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D. W. Wang
,
H. W. Wijesekera
,
E. Jarosz
,
W. J. Teague
, and
W. S. Pegau

Abstract

Breaking surface waves generate layers of bubble clouds as air parcels entrain into the upper ocean through the action of turbulent motions. The turbulent diffusivity in the bubble cloud layer is investigated by combining measurements of surface winds, waves, bubble acoustic backscatter, currents, and hydrography. These measurements were made at water depths of 60–90 m on the shelf of the Gulf of Alaska near Kayak Island during late December 2012, a period when the ocean was experiencing winds and significant wave heights up to 22 m s−1 and 9 m, respectively. Vertical profiles of acoustic backscatter decayed exponentially from the wave surface with e-folding lengths of about 0.6 to 6 m, while the bubble penetration depths were about 3 to 30 m. Both e-folding lengths and bubble depths were highly correlated with surface wind and wave conditions. The turbulent diffusion coefficients, inferred from e-folding length and bubble depth, varied from about 0.01 to 0.4 m2 s−1. Analysis suggests that the turbulent diffusivity in the bubble layer can be parameterized as a function of the cube of the wind friction velocity with a proportionality coefficient that depends weakly on wave age. Furthermore, in the bubble layer, on average, the shear production of the turbulent kinetic energy estimated by the diffusion coefficients is a similar order of magnitude as the dissipation rate predicted by the wall boundary layer theory.

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W. J. Teague
,
E. Jarosz
,
D. W. Wang
, and
D. A. Mitchell

Abstract

Hurricane Ivan passed directly over an array of 14 acoustic Doppler current profilers deployed along the outer continental shelf and upper slope in the northeastern Gulf of Mexico. Currents in excess of 200 cm s−1 were generated during this hurricane. Shelf currents followed Ekman dynamics with overlapping surface and bottom layers during Ivan’s approach and transitioned to a dominant surface boundary layer as the wind stress peaked. Slope currents at the onset of Ivan were wind driven near the surface, but deeper in the water column they were dominated during and after the passage of Ivan by subinertial waves with a period of 2–5 days that had several characteristics of topographic Rossby waves. Currents on the slope at 50 m and greater depths commonly exceeded 50 cm s−1. Surprisingly, the strongest currents were present to the left of the storm track on the shelf while more energetic currents were to the right of the hurricane path on the slope during the forced stage. Near-inertial motion lasting for a time period of about 10 days was excited by the storm on the shelf and slope. Record wave heights were measured near the eyewall of Hurricane Ivan and were shown not to be rogue waves. The large surface waves and strong near-bottom currents caused significant bottom scour on the outer shelf at water depths as deep as 90 m.

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H. W. Wijesekera
,
D. W. Wang
,
W. J. Teague
,
E. Jarosz
,
W. E. Rogers
,
D. B. Fribance
, and
J. N. Moum

Abstract

Several acoustic Doppler current profilers and vertical strings of temperature, conductivity, and pressure sensors, deployed on and around the East Flower Garden Bank (EFGB), were used to examine surface wave effects on high-frequency flows over the bank and to quantify spatial and temporal characteristic of these high-frequency flows. The EFGB, about 5-km wide and 10-km long, is located about 180-km southeast of Galveston, Texas, and consists of steep slopes on southern and eastern sides that rise from water depths over 100 m to within 20 m of the surface. Three-dimensional flows with frequencies ranging from 0.2 to 2 cycles per hour (cph) were observed in the mixed layer when wind speed and Stokes drift at the surface were large. These motions were stronger over the bank than outside the perimeter. The squared vertical velocity w 2 was strongest near the surface and decayed exponentially with depth, and the e-folding length of w 2 was 2 times larger than that of Stokes drift. The 2-h-averaged w 2 in the mixed layer, scaled by the squared friction velocity, was largest when the turbulent Langmuir number was less than unity and the mixed layer was shallow. It is suggested that Langmuir circulation is responsible for the generation of vertical flows in the mixed layer, and that the increase in kinetic energy is due to an enhancement of Stokes drift by wave focusing. The lack of agreement with open-ocean Langmuir scaling arguments is likely due to the enhanced kinetic energy by wave focusing.

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H. W. Wijesekera
,
E. Jarosz
,
W. J. Teague
,
D. W. Wang
,
D. B. Fribance
,
J. N. Moum
, and
S. J. Warner

Abstract

Pressure differences across topography generate a form drag that opposes the flow in the water column, and viscous and pressure forces acting on roughness elements of the topographic surface generate a frictional drag on the bottom. Form drag and bottom roughness lengths were estimated over the East Flower Garden Bank (EFGB) in the Gulf of Mexico by combining an array of bottom pressure measurements and profiles of velocity and turbulent kinetic dissipation rates. The EFGB is a coral bank about 6 km wide and 10 km long located at the shelf edge that rises from 100-m water depth to about 18 m below the sea surface. The average frictional drag coefficient over the entire bank was estimated as 0.006 using roughness lengths that ranged from 0.001 cm for relatively smooth portions of the bank to 1–10 cm for very rough portions over the corals. The measured form drag over the bank showed multiple time-scale variability. Diurnal tides and low-frequency motions with periods ranging from 4 to 17 days generated form drags of about 2000 N m−1 with average drag coefficients ranging between 0.03 and 0.22, which are a factor of 5–35 times larger than the average frictional drag coefficient. Both linear wave and quadratic drag laws have similarities with the observed form drag. The form drag is an important flow retardation mechanism even in the presence of the large frictional drag associated with coral reefs and requires parameterization.

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H. W. Wijesekera
,
D. W. Wang
,
E. Jarosz
,
W. J. Teague
,
W. S. Pegau
, and
J. N. Moum

Abstract

Momentum transport by energy-containing turbulent eddies in the oceanic mixed layer were investigated during high-wind events in the northern Gulf of Alaska off Kayak Island. Sixteen high-wind events with magnitudes ranging from 7 to 22 m s−1 were examined. Winds from the southeast prevailed from one to several days with significant wave heights of 5–9 m and turbulent Langmuir numbers of about 0.2–0.4. Surface buoyancy forcing was much weaker than the wind stress forcing. The water column was well mixed to the bottom depth of about 73 m. Spectral analyses indicate that a major part of the turbulent momentum flux was concentrated on 10–30-min time scales. The ratio of horizontal scale to mixed layer depth was from 2 to 8. Turbulent shear stresses in the mixed layer were horizontally asymmetric. The downwind turbulent stress at 10–20 m below the surface was approximately 40% of the averaged wind stress and was reduced to 5%–10% of the wind stress near the bottom. Turbulent kinetic energy in the crosswind direction was 30% larger than in the downwind direction and an order of magnitude larger than the vertical component. The averaged eddy viscosity between 10- and 30-m depth was ~0.1 m2 s−1, decreased with depth rapidly below 50 m, and was ~5 × 10−3 m2 s−1 at 5 m above the bottom. The divergence of turbulent shear stress accelerated the flow during the early stages of wind events before Coriolis and pressure gradient forces became important.

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J. Wang
,
M. R. Hjelmfelt
,
W. J. Capehart
, and
R. D. Farley

Abstract

Numerical simulations of two snowfall events over the Black Hills of South Dakota are made to demonstrate the use and potential of a coupled atmospheric and land surface model. The Coupled Atmospheric–Hydrologic Model System was used to simulate a moderate topographic snowfall event of 10–11 April 1999 and a blizzard event of 18–23 April 2000. These two cases were chosen to provide a contrast of snowfall amounts, locations, and storm dynamics. The model configuration utilized a nested grid with an outer grid of 16-km spacing driven by numerical forecast model data and an inner grid of 4 km centered over the Black Hills region. Simulations for the first case were made with the atmospheric model, the Advanced Regional Prediction System (ARPS) alone, and with ARPS coupled with the National Center for Atmospheric Research Land Surface Model (LSM). Results indicated that the main features of the precipitation pattern were captured by ARPS alone. However, precipitation amounts were greatly overpredicted. ARPS coupled with LSM produced a very similar precipitation pattern, but with precipitation amounts much closer to those observed. The coupled model also permits simulation of the resulting snow cover and snowmelt. Simulated percentage snow melting occurred somewhat more rapidly than that of the observed. Snow–rain discrimination may be taken from the precipitation type falling out of the atmospheric model based on the microphysical parameterization, or by the use of a surface temperature criteria, as used in most large-scale models. The resulting snow accumulation patterns and amounts were nearly identical. The coupled model configuration was used to simulate the second case. In this case the simulated precipitation and snow depth maximum over the eastern Black Hills were biased to the east and north by about 24 km. The resulting spatial correlation of the simulated snowfall and observations was only 0.37. If this bias is removed, the shifted pattern over the Black Hills region has a correlation of 0.68. Snow-melting patterns for 21 and 22 April appeared reasonable, given the spatial bias in the snowfall simulation.

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D. Y. Wang
,
W. E. Ward
,
G. G. Shepherd
, and
Dongs-Liang Wu

Abstract

A climatology of stationary planetary waves (SPWs) in horizontal winds at latitudes 70°S–70°N and altitudes 90–120 km is obtained from Wind-Imaging Interferometer (WINDII) green line measurements in December–January and March–April of 1991–96. The observed solstitial SPW fields are relatively stronger and dominated by zonal wavenumber-1 variations. In contrast, the equinoctial SPW fields are weaker and characterized by zonal wavenumber-2 variations. The zonal amplitude maxima of 10–25 m s−1 are generally centered at the midlatitudes of 35°–40° in both hemispheres around 96 km, with the eastward perturbation velocity maxima around 90°E for wavenumber 1 and 60° and 240°E for wavenumber 2. The meridional amplitude maxima are about 5–15 m s−1 and show more variabilities in their latitude–height distributions. The meridional phases indicated that Eliassen–Palm (EP) fluxes were downward–poleward for the winter maxima, vertically varying poleward for the summer maxima, and more variable during March–April. The hemispheric–seasonal–interannual variations in amplitude and phase are of 10 m s−1 and 30°, respectively. In particular, a distinguishable local summer maximum with an amplitude of 10–20 m s−1 is found to exist in the wavenumber-1 variation of zonal wind component. The hemispheric asymmetry is also characterized by the nodal phase (or phase jump) lines shifted toward the winter hemisphere by 10°–30°. Wave penetrations across the equator are observed with amplitudes of 5 m s−1 at 97–100 km. While the summer maximum of the wavenumber-1 component persisted during the four years, large variability is found in the winter hemisphere where the wavenumber-2 component became significant at the 90–105-km region during December 1992–January 1993 and December 1993–January 1994 and at the 105–120-km region during December 1991–January 1992. The excitation due to in situ forcing of azonal gravity wave drag, which varies longitudinally, is thought to be largely responsible for the observed SPW, particularly for the summer maximum, while the leakage of upward propagating SPW from the lower to the higher atmosphere also plays a role, especially in the winter and the equinoctial periods.

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Jayarathna W. N. D. Sandaruwan
,
Wen Zhou
,
Paxson K. Y. Cheung
,
Yan Du
, and
Xuan Wang

Abstract

Marine heatwaves (MHWs) are extreme climatic events that can have a significant impact on marine ecosystems and their services across the world. We examine the spatiotemporal variation of summer MHWs in the north Indian Ocean (NIO) and find that the whole NIO Basin exhibits a pronounced spatial variability as well as a significant increasing trend in MHW frequency. We show that the NIO has two leading MHW modes linked to two distinct sea surface temperature (SST) patterns during summer. The first MHW mode is associated with basinwide warming, which is preconditioned by a decaying El Niño–Southern Oscillation (ENSO) and sustained throughout the summer by anomalous northeasterlies extending from the anticyclonic circulation of the western North Pacific subtropical high (WNPSH). The combined effect of thermocline warming due to downwelling oceanic planetary waves, decreased wind-induced evaporative cooling, and enhanced insolation cause basinwide summer MHWs. The second MHW mode exhibits a zonal dipole pattern, which has unfavorable cooling conditions in the previous seasons. The second MHW mode is associated with a phase change of ENSO and is greatly influenced by the formation of an interhemispheric pressure difference (IHPD) due to strengthening of the Australian high (AH) and weakening of the WNPSH. The IHPD induces cross-equatorial southerly winds across the eastern Indian Ocean. These winds favor the transformation of basinwide cooling conditions into zonal SST patterns via wind–evaporation–SST and thermocline–SST feedback, causing MHWs with a zonal dipole pattern. These MHW modes have a significant influence on the distribution and intensity of summer precipitation in the NIO.

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J. R. Wang
,
J. D. Spinhirne
,
P. Racette
,
L. A. Chang
, and
W. Hart

Abstract

Simultaneous measurements with the millimeter-wave imaging radiometer (MIR), cloud lidar system (CLS), and the MODIS airborne simulator (MAS) were made aboard the NASA ER-2 aircraft over the western Pacific Ocean on 17–18 January 1993. These measurements were used to study the effects of clouds on water vapor profile retrievals based on millimeter-wave radiometer measurements. The CLS backscatter measurements (at 0.532 and 1.064 μm) provided information on the heights and a detailed structure of cloud layers; the types of clouds could be positively identified. All 12 MAS channels (0.6–13 μm) essentially respond to all types of clouds, while the six MIR channels (89–220 GHz) show little sensitivity to cirrus clouds. The radiances from the 12-μm and 0.875-μm channels of the MAS and the 89-GHz channel of the MIR were used to gauge the performance of the retrieval of water vapor profiles from the MIR observations under cloudy conditions. It was found that, for cirrus and absorptive (liquid) clouds, better than 80% of the retrieval was convergent when one of the three criteria was satisfied; that is, the radiance at 0.875 μm is less than 100 W cm−3 sr−1, or the brightness at 12 μm is greater than 260 K, or brightness at 89 GHz is less than 270 K (equivalent to cloud liquid water of less than 0.04 g cm−2). The range of these radiances for convergent retrieval increases markedly when the condition for convergent retrieval was somewhat relaxed. The algorithm of water vapor profiling from the MIR measurements could not perform adequately over the areas of storm-related clouds that scatter radiation at millimeter wavelengths.

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D. Kim
,
K. Sperber
,
W. Stern
,
D. Waliser
,
I.-S. Kang
,
E. Maloney
,
W. Wang
,
K. Weickmann
,
J. Benedict
,
M. Khairoutdinov
,
M.-I. Lee
,
R. Neale
,
M. Suarez
,
K. Thayer-Calder
, and
G. Zhang

Abstract

The ability of eight climate models to simulate the Madden–Julian oscillation (MJO) is examined using diagnostics developed by the U.S. Climate Variability and Predictability (CLIVAR) MJO Working Group. Although the MJO signal has been extracted throughout the annual cycle, this study focuses on the boreal winter (November–April) behavior. Initially, maps of the mean state and variance and equatorial space–time spectra of 850-hPa zonal wind and precipitation are compared with observations. Models best represent the intraseasonal space–time spectral peak in the zonal wind compared to that of precipitation. Using the phase–space representation of the multivariate principal components (PCs), the life cycle properties of the simulated MJOs are extracted, including the ability to represent how the MJO evolves from a given subphase and the associated decay time scales. On average, the MJO decay (e-folding) time scale for all models is shorter (∼20–29 days) than observations (∼31 days). All models are able to produce a leading pair of multivariate principal components that represents eastward propagation of intraseasonal wind and precipitation anomalies, although the fraction of the variance is smaller than observed for all models. In some cases, the dominant time scale of these PCs is outside of the 30–80-day band.

Several key variables associated with the model’s MJO are investigated, including the surface latent heat flux, boundary layer (925 hPa) moisture convergence, and the vertical structure of moisture. Low-level moisture convergence ahead (east) of convection is associated with eastward propagation in most of the models. A few models are also able to simulate the gradual moistening of the lower troposphere that precedes observed MJO convection, as well as the observed geographical difference in the vertical structure of moisture associated with the MJO. The dependence of rainfall on lower tropospheric relative humidity and the fraction of rainfall that is stratiform are also discussed, including implications these diagnostics have for MJO simulation. Based on having the most realistic intraseasonal multivariate empirical orthogonal functions, principal component power spectra, equatorial eastward propagating outgoing longwave radiation (OLR), latent heat flux, low-level moisture convergence signals, and vertical structure of moisture over the Eastern Hemisphere, the superparameterized Community Atmosphere Model (SPCAM) and the ECHAM4/Ocean Isopycnal Model (OPYC) show the best skill at representing the MJO.

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