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Frank Richards and Phil Arkin

Abstract

The effect of averaging over various spatial scales (0.5–2.5° latitude) and times (1–24 h) on the relationship between the mean fraction of the averaging area covered by clouds colder than various IR equivalent blackbody temperature thresholds and the precipitation over that area is examined. While a linear relationship between fractional coverage and rainfall amount shows considerable scatter at the smallest scale, there is much better correspondence at the larger scales, with linear correlation coefficients often exceeding 0.8. Large-scale rainfall estimates based on linear regression coefficients detect the timing and magnitudes of major rainfall events during GATE. For scales on the order of 2–3° of latitude, estimates based on a linear model are comparable to those found by Stout et al. (1979) and Griffith et al. (1980) for the GATE area. This simple model appears to be limited to scales considerably larger than the convective scale. Averaging over these scales minimizes the effects of the spatial and temporal details of the convective fields. The linear model can be interpreted as the application of an effective mean rainfall rate to the entire precipitating cloudy area. While such an approach does not provide detailed resolution of the field of precipitation, estimation procedures based on linear models may be useful for large-scale budget studies and certain hydrologic and agricultural applications.

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Saulo M. Soares, Kelvin J. Richards, Frank O. Bryan, and Kunio Yoneyama

Abstract

Scale interactions in the coupled ocean and atmosphere of the tropics play a crucial role in shaping the climate state and its spatial and temporal variability. The mechanisms driving the seasonal cycles of mixed layer (ML) temperature and salinity in the tropical south Indian Ocean (TSIO) are revisited and quantified using model and observations to form a basis on which to assess the cycle’s impact on shorter and longer time scale variability in the region. Budgets of ML heat for the western, central, and eastern TSIO in both model and observations indicate that seasonality in ML temperature is driven by surface heat fluxes in all regions; ocean processes, however, are essential to explain east–west differences in the cycle. In contrast, the salt budgets show that ML salinity in the west and central regions of the TSIO is driven by horizontal advection, with salinity increasing during austral winter mainly due to meridional advection, and freshening during spring–summer due to zonal advection; in the east, no single mechanism appears to dominate ML salinity seasonality. The ML seasonal cycle across the entire region is very much influenced by the basin-scale adjustment that occurs in response to monsoon winds in the eastern side of the basin. Zonal advection, as part of the adjustment process, is the key mechanism responsible for bringing fresher/colder waters from the east to the central and western TSIO during austral spring, leading to a lag in the coldest ML temperatures in the east relative to the west/central TSIO, and effectively coupling the eastern and western TSIO beyond simply Rossby wave dynamics.

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