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LUTZ HASSE and VOLKER WAGNER

Abstract

The ratio between observed surface and geostrophic wind speed has been investigated from observations at the German Bight, taking geostrophic wind and the air-sea temperature difference as parameters. The ratio decreases with increasing geostrophic wind and increasing stability. While stability is an important parameter for light to moderate winds, variation of the ratio with geostrophic wind speed cannot be neglected, taking the full range of geostrophic wind speeds into consideration. From the Navier-Stokes equations, such a variation is to be expected. For light winds, the (local) surface wind may exceed the (mesoscale) geostrophic wind. Both effects together can be described approximately by a linear relation between the surface wind and geostrophic wind, with a slope of 0.56 and a constant term b>0 varying with stability. The residual error was 2 m/s. Variation with latitude is inferred from the Navier-Stokes equations.

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Hans-Jörg Isemer and Lutz Hasse

Abstract

The Beaufort equivalent scale of the World Meteorological Organization (WMO), used for decades to transform marine Beaufort estimates to surface wind speeds over the oceans, contains systematic errors that depend nonlinearly on the wind speed. Applying a revised scientific equivalent scale instead of the WMO scale produces significant changes in statistics of surface wind speed U over the ocean and, consequently, in all air-sea fluxes that are related to U.

For the North Atlantic Ocean these biases are quantified as follows. The WMO scale underestimates climatological monthly means of U significantly: up to 1.6 m s−1in tropical latitudes throughout the year. In subpolar regions, differences are significant from spring through autumn and reach 1.3 m s−1. These regionally and seasonally different monthly biases are equivalent to an overestimate of the annual variation of U, which reaches 1.5 m s−1 in the westerlies. Local standard deviations may be overestimated up to 1.2 m s−1. The WMO scale underestimates climatological monthly estimates of latent heat flux up to 50 W m−2. (up to 25%). The bias of the mean annual North Atlantic evaporation rate is 0.3 m yr−1. The bias in annual net air-sea heat flux amounts to 27 W m−2, equivalent to an underestimate of the transequatorial oceanic heat transport by 1.15 PW (1 PW = 1015 W). Climatological monthly wind stress at the ocean surface is underestimated by more than 4.5 × 10−2 N m−2 (up to 50%) in the trade-wind region.

Most existing regional and global air-sea flux compilations (including COADS) have been derived using the WMO scale. Hence, large biases are included in these compilations, although they can be partially hidden by an artificial increase of parameterization coefficients. The wind statistics revised according to a more accurate scale allow the application of bulk coefficients in accordance with newer experimental results from the open ocean. Therefore means and statistics of wind speed and climatological estimates of air-sea fluxes over the World Ocean need revision.

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Sergey K. Gulev and Lutz Hasse

Abstract

On the basis of the collection of individual marine observations available from the Comprehensive Ocean–Atmosphere Data Set, major parameters of the sea state were evaluated. Climatological fields of wind waves and swell height and period, as well as significant wave height and resultant period are obtained for the North Atlantic Ocean for the period from 1964 to 1993. Validation of the results against instrumental records from National Data Buoy Center buoys and ocean weather station measurements indicate relatively good agreement for wave height and systematic biases in the visually estimated periods that were corrected. Wave age, which is important for wind stress estimates, was evaluated form wave and wind observations. The climatology of wave age indicates younger waves in winter in the North Atlantic midlatitudes and Tropics. Wave age estimates were applied to the calculations of the wind stress using parameterizations from field experiments. Differences between wave-age-based and traditional estimates are not negligible in wintertime in midlatitudes and Tropics where wave-induced stress contributes from 5% to 15% to the total stress estimates. Importance of the obtained effects for ocean circulation and climate variability is discussed.

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Lutz Hasse and Stuart D. Smith

Abstract

Parameterization of turbulent wind stress and sensible and latent heat fluxes is reviewed in the context of climate studies and model calculations, and specific formulas based on local measurements are recommended. Wind speed is of key importance, and in applying experimental results, the differences between local and modeled winds must be considered in terms of their method of observation or calculation. Climatological wind data based on Beaufort wind force reports require correction for historical trends. Integrated long-term net turbulent and radiative heat fluxes at the sea surface, calculated from archived data, are consistent with meridional heat transport through oceanographic sections; this lends support to the methods used.

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Martin Grossklaus, Klaus Uhlig, and Lutz Hasse

Abstract

A new optical disdrometer has been developed that is optimized for use in high wind speeds, for example, on board ships. The minimal detectable size of droplets is 0.35 mm. Each drop is measured separately with regard to its size and residence time within the sensitive volume. From the available information, the drop size distribution can be calculated with a resolution of 0.05 mm in diameter either by evaluation of the residence time of drops or by drop counting knowing the local wind. Experience shows that using the residence time leads to better results. Rain rates can be determined from the droplet spectra by assuming terminal fall velocity of the drops according to their size. Numerical modeling of disdrometer measurements has been performed, allowing the study of the effect of multiple occupancy of the sensitive volume and grazing incidences on disdrometer measurements. Based on these studies an iterative procedure has been developed to eliminate the impact of these effects on the calculated drop size distributions. This technique may also be applied to any other kind of disdrometer. Long-term simultaneous measurements of the disdrometer and a conventional rain gauge have been used to validate this procedure.

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Hans-Jörg Isemer, Jürgen Willebrand, and Lutz Hasse

Abstract

An inverse technique is used to adjust uncertain coefficients and parameters in the bulk formulae of climatological air-sea energy fluxes in order to obtain an agreement of indirect estimates of meridional heat transport with direct estimates in the North Atlantic Ocean. Three oceanographic estimates of ocean heat transport at the equator, at 25°N, and 32°N are compatible with meteorological evidence provided that the uncertainties of both direct and indirect estimates are taken into account. The transport coefficient CE for estimation of the latent heat flux is the major contributor to the overall uncertainty in estimates of ocean heat transport. The constraint of 1 PW northward transport across 25°N leads to a set of parameterizations for which the parameter adjustments are only less than half as large as the estimated uncertainties. Based on this set of constrained parameterizations monthly climatological fields of the individual fluxes in the North Atlantic Ocean are computed which are consistent with direct transport estimates.

With a larger set of heat transport observations this method will provide a possibility to discriminate between various bulk formulations, and to obtain more accurate estimates of the air-sea energy flux.

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