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B. Curry, C. M. Lee, and B. Petrie

Abstract

Davis Strait volume [−2.3 ± 0.7 Sv (1 Sv ≡ 106 m3 s−1); negative sign indicates southward transport], freshwater (−116 ± 41 mSv), and heat (20 ± 9 TW) fluxes estimated from objectively mapped 2004–05 moored array data do not differ significantly from values based on a 1987–90 array but are distributed differently across the strait. The 2004–05 array provided the first year-long measurements in the upper 100 m and over the shelves. The upper 100 m accounts for 39% (−0.9 Sv) of the net volume and 59% (−69 mSv) of the net freshwater fluxes. Shelf contributions are small: 0.4 Sv (volume), 15 mSv (freshwater), and 3 TW (heat) from the West Greenland shelf and −0.1 Sv, −7 mSv, and 1 TW from the Baffin Island shelf. Contemporaneous measurements of the Baffin Bay inflows and outflows indicate that volume and freshwater budgets balance to within 26% and 4%, respectively, of the net Davis Strait outflow. Davis Strait volume and freshwater fluxes nearly equal those from Fram Strait, indicating that both are significant Arctic freshwater pathways.

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Craig M. Lee and Charles C. Eriksen

Abstract

The upper-ocean to forcing by pressure gradients and wind stress is examined using observations from the Frontal Air–Sea Interaction Experiment. A moored way acquired time series of winds. upper-ocean currents, temperatures, and salinities between winter and late spring in a region of the Sargasso Sea known for the presence of upper-ocean fronts. These fronts have timescales of 10 days and dominate current variance, while winds varied with the 4-day timescale of passing weather systems. Employing a frequency domain regression model. it is found that gestrophy accounts for most of the low-frequency (>100 h)current variance in the seasonal pycnoclino, but wind-forced shear becomes important nearer the surface. In particular, currents oriented in the typical NE–SW alonfront direction display geostrophic balance, while those perpendicular to them do not.

Wind forcing can produce geostrophic currents indirectly through Ekman pumping. and knowledge of the geostrophic shear is required to distinguish between this and currents driven directly by the wind through turbulent shear stress. Previous investigations rely on the assumption that no wind-driven stress penetrates below the mixed layer to remove the wind-coherent geostrophic flow. Baroclinic pressure gradients are calculated using estimates of density across the moored array. A linear regression model uses pressure gradients record to explicitly remove the geostrophic shear and isolate the directly wind-driven acceleration at timescales longer than 10 days. The resulting response satisfies the Ekman transport relation, penetrates well into the stratified fluid spirals to the right, and decays with depth.

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Alan M. Davies, Simon C. M. Kwong, and Jong Chan Lee

Abstract

A three-dimensional hydrodynamic model of the Faeroe–Shetland Channel and northern North Sea is used to investigate the spatial variability of M 2 tidal elevations and currents in the region. This area is chosen because it covers a range of water depths. Also, there is a significant database of tidal elevations (namely 41 gauges) and current meters (namely 89 observations) with which comparisons can be made. With the exception of a couple of measurements made at the shelf edge, which may be influenced by the internal tide, namely a 180° phase shift across the thermocline due to a first mode internal tide, the observations correspond to those of a barotropic tide. Two different approaches are used to represent the profile of tidal currents in the vertical. In the first a spectral/functional method is used, while in the second a finite difference grid is applied. A range of parameterizations of vertical eddy viscosity (suitable for deep water regions) are used, from ones in which viscosity is related to the flow field and water depth, to the flow field only, with a final calculation involving a Prandtl mixing length formulation. Calculations with the flow and depth dependent viscosity model show that in deep water, this parameterization leads to an artificially high viscosity and hence to a boundary layer thickness that is too large. Both the Prandtl mixing length model and the one in which viscosity is related to only the flow field give low viscosity in deep water, with tidal current profiles showing a high sheared bottom boundary layer with little shear above this. In shallow water comparable viscosity values and current profiles are computed with all the various parameterizations of eddy viscosity.

On average the mean error in tidal elevation amplitude was 3.6 cm, with a phase error of −8 deg, although there was a bias to underpredict tidal elevations. For tidal currents in general there was a slight bias to overpredict currents, with the magnitude of the semimajor axis being reproduced on average with an rms error of 2.3 cm s−1. A calculation in which the open boundary input was adjusted to give a mean elevation amplitude error of zero, with a phase error of −1.9°E, and no significant bias in the elevations, did however show a bias to underpredict tidal currents, with an rms error of 3.6 cm s−1 in the semimajor axis.

Model calculations showed that the most sensitive test of the model's accuracy was a detailed comparison of tidal current profiles, in the near-bed region. Also the accuracy of computed tidal currents in deep water was determined more by the uncertainty in the boundary forcing to the model than the exact form of the eddy viscosity parameterization.

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C. M. Shun, S. Y. Lau, and O. S. M. Lee

Abstract

A Terminal Doppler Weather Radar (TDWR) started operation in Hong Kong, China, in 1997 for monitoring wind shear associated with thunderstorms affecting the Hong Kong International Airport. The airport was built on land reclaimed from the sea and lies to the immediate north of the mountainous Lantau Island, which has hills rising to nearly 1000 m. Since 1997, the airport experienced a number of tropical cyclone passages, some bringing strong southerly winds across these hills. Under these conditions the TDWR captured interesting but complex flow patterns in the lower atmosphere. The TDWR Doppler velocity datasets reveal features not previously observed with conventional instruments. These include shear lines, reverse flow, small-scale vortices, streaks of low-speed flow set against a high-speed background, as well as gap-related downslope high-speed flow. Hovmöller diagrams constructed from the Doppler velocity data bring out in considerable detail periodic shedding of vortices and transient wind patterns in the wake of the hills.

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Noel A. Pelland, Charles C. Eriksen, and Craig M. Lee

Abstract

In the California Current System, subthermocline, lenslike anticyclonic eddies generated within the California Undercurrent (CU) are one mechanism for lateral transport of the warm, saline waters of the CU. Garfield et al. established the name “Cuddies” for eddies of this type and hypothesized that they account for a significant fraction of the offshore transport of CU water. This study presents observations of subthermocline eddies collected from a time series of Seaglider surveys in the northern California Current System. Gliders made 46 crossings of subthermocline anticyclones and 17 crossings of subthermocline cyclones over 5.5 yr. Close inspection grouped these into 20 distinct anticyclones and 10 distinct cyclones. Water properties at the core of anticyclonic eddies were similar to those in the core of the CU over the continental slope; these anticyclones are examples of Cuddies. Anticyclonic (cyclonic) eddies had average radii of 20.4 (20.6) km, peak azimuthal current speeds of 0.25 (0.23) m s−1, and average core anomalies of potential vorticity 65% below (125% above) ambient values. Anticyclones contained an order of magnitude greater available heat and salt anomaly relative to background conditions than cyclones on average. Circumstantial evidence of eddy decay through lateral intrusions was found although this was not observed consistently. Observed eddy properties and the geometry of flow over the continental slope were consistent with eddy formation due to frictional torque acting on the CU. Loss of heat and salt from the CU due to subthermocline eddies is estimated to account for 44% of the freshening and cooling of the CU as it flows poleward.

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D. N. Fox, W. J. Teague, C. N. Barron, M. R. Carnes, and C. M. Lee

Abstract

The Modular Ocean Data Assimilation System (MODAS) is used by the U.S. Navy for depiction of three-dimensional fields of temperature and salinity over the global ocean. MODAS includes both a static climatology and a dynamic climatology. While the static climatology represents the historical averages, the dynamic climatology assimilates near-real-time observations of sea surface height and sea surface temperature and provides improved temperature and salinity fields. The methodology for the construction of the MODAS climatology is described here. MODAS is compared with Levitus and Generalized Digital Environmental Model climatologies and with temperature and salinity profiles measured by SeaSoar in the Japan/East Sea to illustrate MODAS capabilities. MODAS with assimilated remotely sensed data is able to portray time-varying dynamical features that cannot be represented by static climatologies.

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E. Kunze, J. M. Klymak, R.-C. Lien, R. Ferrari, C. M. Lee, M. A. Sundermeyer, and L. Goodman

Abstract

Submesoscale stirring contributes to the cascade of tracer variance from large to small scales. Multiple nested surveys in the summer Sargasso Sea with tow-yo and autonomous platforms captured submesoscale water-mass variability in the seasonal pycnocline at 20–60-m depths. To filter out internal waves that dominate dynamic signals on these scales, spectra for salinity anomalies on isopycnals were formed. Salinity-gradient spectra are approximately flat with slopes of −0.2 ± 0.2 over horizontal wavelengths of 0.03–10 km. While the two to three realizations presented here might be biased, more representative measurements in the literature are consistent with a nearly flat submesoscale passive tracer gradient spectrum for horizontal wavelengths in excess of 1 km. A review of mechanisms that could be responsible for a flat passive tracer gradient spectrum rules out (i) quasigeostrophic eddy stirring, (ii) atmospheric forcing through a relict submesoscale winter mixed layer structure or nocturnal mixed layer deepening, (iii) a downscale vortical-mode cascade, and (iv) horizontal diffusion because of shear dispersion of diapycnal mixing. Internal-wave horizontal strain appears to be able to explain horizontal wavenumbers of 0.1–7 cycles per kilometer (cpkm) but not the highest resolved wavenumbers (7–30 cpkm). Submesoscale subduction cannot be ruled out at these depths, though previous observations observe a flat spectrum well below subduction depths, so this seems unlikely. Primitive equation numerical modeling suggests that nonquasigeostrophic subinertial horizontal stirring can produce a flat spectrum. The last need not be limited to mode-one interior or surface Rossby wavenumbers of quasigeostrophic theory but may have a broaderband spectrum extending to smaller horizontal scales associated with frontogenesis and frontal instabilities as well as internal waves.

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Richard C. Y. Li, Wen Zhou, C. M. Shun, and Tsz Cheung Lee

Abstract

This study investigates changes in the destructiveness of landfalling tropical cyclones (TCs) over China during 1975–2014. Using four different TC datasets, it is found that TCs making landfall over east China (TCEC) have tended to be more destructive in recent decades, with a significant increase in the power dissipation index (PDI) after landfall. Both time series analysis and diagnostic analysis reveal that such an increase in the PDI of TCEC is associated with concomitant enhancement in landfall frequency as well as landfall intensity over east China. In contrast, changes in the PDI of TCs making landfall over south China (TCSC) are less apparent. Examination of different TC-related parameters shows no obvious changes in terms of landfall frequency, duration, and maximum intensity of TCSC. Diagnostic analysis further suggests that the reduction in TC occurrence over south China offsets considerably the positive effects of the intensity and the nonlinear term.

Further examination of the environmental parameters reveals significant changes in the large-scale steering flow in recent decades, which is characterized by a prominent cyclonic circulation centered over southeast China. The southeasterly flows on the eastern flank of the cyclonic circulation tend to favor subsequent landfall of TCs over east China, resulting in an increase in landfall frequency, which contributes in part to the enhanced PDI of TCs over this region. Meanwhile, the slowing down of the mean translation speed of TCEC and the weakening of vertical wind shear coupled with warmer SSTs in the WNP tend to favor the intensification of TCEC, leading to an increase in intensity and hence the PDI of TCs over east China.

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Daniel C. Hartung, Justin M. Sieglaff, Lee M. Cronce, and Wayne F. Feltz

Abstract

The University of Wisconsin Convective Initiation (UWCI) algorithm utilizes geostationary IR satellite data to compute cloud-top cooling (UW-CTC) rates and assign CI nowcasts to vertically growing clouds. This study is motivated by National Weather Service (NWS) forecaster reviews of the algorithm output, which hypothesized that more intense cloud-top cooling corresponds to more vigorous short-term (0–60 min) convective development. An objective validation of UW-CTC rates using a satellite-based object-tracking methodology is presented, along with a prognostic evaluation of such cloud-top cooling rates for use in forecasting the growth and development of deep convection. In general, both a cloud object’s instantaneous and maximum cooling rate(s) are shown to be useful prognostic tools in predicting future radar intensification. UW-CTC rates are shown to be most skillful in detecting convective clouds that achieved intense radar signatures. The UW-CTC rate lead time ahead of the various radar fields is also shown, along with an illustration of the benefit of UW-CTC rates in operational forecasting. The results of this study suggest that convective clouds with the strongest UW-CTC rates are more likely to achieve significant near-term (0–60 min) radar signatures in such fields as composite reflectivity, vertically integrated liquid (VIL), and maximum estimated size of hail (MESH) compared to clouds that exhibit only weak UW-CTC rates.

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Justin M. Sieglaff, Daniel C. Hartung, Wayne F. Feltz, Lee M. Cronce, and Valliappa Lakshmanan

Abstract

Studying deep convective clouds requires the use of available observation platforms with high temporal and spatial resolution, as well as other non–remote sensing meteorological data (i.e., numerical weather prediction model output, conventional observations, etc.). Such data are often at different temporal and spatial resolutions, and consequently, there exists the need to fuse these different meteorological datasets into a single framework. This paper introduces a methodology to identify and track convective cloud objects from convective cloud infancy [as few as three Geostationary Operational Environmental Satellite (GOES) infrared (IR) pixels] into the mature phase (hundreds of GOES IR pixels) using only geostationary imager IR window observations for the purpose of monitoring the initial growth of convective clouds.

The object tracking system described within builds upon the Warning Decision Support System-Integrated Information (WDSS-II) object tracking capabilities. The system uses an IR-window-based field as input to WDSS-II for cloud object identification and tracking and a Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin (UW-CIMSS)-developed postprocessing algorithm to combine WDSS-II cloud object output. The final output of the system is used to fuse multiple meteorological datasets into a single cloud object framework. The object tracking system performance analysis shows improved object tracking performance with both increased temporal resolution of the geostationary data and increased cloud object size. The system output is demonstrated as an effective means for fusing a variety of meteorological data including raw satellite observations, satellite algorithm output, radar observations, and derived output, numerical weather prediction model output, and lightning detection data for studying the initial growth of deep convective clouds and temporal trends of such data.

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