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Hung-Chi Kuo, George T-J. Chen, and Chung-Hsien Lin

Abstract

The merger of tropical cyclones Zeb and Alex is described. The process includes mutual cyclonic rotation, followed by merger of Zeb and Alex. The cyclonic rotation of Alex around Zeb accelerated as the separation distance decreased to 850 km. During the merger process, Alex was quickly elongated and wrapped cyclonically around Zeb to become a spiral band of Zeb. The final merger occurred at a distance of 450 km. The observed merger processes appear to be in good agreement with the potential vorticity theories of vortex interaction and the formation of spiral bands. Despite the presence of moist convections, the straining-out regime of Dritschel and Waugh appears to be applicable to the interaction between Alex and Zeb.

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Shu-Hsien Chou, Robert M. Atlas, Chung-Lin Shie, and Joe Ardizzone

Abstract

Monthly averages of daily latent heat fluxes over the oceans for February and August 1988 are estimated using a stability-dependent bulk scheme. Daily fluxes are computed from daily SSM/I (Special Sensor Microwave/Imager) wind speeds and EOF-retrieved SSM/I surface humidity, National Meteorological Center sea surface temperatures, and the European Centre for Medium-Range Weather Forecasts analyzed 2-m temperatures. Daily surface specific humidity (Q) is estimated from SSM/I precipitable water of total (W) and a 500-m bottom layer (W B) using an EOF (empirical orthogonal function) method. This method has six W-based categories of EOFs (independent of geographical locations) and is developed using 23 177 FGGE IIb humidity soundings over the global oceans. For 1200 FGGE IIb humidity soundings, the accuracy of EOF-retrieved Q is 0.75 g kg−1 for the case without errors in W and W B, and increases to 1.16 g kg−1 for the case with errors in W and W B. Compared to 342 collocated radiosonde observations, the EOF-retrieved SSM/I Q has an accuracy of 1.7 g kg−1. The method improves upon the humidity retrieval of Liu and is competitive with that of Schulz et al.

The SSM/I surface humidity and latent heat fluxes of these two months agree reasonably well with those of COADS (Comprehensive Ocean–Atmosphere Data Set). Compared to the COADS, the sea–air humidity difference of SSM/I has a positive bias of approximately 1–3 g kg−1 (an overestimation of flux) over the wintertime trade wind belts and wintertime extratropical oceans. In the summertime extratropical Pacific and summertime eastern equatorial Pacific Ocean, it has a negative bias of about 1–2 g kg−1 (an underestimation of flux). The results further suggest that the two monthly flux estimates, computed from daily and monthly mean data, do not differ significantly over the oceans.

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Shu-Hsien Chou, Eric Nelkin, Joe Ardizzone, Robert M. Atlas, and Chung-Lin Shie

Abstract

Information on the turbulent fluxes of momentum, latent heat, and sensible heat at the air–sea interface is essential in improving model simulations of climate variations and in climate studies. A 13.5-yr (July 1987–December 2000) dataset of daily surface turbulent fluxes over global oceans has been derived from the Special Sensor Microwave Imager (SSM/I) radiance measurements. This dataset, Goddard Satellite-based Surface Turbulent Fluxes, version 2 (GSSTF2), has a spatial resolution of 1° × 1° latitude–longitude and a temporal resolution of 1 day. Turbulent fluxes are derived from the SSM/I surface winds and surface air humidity, as well as the 2-m air and sea surface temperatures (SST) of the NCEP–NCAR reanalysis, using a bulk aerodynamic algorithm based on the surface layer similarity theory.

The GSSTF2 bulk flux model is validated by comparing hourly turbulent fluxes computed from ship data using the model with those observed fluxes of 10 field experiments over the tropical and midlatitude oceans during 1991–99. In addition, the GSSTF2 daily wind stress, latent heat flux, wind speed, surface air humidity, and SST compare reasonably well with those of the collocated measurements of the field experiments. The global distributions of 1988–2000 annual- and seasonal-mean turbulent fluxes show reasonable patterns related to the atmospheric general circulation and seasonal variations. Zonal averages of latent heat fluxes and input parameters over global oceans during 1992–93 have been compared among several flux datasets: GSSTF1 (version 1), GSSTF2, the Hamburg Ocean–Atmosphere Parameters and Fluxes from Satellite Data (HOAPS), NCEP–NCAR reanalysis, and one based on the Comprehensive Ocean–Atmosphere Data Set (COADS). Significant differences are found among the five. These analyses suggest that the GSSTF2 latent heat flux, surface air humidity, and winds are likely to be more realistic than the other four flux datasets examined, although those of GSSTF2 are still subject to regional biases. The GSSTF2 is useful for climate studies and has been submitted to the sea surface turbulent flux project (SEAFLUX) for intercomparison studies.

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