatial coherence technique” was employed to detect cloud, relying on the assumption that the cloud tops do not have a uniform temperature over small distances (∼20 km). The monthly mean cloudiness and mean diurnal variability for three months (April, July and October 1983) computed from the radiance data are described here. Significant seasonal variations in cloudiness were observed, such as the latitudinal movement of the ITCZ and the enhancement of the southern subtropical jet during July. The diurnal cycle of cloudiness was observed over equatorial Africa, particularly for the high cloud coverage.

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R. W. Saunders
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C. P. R. Saunders
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C. P. R. Saunders

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C. P. R. Saunders

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C. P. R. Saunders

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Recent developments in the area of thunderstorm electrification processes are reviewed. These processes have two main divisions; (a) convective, in which particles charged by ion capture are moved by convection currents to strengthen the electric field in the cloud, and (b) processes involving charge transfer during particle interactions, following which oppositely charged particles move apart in the updraft to form the observed charge centers. Type-b processes are further subdivided into inductive (relying on the preexistence of an electric field) and noninductive charge-transfer mechanisms. Field and laboratory evidence points to the importance of interactions between particles of the ice phase, in the presence of liquid water droplets, in separating electric charge in thunderstorms. Recent experimental studies have investigated the dependence of charge transfer on the size and relative velocity of the interacting particles and have determined the dependence of the sign of the charge transfer on temperature and cloud liquid water content. Field data upon which the laboratory simulations are based are obtained by increasingly sophisticated airborne and ground-based means. Calculations of electric field growth using experimental charge-transfer data in numerical models of the dynamical and microphysical development of thunderstorms show agreement with observations, although further refinement is required. Some directions for future research are outlined.

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M. Tomassini, D. LeMeur, and R. W. Saunders

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During August–September 1995 new near-surface wind datasets over the tropical Atlantic from both the ERS-1 scatterometer and Meteosat satellites were available at the European Centre for Medium-Range Weather Forecasts. At this time there was an unusually high number of hurricanes present in the tropical Atlantic and so the impact of these data on analyzing and forecasting the main cyclones was investigated. Assimilation experiments using a new variational scheme, with the ERS-1 winds, showed clear improvements both in the analyses and short-range forecasts, compared with the optimal interpolation scheme without these data. For example, the forecast positions for Hurricane Iris were reduced by almost 50% when the scatterometer data was included. For Hurricane Luis the improvement was for a higher percentage of cases when the model identified the cyclone in the 24- and 48-h forecasts. For the 72-h forecasts 80% of the reported cyclones were detected compared with only 33% for the analyses without ERS-1 data.

The impact of the Meteosat lower-tropospheric cloud motion winds was found to be small due to lack of coverage in the vicinity of the center of the hurricanes at this time. The impact of one profile from a ship in the vicinity of Hurricane Luis just before its approach to the Caribbean Islands was clearly demonstrated by large improvements to both analyses with and without the scatterometer winds.

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P. M. Saunders and S. R. Thompson

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Results are presented from the integration of a fine-resolution numerical model of the ocean operating in a diagnostic mode. The region covered lies south of 24°S, as depicted in the FRAM Atlas. Here transports, heat, salt, and freshwater fluxes are examined at 60°S and near 30°S in all three oceans. Results are found to be generally realistic.

At midlatitude the meridional heat flux is largely determined by the structure of the mean meridional motions and to a lesser degree by the gyre-scale horizontal motions. These roles are reversed for freshwater fluxes. At a fixed high latitude the freshwater flux is determined by the mean meridional motions and the heat flux principally by the large-scale wandering of the circumpolar current across the latitude. Only in this latter case, namely for the heat flux at 60°S, do mesoscale motions contribute to a significant extent.

The model underestimates the production and export of abyssal water; the climatological state with which it is initialized is identified as the likely cause. A suggestion is offered for assessing the accuracy of diagnostic integrations.

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J. Hallett and C. P. R. Saunders

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Laboratory studies of rime growth on a moving rod under conditions of secondary ice crystal production show that the rod acquires a positive charge, equivalent to charge associated with each ejected particle of 5 × 10−4C. Ice crystals produced by seeding also impart a positive charge to the rime, equivalent to a charge per particle of 5 × 10−16C. As the water vapor supply is cut off, the charge sign reverses. The results suggest that the sign of the charge transfer depends on the physical state of the rime surface and its vapor pressure excess or deficit relative to the environment. Charge separation in convective clouds is critically dependent on the changing proportion of graupel and small secondary ice crystals.

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