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Weifeng G. Zhang and Glen G. Gawarkiewicz

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Through combining analytical arguments and numerical models, this study investigates the finite-amplitude meanders of shelfbreak fronts characterized by sloping isopycnals outcropping at both the surface and the shelfbreak bottom. The objective is to provide a formula for the meander length scale that can explain observed frontal length scale variability and also be verified with observations. Considering the frontal instability to be a mixture of barotropic and baroclinic instability, the derived along-shelf meander length scale formula is [b 1/(1 + a 1 S 1/2)]NH/f, where N is the buoyancy frequency; H is the depth of the front; f is the Coriolis parameter; S is the Burger number measuring the ratio of energy conversion associated with barotropic and baroclinic instability; and a 1 and b 1 are empirical constants. Initial growth rate of the frontal instability is formulated as [b 2(1 + a 1 S 1/2)/(1 + a 2 α S 1/2)]NH/L, where α is the bottom slope at the foot of the front, and a 2 and b 2 are empirical constants. The formulas are verified using numerical sensitivity simulations, and fitting of the simulated and formulated results gives a 1 = 2.69, b 1 = 14.65, a 2 = 5.1 × 103, and b 2 = 6.2 × 10−2. The numerical simulations also show development of fast-growing frontal symmetric instability when the minimum initial potential vorticity is negative. Although frontal symmetric instability leads to faster development of barotropic and baroclinic instability at later times, it does not significantly influence the meander length scale. The derived meander length scale provides a framework for future studies of the influences of external forces on shelfbreak frontal circulation and cross-frontal exchange.

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Yuying Zhang and Gerald G. Mace

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Algorithms are developed to convert data streams from multiple airborne and spaceborne remote sensors into layer-averaged cirrus bulk microphysical properties. Radiometers such as the Moderate-Resolution Imaging Spectroradiometer (MODIS) observe narrowband spectral radiances, and active remote sensors such as the lidar on the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite and the millimeter radar on CloudSat will provide vertical profiles of attenuated optical backscatter and radar reflectivity. Equivalent airborne remote sensors are also routinely flown on the NASA WB-57F and ER-2 aircraft. Algorithms designed to retrieve cirrus microphysical properties from remote sensor data must be able to handle the natural variability of cirrus that can range from optically thick layers that cause lidar attenuation to tenuous layers that are not detected by the cloud radar. An approach that is adopted here is to develop an algorithm suite that has internal consistency in its formulation and assumptions. The algorithm suite is developed around a forward model of the observations and is inverted for layer-mean cloud properties using a variational technique. The theoretical uncertainty in the retrieved ice water path retrieval is 40%–50%, and the uncertainty in the layer-mean particle size retrieval ranges from 50% to 90%. Two case studies from the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL) Florida Area Cirrus Experiment (FACE) field campaign as well as ground-based cases from the Atmospheric Radiation Measurement Program (ARM) are used to show the efficacy and error characteristics of the algorithms.

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Weifeng G. Zhang and Claudia Cenedese

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This study examines the dispersal of dense water formed in an idealized coastal polynya on a sloping shelf in the absence of ambient circulation and stratification. Both numerical and laboratory experiments reveal two separate bottom pathways for the dense water: an offshore plume moving downslope into deeper ambient water and a coastal current flowing in the direction of Kelvin wave propagation. Scaling analysis shows that the velocity of the offshore plume is proportional not only to the reduced gravity, bottom slope, and inverse of the Coriolis parameter, but also to the ratio of the dense water depth to total water depth. The dense water coastal current is generated by the along-shelf baroclinic pressure gradient. Its dynamics can be separated into two stages: (i) near the source region, where viscous terms are negligible, its speed is proportional to the reduced gravity wave speed and (ii) in the far field, where bottom drag becomes important and balances the pressure gradient, the velocity is proportional to Hc[g′/(LC d)]1/2 in which H c is the water depth at the coast, g′ the reduced gravity, C d the quadratic bottom drag coefficient, and L the along-shelf span of the baroclinic pressure gradient. The velocity scalings are verified using numerical and laboratory sensitivity experiments. The numerical simulations suggest that only 3%–23% of the dense water enters the coastal pathway, and the percentage depends highly on the ratio of the velocities of the offshore and coastal plumes. This makes the velocity ratio potentially useful for observational studies to assess the amount of dense water formed in coastal polynyas.

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Randy G. Brown and Chidong Zhang

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The tropical western Pacific warm pool is often generalized to be a region of heavy precipitation. This conceptis useful in constructing simplified models of the tropical circulation. However, the warm pool region is oftenpunctuated by periods of little rain. Such drought periods may last up to 10 days over an area of at least 6 ×105 km2. Other common features of the drought periods include an extremely dry midtroposphere, few deepclouds typically associated with mesoscale convective systems, and a substantial amount of clouds that are tootall to be categorized as trade cumuli but too short to fall into the category of deep convective clouds. Midtropospheric moisture varies substantially (60% in relative humidity, 4 g kg−1 in water vapor mixing ratio) betweenrainy and drought periods. The frequency distributions of humidity exhibit bimodal structures at certain levelsabove the freezing level. In either rainy or drought periods, or in a long period including both, the time-meanhumidity above the boundary layer deviates substantially from the most frequent profile of humidity, definedas the relative humidity corresponding to the maximum frequency distribution at each level. Mean soundings,therefore, do not accurately represent the overall vertical structure of moisture in the warm pool. Calculationsof a simple parcel model demonstrate that the warm pool atmosphere above the boundary layer can be dryenough to discourage the growth of deep convective clouds by depleting parcel buoyancy through entrainment.These results were drawn from an analysis of soundings collected during the Intensive Observing Period (1November 1992–28 February 1993) of the Tropical Ocean Global Atmosphere Coupled Ocean–AtmosphereResponse Experiment.

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Robert G. Nystrom and Fuqing Zhang

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Hurricane Patricia (2015) was a record-breaking tropical cyclone that was difficult to forecast in real time by both operational numerical weather prediction models and operational forecasters. The current study examines the potential for improving intensity prediction for extreme cases like Hurricane Patricia. We find that Patricia’s intensity predictability is potentially limited by both initial conditions, related to the data assimilation, and model errors. First, convection-permitting assimilation of airborne Doppler radar radial velocity observations with an ensemble Kalman filter (EnKF) demonstrates notable intensity forecast improvements over assimilation of conventional observations alone. Second, decreasing the model horizontal grid spacing to 1 km and reducing the surface drag coefficient at high wind speed in the parameterization of the sea surface–atmosphere exchanges is also shown to notably improve intensity forecasts. The practical predictability of Patricia, its peak intensity, rapid intensification, and the underlying dynamics are further investigated through a high-resolution 60-member ensemble initialized with realistic initial condition uncertainties represented by the EnKF posterior analysis perturbations. Most of the ensemble members are able to predict the peak intensity of Patricia, but with greater uncertainty in the timing and rate of intensification; some members fail to reach the ultimate peak intensity before making landfall. Ensemble sensitivity analysis shows that initial differences in the region beyond the radius of maximum wind contributes the most to the differences between ensemble members in Patricia’s intensification. Ensemble members with stronger initial primary and secondary circulations beyond the radius of maximum wind intensify earlier, are able to maintain the intensification process for longer, and thus reach a greater and earlier peak intensity.

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Yanwu Zhang, James G. Bellingham, and Yi Chao

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For estimating lateral flux in the ocean using fixed or mobile platforms, the authors present a method of analyzing the estimation error and designing the sampling strategy. When an array of moorings is used, spatial aliasing leads to an error in flux estimation. When an autonomous underwater vehicle (AUV) is run, measurements along its course are made at different times. Such nonsynopticity in the measurements leads to an error in flux estimation. It is assumed that the temporal–spatial autocovariance function of the flux variable can be estimated from historical data or ocean models (as in this paper). Using the temporal–spatial autocovariance function of the flux variable, the mean-square error of the flux estimate by fixed or mobile platforms is derived. The method is used to understand the relative strengths of moorings and AUVs (assumed here to be able to maintain constant speed) under different scenarios of temporal and spatial variabilities. The flux estimate by moorings through trapezoidal approximation generally carries a bias that drops quadratically with the number of moorings. The authors also show that a larger number of slower AUVs may achieve a more accurate flux estimate than a smaller number of faster AUVs under the same cumulative speed, but the performance margin shrinks with the increase of the cumulative speed. Using the error analysis results, one can choose the type of platforms and optimize the sampling strategy under resource constraints. To verify the theoretical analysis, the authors run simulated surveys in synthesized ocean fields. The flux estimation errors agree very well with the analytical predictions. Using an ocean model dataset, the authors estimate the lateral heat flux across a section in Monterey Bay, California, and also compare the flux estimation errors with the analytical predictions.

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Weifeng G. Zhang and Timothy F. Duda

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To quantify dynamical aspects of internal-tide generation at the Mid-Atlantic Bight shelf break, this study employs an idealized ocean model initialized by climatological summertime stratification and forced by monochromatic barotropic tidal currents at the offshore boundary. The Froude number of the scenario is subunity, and the bathymetric slope offshore of the shelf break is supercritical. A barotropic-to-baroclinic energy conversion rate of 335 W m−1 is found, with 14% of the energy locally dissipated through turbulence and bottom friction and 18% radiated onto the shelf. Consistent with prior studies, nonlinear effects result in additional super- and subharmonic internal waves at the shelf break. The subharmonic waves are subinertial, evanescent, and mostly trapped within a narrow beam of internal waves at the forcing frequency. They likely result from nonresonant triad interaction associated with strong nonlinearity. Strong vertical shear associated with the subharmonic waves tends to enhance local energy dissipation and turbulent momentum exchange (TME). A simulation with reduced tidal forcing shows an expected diminished level of harmonic energy. A quasi-linear simulation verifies the role of momentum advection in controlling the relative phases of internal tides and the efficiency of barotropic-to-baroclinic energy conversion. The local TME is tightly coupled with the internal-wave dynamics: for the chosen configuration, neglecting TME causes the internal-wave energy to be overestimated by 12%, and increasing it to high levels damps the waves on the continental shelf. This work implies a necessity to carefully consider nonlinearity and turbulent processes in the calculation of internal tidal waves generated at the shelf break.

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Wuyin Lin, Minghua Zhang, and Norman G. Loeb

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Marine boundary layer (MBL) clouds can significantly regulate the sensitivity of climate models, yet they are currently poorly simulated. This study aims to characterize the seasonal variations of physical properties of these clouds and their associated processes by using multisatellite data. Measurements from several independent satellite datasets [International Satellite Cloud Climatology Project (ISCCP), Clouds and the Earth’s Radiant Energy System–Moderate Resolution Imaging Spectroradiometer (CERES–MODIS), Geoscience Laser Altimeter System (GLAS), and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)], in conjunction with balloon soundings from the mobile facility of the Atmospheric Radiation Measurement (ARM) program at Point Reyes and reanalysis products, are used to characterize the seasonal variations of MBL cloud-top and cloud-base heights, cloud thickness, the degree of decoupling between clouds and MBL, and inversion strength off the California coast.

The main results from this study are as follows: (i) MBL clouds over the northeast subtropical Pacific in the summer are more prevalent and associated with a larger in-cloud water path than in winter. The cloud-top and cloud-base heights are lower in the summer than in the winter. (ii) Although the lower-tropospheric stability of the atmosphere is higher in the summer, the MBL inversion strength is only weakly stronger in the summer because of a negative feedback from the cloud-top altitude. Summertime MBL clouds are more homogeneous and are associated with lower surface latent heat flux than those in the winter. (iii) Seasonal variations of low-cloud properties from summer to winter resemble the downstream stratocumulus-to-cumulus transition of MBL clouds in terms of MBL depth, cloud-top and cloud-base heights, inversion strength, and spatial homogeneity. The “deepening–warming” mechanism of Bretherton and Wyant for the stratocumulus-to-trade-cumulus transition downstream of the cold eastern ocean can also explain the seasonal variation of low clouds from the summer to the winter, except that warming of the sea surface temperature needs to be taken as relative to the free-tropospheric air temperature, which occurs in the winter. The observed variation of low clouds from summer to winter is attributed to the much larger seasonal cooling of the free-tropospheric air temperature than that of the sea surface temperature.

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J. Vivekanandan, G. Zhang, and M. K. Politovich

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Multiple parameters are needed to adequately describe the icing condition of clouds, yet those that can be accurately measured using remote sensors are usually limited. In this paper, a detection and classification method using two parameters derived from dual-wavelength (Ka and X bands) radar measurements is explored. The first of these is the radar-estimated size (RES), defined as the cube root of the sixth and third moment size distribution ratio. The RES is shown to be useful in characterizing icing environments through simulations using modified gamma droplet size distributions with realistic bounds based on several sets of in situ measurements. The RES is also relatively easy to retrieve using dual-wavelength radar measurements. The second parameter is the liquid water content (LWC), the total mass of liquid droplets of all sizes. The LWC may be estimated by taking advantage of the difference in attenuation due to the presence of liquid water between the X- and Ka-band radars. In situ measurements of droplet spectra in various research projects were analyzed for quantifying the utility of LWC, RES, and median volume diameter for describing aircraft icing hazard categories. It is shown that a combination of RES and LWC could distinguish environments with a combination of large droplets and higher liquid water contents.

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