Henry Potter

In situ Measurements of Momentum Fluxes in Typhoons

AbstractOne of the scientific objectives of the U.S. Office of Naval Research–sponsored Impact of Typhoons on the Ocean in the Pacific (ITOP) campaign was improved understanding of air–sea fluxes at high wind speeds. Here the authors present the first-ever direct measurements of momentum fluxes recorded in typhoons near the surface. Data were collected from a moored buoy over 3 months during the 2010 Pacific typhoon season. During this period, three typhoons and a tropical storm were encountered. Maximum 30-min sustained wind speeds above 26 m s−1 were recorded. Data are presented for 1245 h of direct flux measurements. The drag coefficient shows evidence of a rolloff at wind speeds greater than 22 m s−1, which occurred during the passage of a single typhoon. This result is in agreement with other studies but occurs at a lower wind speed than previously measured. The authors conclude that this rolloff was caused by a reduction in the turbulent momentum flux at the frequency of the peak waves during strong...

EASI: An Air–Sea Interaction Buoy for High Winds

AbstractThis paper describes the new Extreme Air–Sea Interaction (EASI) buoy designed to measure direct air–sea fluxes, as well as mean properties of the lower atmosphere, upper ocean, and surface waves in high wind and wave conditions. The design of the buoy and its associated deep-water mooring are discussed. The performance of EASI during its 2010 deployment off Taiwan, where three typhoons were encountered, is summarized.

Upper ocean cooling and air‐sea fluxes under typhoons: A case study

Direct observations of ocean temperatures and air-sea energy exchange underneath three typhoons and a tropical storm encountered in the Philippine Sea during the 2010 Pacific typhoon season are examined. Data are reported from two buoys 180 km apart with ocean temperatures recorded to 150 m and wind speeds up to 26 m s−1. A detailed examination of the cold wakes is used to determine the mechanisms though which the ocean cools. The result show that net cooling varied between storms by two orders of magnitude, accounting for between 9 and 1000 MJ m−2 of heat loss, and were a result of entrainment, advection, and surface fluxes. In some cases a marked temperature increase below the mixed layer occurred due to entrainment of warm water across the thermocline. Mixed layer temperature decreases ranged from 0.35 to 1.6°C and found to be well predicted by typhoon translation speed and wind speed. Of the mixed layer heat loss, 12–47% was attributed to enthalpy fluxes, the upper range of which is much greater than previous reports. Results are discussed in terms of their relevance to tropical cyclone and climate modeling.

Whitecap lifetime stages from infrared imagery with implications for microwave radiometric measurements of whitecap fraction

Quantifying active and residual whitecap fractions separately can improve parameterizations of air-sea fluxes associated with breaking waves. We use data from a multi-instrumental field campaign on FLoating Instrument Platform (FLIP) to simultaneously capture the signatures of active and residual whitecaps at visible, infrared (IR) and microwave wavelengths using, respectively, video camera, mid-IR camera, and a radiometer at 10 GHz. We present results from processing and analyzing IR images and correlating this information with radiometric time series of brightness temperature at horizontal and vertical polarizations TBH and TBV. The results provide evidence that breaking crests and decaying foam appear in mid-IR as bright and dark pixels clearly distinguishing active from residual whitecaps. We quantify the durations of whitecap lifetime stages from the IR images and identify their corresponding signatures in TB time series. Results show that TBH and TBV vary in phase during the active and in anti-phase during the residual whitecap stages. A methodology to distinguish active and residual whitecaps in radiometric time series without a priori IR information has been developed and verified with corresponding IR and video images. The method uses the degree of polarization P (the ratio between the sum and difference of TBV and TBH) to capture whitecaps as prominent spikes. The maximum and zero-crossing of the first derivative of P serve to identify the presence of active whitecaps, while the minimum of dP marks the transition from active to residual whitecap stage. The findings have implications for radiometric measurements of active and total whitecap fractions. This article is protected by copyright. All rights reserved.