Downloaded 10/01/21 04:34 PM UTC JANUARY 2015 P O T T E R E T a L
Total Page：16
File Type：pdf, Size：1020Kb
104 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72 In situ Measurements of Momentum Fluxes in Typhoons 1 HENRY POTTER,* HANS C. GRABER,NEIL J. WILLIAMS,CLARENCE O. COLLINS III, # RAFAEL J. RAMOS, AND WILLIAM M. DRENNAN Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida (Manuscript received 7 February 2014, in final form 8 September 2014) ABSTRACT One 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 2 speeds above 26 m s 1 were recorded. Data are presented for 1245 h of direct flux measurements. The drag 2 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 mo- mentum flux at the frequency of the peak waves during strongly forced conditions. 1. Introduction terms of nondimensional bulk transfer coefficients for drag C and enthalpy C . Ooyama (1969) and Emanuel The marine boundary layer is a dynamic region of D K (1986) both indicated the importance of C and C , and Earth in which the ocean constantly interacts with the D K the latter theorized that the intensity of a tropical cy- atmosphere. This interaction facilitates the exchange of clone is proportional to the ratio of these bulk transfer momentum, mass, and heat between these fluids through coefficients, C /C . Comparing observations to results turbulent processes. Tropical cyclones gain their energy K D obtained from a simple axisymmetric model with ide- from this interaction by extracting heat from the un- alized environmental conditions led Emanuel (1995) to derlying ocean through enthalpy fluxes (Riehl 1950) hypothesize that the most likely range of C /C during and lose energy from wind stress on the surface of the K D a tropical cyclone is 1.2–1.5, with a lower bound of 0.75. water (Chen et al. 2007). Consequently, at high wind Few campaigns have set out to directly measure fluxes speeds, turbulent momentum and enthalpy fluxes are in tropical cyclone conditions; thus, the data available to responsible for the genesis, conservation, and dissipa- determine the value of these bulk transfer coefficients is tion of tropical cyclones (Malkus and Riehl 1960; limited. For wind speeds between approximately 5 and Emanuel 1986). The fluxes of momentum and enthalpy 2 20 m s 1, there is general agreement as to the behavior across the air–sea interface are typically represented in of CK and CD. However, at high wind speeds, where measurements are made increasingly difficult because of harsh environmental conditions, data are scarce and * Current affiliation: Remote Sensing Division, Naval Research have a large amount of scatter. Laboratory, Washington, DC. 1 The dedicated campaigns that have investigated en- Current affiliation: Oceanography Division, Naval Research thalpy fluxes [e.g., Humidity Exchange Over the Sea Laboratory, Stennis Space Center, Mississippi. # Current affiliation: Woods Hole Group–Houston, Stafford, (HEXOS; DeCosmo et al. 1996), GasEx (McGillis et al. Texas. 2004), and Surface Wave Dynamics Experiment (SWADE; Katsaros et al. 1993)] have general agreement that CK has 21 Corresponding author address: Henry Potter, Naval Research no dependence on wind speeds between 5 and 20 m s . 21 Laboratory, 4555 Overlook Ave. SW, Washington, DC 20375. This result was extended to 30 m s by the Coupled E-mail: [email protected] Boundary Layer Air–Sea Transfer (CBLAST) field DOI: 10.1175/JAS-D-14-0025.1 Ó 2015 American Meteorological Society Unauthenticated | Downloaded 10/01/21 04:34 PM UTC JANUARY 2015 P O T T E R E T A L . 105 2 program, during which the first direct measurements of about 30 m s 1 wind speeds, but because of uncertainties enthalpy fluxes in the hurricane boundary layer were in the estimate, could not rule out entirely the possibility recorded using measurements from aircraft (Zhang et al. of some continued increase. Wada and Kohno (2012) 2008a). Laboratory experiments reported by Haus et al. also reported a leveling of CD when applying the (2010) and Jeong et al. (2012) extend the findings to roughness length scheme of Taylor and Yelland (2001) 2 40 m s 1. Using a total energy budget approach, to their numerical simulation of Typhoon Fanapi. CBLAST data were also used to deduce CK for wind Evidence is mounting that shows CD plateaus or de- 2 speeds between 52 and 72 m s 1. These results suggest creases at high wind speeds. Even beyond the rolloff that it is probable that the magnitude of CK is not de- limits of Donelan et al. (2004) and Powell et al. (2003), pendent on wind speed in major hurricane conditions CK/CD remains around 0.5, much lower than the 0.75 (Bell et al. 2012). threshold suggested by Emanuel (1995). Zhang et al. 2 For wind speeds between 5 and 20 m s 1, it has been (2008a), combining their results with French et al. shown by multiple campaigns that CD increases linearly (2007), found the mean value of CK/CD 5 0.63. Bell with wind speed [e.g., Adverse Weather Experiment et al. (2012), using a budget analysis, found CK/CD likely (AWE; Drennan and Shay 2006), HEXOS (Smith et al. to be less than 1.0 and perhaps as low as 0.4 for wind 2 1992), SWADE (Donelan et al. 1997), Large and Pond speeds to 72 m s 1. The significant variability of this ratio (1981), Smith (1980)]. Outputs from the Coupled highlights the uncertainty in our understanding of the Ocean–Atmosphere Response Experiment (COARE) drag coefficient at high wind speeds. Much of this un- 3.5 algorithm (Edson et al. 2013) have a roughly linear certainty is due to the lack of direct flux measurements wind speed dependence on CD under neutral conditions made at the air–sea interface during tropical cyclones. 21 for wind speeds up to 25 m s . To ascertain the behavior of CD in tropical cyclones, The few measurements of momentum fluxes at or there remains an imperative need for the direct mea- approaching tropical cyclone force winds indicate that surement of momentum fluxes at the air–sea interface. CD may reach saturation or even decrease at higher One of the objectives of the Impact of Typhoons on wind speeds. Powell et al. (2003) used GPS dropsonde the Ocean in the Pacific (ITOP) campaign was to make profiles of wind speeds extrapolated to the surface to flux measurements in typhoon conditions to understand determine CD. This study was the first to report a level- the behavior of bulk exchange coefficients at high wind ing off, or possible decrease, of CD at wind speeds above speeds. This was achieved using moored surface buoys 2 hurricane force (33 m s 1). Further evidence was put deployed for approximately 3 months during the 2010 forth by Donelan et al. (2004), who, from measurements Pacific typhoon season. The work presented in this pa- made in a wind wave tank, also supported a saturation of per focuses on momentum fluxes collected at the air–sea 21 CD at wind speeds above 33 m s . As part of the interface during the ITOP campaign. In section 2,we CBLAST campaign, French et al. (2007) reported the present the measurement theory. Section 3 is a discus- first open-ocean eddy covariance measurements in sion of the experiment, instruments, and data process- hurricanes. This study determined the drag coefficient ing. Results are shown in section 4, and section 5 is using a Rosemount 858Y probe and a Best Aircraft a discussion. Conclusions are reserved for section 6. Turbulence gust probe mounted on a boom on an air- craft that obtained high-frequency measurements of 2. Theory pressure distribution and temperature. These measure- ments were used to determine three-dimensional wind In stationary and homogeneous conditions, the mo- velocities during a series of stepped descents within the mentum flux t is assumed to be constant within the boundary layer that were extrapolated to the surface. surface flux layer. Using the eddy covariance method, 2 They found that, for wind speeds greater than 22 m s 1 the momentum flux is calculated above the viscous 21 up to 30 m s , CD begins to level off or decrease. sublayer from high-frequency measurements of the However, because of the highly variable nature of the wind velocity components: measurements and the limited number of individual flux estimates, they were not able to provide a definitive t 5 r[(2u0w0)i 1 (2y0w0)j]. (1) description of the behavior of CD at tropical storm–force winds. CBLAST data were also used by Bell et al. Here, r is the air density, i and j are unit vectors along (2012), who deduced the momentum exchange for wind and perpendicular to the mean wind direction; and u, y, 2 speeds between 52 and 72 m s 1 using the conservation and w correspond to horizontal-downwind, horizontal- of azimuthally averaged angular momentum. They es- crosswind, and vertical components of wind velocity, timated that CD does not continue to increase beyond respectively.