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W. D. Hibler III and K. Bryan

Abstract

A coupled ice–ocean model suitable for simulating ice–ocean circulation over a seasonal cycle is developed by coupling the dynamic thermodynamic sea ice model of Hibler with a multilevel baroclinic ocean model (Bryan). This model is used to investigate the effect of ocean circulation on seasonal sea ice simulations by carrying out a simulation of the Arctic, Greenland and Norwegian seas. The ocean model contains a linear term that damps the ocean's temperature and salinity towards climatology. The damping term was chosen to have a three-year relaxation time, equivalent to the adjustment time of the pack ice. No damping, however, was applied to the uppermost layer of the ocean model, which is in direct contact with the moving pack ice. This damping procedure allows seasonal and shorter time-scale variability to be simulated in the ocean, but does not allow the model to drift away from ocean climatology on longer time scales.

For the standard experiment, an initial integration of five years was performed at one-day time steps and a 1.45° by 1.45° resolution in order to obtain a cycle equilibrium. For comparison, a five-year simulation with an ice-only model, and shorter one-year sensitivity simulations without surface salt fluxes and without ocean currents, were also carried out. Input fields consisted of climatological surface air temperatures and mixing ratios, together with daily geostrophic winds from 1979.

The surface current structure at the end of the five-year simulation exhibits a stronger East Greenland Current and Beaufort Sea Gyre than the initial geostrophic estimates, and is in better agreement with observation. in the Greenland/Norwegian Sea the upper 0.5 km of the ocean becomes more isothermal, with a noticeable seasonal variation in temperature. This neutral density allows monthly averaged winter heat fluxes as large as 350 W m−2 to be delivered to the upper ocean, thus yielding a much more realistic ice edge than is obtainable by the ice-only model. Spatial variations in ice thickness and ice drift prediction are also in better agreement in the full ice–ocean model as compared to the ice-only model. Except in very shallow regions, month-to-month fluctuations in ice motion are much larger than upper ocean current fluctuations, which also tend to be smaller than mean annual currents. In the central basin, the ice interaction is found to reduce by about 40% the wind stress transferred into the ocean.

Analysis of the advance and retreat of the East Greenland ice edge shows that while there is some initial freezing in the fall, on a monthly averaged basis the ice tends to melt during the winter, thus partially off-setting the advection of ice into the region. The amount of melt tends to oscillate from month to month, with large melt ratios coinciding with large oceanic heat fluxes and vice versa. Examination of shorter sensitivity simulations shows this realistic ice edge to be especially dependent on the inclusion of the full three-dimensional circulation in the ocean, and to a lesser degree sensitive to the inclusion of ice melt fluxes. Analysis of the global budgets shows that an annual northward heat transport across the Denmark Strait and Iceland-Faeroe-Shetland passages of about 0.18 × 1015 W is required to balance the atmospheric heat gain.

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Bryan K. Woods and Ronald B. Smith

Abstract

Recent stratospheric mountain wave measurements over the Sierra Nevada indicate that downgoing secondary waves may be common or even ubiquitous in large wave events. Because of their short wavelengths, they may dominate the vertical velocity field near the tropopause, and they give a remote indicator of wave breaking farther aloft. Using a 2D numerical model, the authors have simulated the secondary wave generation process with qualitative agreement in the wave location, phase speed, wavelength (i.e., 10–20 km), and amplitude. A key to the analysis was the use of Morlet wavelet cross spectra on both the observational and simulated fields.

Several characteristics of the simulated secondary waves were unexpected. First, the secondary waves are generated with good efficiency, approaching 20% of the primary upgoing wave momentum flux. Second, whereas most of the secondary waves are downward, the shorter components reflect upward from the tropopause, giving a kind of lee wave trapping in the lower stratosphere. Long waves are also observed propagating upward and downward away from the wave breaking region. Third, the phase speed of the secondary waves is nearly zero so the Eliassen–Palm relationship between momentum and energy flux is satisfied. While the 2D results are robust to grid size and subgrid parameterization, an extension of the modeling to three dimensions is disappointing. The secondary waves’ amplitudes in the 3D runs are much smaller than observed.

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Bryan K. Woods and Ronald B. Smith

Abstract

In recent years, aircraft data from mountain waves have been primarily analyzed using velocity and temperature power spectrum and momentum flux estimation. Herein it is argued that energy flux wavelets (i.e., pressure–velocity wavelet cross-spectra) provide a more powerful tool for locating and classifying different types of mountain waves. In the first part of the paper, pressure–velocity cross-spectra using various linear mountain-wave solutions are shown to be capable of disentangling collocated waves with different propagation directions and wavelength. A field of group velocity vectors can also be determined.

In the second part, the energy flux wavelet technique is applied to five cases of mountain waves entering the stratosphere from the Terrain-Induced Rotor Experiment (T-REX) in 2006. Perturbation pressure along the flight track is determined using aircraft static pressure corrected hydrostatically with GPS altitude. In four of the cases, collocated long up-propagating and short down-propagating waves are seen in the stratosphere. These waves have strong, but opposite, pw′ cospectra. In one of these cases, a patch of turbulence is collocated with the up and down waves. In two other cases, trapped waves riding on the tropopause inversion layer (TIL) are seen. These trapped waves have pw′ quadrature spectra that reverse sign across the tropopause. These newly discovered wave types may arise from secondary wave generation (i.e., a nonlinear transfer of energy from the long vertically propagating waves to shorter modes).

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K. Bryan, S. Manabe, and M. J. Spelman

Abstract

Numerical experiments are carried out using a general circulation model of a coupled ocean-atmosphere system with idealized geography, exploring the transient response of climate to a rapid increase of atmospheric carbon dioxide. The computational domain of the model is bounded by meridians 120 degrees apart, and includes two hemispheres. The ratio of land to sea at each latitude corresponds to the actual land-sea ratio for the present geography of the Earth. At the latitude of the Drake Passage the entire sector is occupied by ocean.

In the equivalent of the Northern Hemisphere the ocean delays the climate response to increased atmospheric carbon dioxide. The delay is of the order of several decades, a result corresponding to previous modeling studies. At high latitudes of the equivalent of the ocean-covered Southern Hemisphere, on the other hand, there is no warming at the sea surface, and even a slight cooling over the 50-year duration of the experiment. Two main factors appear to be involved. One is the very large ratio of ocean to land in the Southern Hemisphere. The other factor is the very deep penetration of a meridional overturning associated with the equatorward Ekman transport under the Southern Hemisphere westerlies. The deep cell delays the response to carbon-dioxide warming by upwelling unmodified waters from great depth. This deep cell disappears when the Drake Passage is removed from the model.

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K. R. Bryan, K. P. Black, and R. M. Gorman

Abstract

Three-dimensional point velocity measurements from within and near the surf zone are used to examine changes in turbulent dissipation rate with location relative to the breakpoint and wave conditions. To separate turbulence from wave orbital currents, dissipation rate is derived using the inertial subrange of the wavenumber spectrum. The measured frequency spectrum is transformed into a wavenumber spectrum by generalizing Taylor's hypothesis to advection of turbulence past a sensor by monochromatic water waves in arbitrary water depth. The resulting dissipation rate is compared with that obtained by averaging the dissipation rates calculated from frequency spectra of very short segments of the time series, over which Taylor's hypothesis for steady currents can be used. The dissipation rate calculated using the latter compares well, in the averaged sense, to the dissipation rate calculated using the whole time series, even though the whole time series includes segments that violate Taylor's assumptions. Measured dissipation rates inside and very near the breakpoint show large increases shoreward and lesser increases with deep-water significant wave height. Farther outside the surf zone, dissipation rate depended on frequency, significant wave height, and depth. Simple models show that the surf-zone patterns are explained by shoreward increases in the probability of wave breaking, although measured turbulence levels are significantly less than needed to dissipate the measured wave energy, suggesting that most of the dissipation occurs above trough level or very near the bed.

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Bryan K. Woods, Thomas Nehrkorn, and John M. Henderson

Abstract

A 31-yr time series of boundary layer winds has been developed for a region on the outer continental shelf. This simulated time series was designed to be suitable to study the wind resources for a potential offshore wind farm. Reanalysis data were used to initialize a series of high-resolution numerical simulations. The limited number of high-resolution numerical simulations was repeatedly sampled using an analog matching criterion that is based on the reanalysis data to create a 31-yr time series with 500-m spatial resolution and 10-min temporal resolution. Validation against buoy data indicates that combining the reanalysis and resampled high-resolution numerical simulations produces a much more accurate wind speed distribution than does the reanalysis alone. Both the model physics and downscaled resolution may be contributing to the observed performance gains.

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S. Manabe, R. J. Stouffer, M. J. Spelman, and K. Bryan

Abstract

This study investigates the response of a climate model to a gradual increase or decrease of atmospheric carbon dioxide. The model is a general circulation model of the coupled atmosphere-ocean-land surface system with global geography and seasonal variation of insulation. To offset the bias of the coupled model toward settling into an unrealistic state, the fluxes of heat and water at the ocean-atmosphere interface are adjusted by amounts that vary with season and geography but do not change from one year to the next. Starting from a quasi-equilibrium climate, three numerical time integrations of the coupled model are performed with gradually increasing, constant, and gradually decreasing concentration of atmospheric carbon dioxide.

It is noted that the simulated response of sea surface temperature is very slow over the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere where vertical mixing of water penetrates very deeply. However, in most of the Northern Hemisphere and low latitudes of the Southern Hemisphere, the distribution of the change in surface air temperature of the model at the time of doubling (or halving) of atmospheric carbon dioxide resembles the equilibrium response of an atmospheric-mixed layer ocean model to CO2 doubling (or halving). For example, the rise of annual mean surface air temperature in response to the gradual increase of atmospheric carbon dioxide increases with latitudes in the Northern Hemisphere and is larger over continents than oceans.

When the time-dependent response of the model oceans to the increase of atmospheric carbon dioxide is compared with the corresponding response to the CO2, reduction at an identical rate, the penetration of the cold anomaly in the latter case is significantly deeper than that of the warm anomaly in the former case. The lack of symmetry in the penetration depth of a thermal anomaly between the two cases is associated with the difference in static stability, which is due mainly to the change in the vertical distribution of salinity in high latitudes and temperature changes in middle and low latitudes.

Despite the difference in penetration depth and accordingly, the effective thermal inertia of the oceans between the two experiments, the time-dependent response of the global mean surface air temperature in the CO2 reduction experiment is similar in magnitude to the corresponding response in the CO2 growth experiment. In the former experiment with a colder climate, snow and sea ice with high surface albedo cover a much larger area, thereby enhancing their positive feedback effect upon surface air temperature. On the other hand, surface cooling is reduced due to the larger effective thermal inertia of the oceans. Because of the compensation between these two effects, the magnitude of surface air temperature response turned out to be similar between the two experiments.

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Thomas Nehrkorn, Bryan K. Woods, Ross N. Hoffman, and Thomas Auligné

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The Feature Calibration and Alignment technique (FCA) has been developed to characterize errors that a human would ascribe to a change in the position or intensity of a coherent feature, such as a hurricane. Here the feature alignment part of FCA is implemented in the Weather Research and Forecasting Data Assimilation system (WRFDA) to correct position errors in background fields and tested in simulation for the case of Hurricane Katrina (2005). The displacement vectors determined by feature alignment can be used to explain part of the background error and make the residual background errors smaller and more Gaussian. Here a set of 2D displacement vectors to improve the alignment of features in the forecast and observations is determined by solving the usual variational data assimilation problem—simultaneously minimizing the misfit to observations and a constraint on the displacements. This latter constraint is currently implemented by hijacking the usual background term for the midlevel u- and υ-wind components. The full model fields are then aligned using a procedure that minimizes dynamical imbalances by displacing only conserved or quasi-conserved quantities. Simulation experiments show the effectiveness of these procedures in correcting gross position errors and improving short-term forecasts. Compared to earlier experiments, even this initial implementation of feature alignment produces improved short-term forecasts. Adding the calculation of displacements to WRFDA advances the key contribution of FCA toward mainstream implementation since all observations with a corresponding observation operator may be used and the existing methodology for estimating the background error covariances may be used to refine the displacement error covariances.

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Kirk Bryan, John K. Dukowicz, and Richard D. Smith

Abstract

Mesoscale eddies in the ocean play an important role in the ocean circulation. In order to simulate the ocean circulation, mesoscale eddies must be included explicitly or parameterized. The eddy permitting calculations of the Los Alamos ocean circulation model offer a special opportunity to test aspects of parameterizations that have recently been proposed. Although the calculations are for a model in level coordinates, averages over a five-year period have been carried out by interpolating to instantaneous isopycnal surfaces. The magnitude of “thickness mixing” or bolus velocity is found to coincide with areas of intense mesoscale activity in the western boundary currents of the Northern Hemisphere and the Antarctic Circumpolar Current. The model also predicts relatively large bolus fluxes in the equatorial region. The analysis does show that the rotational component of the bolus velocity is significant. Predictions of the magnitude of the bolus velocity, assuming downgradient mixing of thickness with various mixing coefficients, have been compared directly with the model. The coefficient proposed by Held and Larichev provides a rather poor fit to the model results because it predicts large bolus velocity magnitudes at high latitudes and in other areas in which there is only a small amount of mesoscale activity. A much better fit is obtained using a constant mixing coefficient or a mixing coefficient originally proposed by Stone in a somewhat different context. The best fit to the model is obtained with a coefficient proportional to λ 2/T, where λ is the radius of deformation, and T is the Eady timescale for the growth of unstable baroclinic waves.

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Anthony J. Schreiner, Steven A. Ackerman, Bryan A. Baum, and Andrew K. Heidinger

Abstract

A technique using the Geostationary Operational Environmental Satellite (GOES) sounder radiance data has been developed to improve detection of low clouds and fog just after sunrise. The technique is based on a simple difference method using the shortwave (3.7 μm) and longwave (11.0 μm) window bands in the infrared range of the spectrum. The time period just after sunrise is noted for the difficulty in being able to correctly identify low clouds and fog over land. For the GOES sounder cloud product this difficulty is a result of the visible reflectance of the low clouds falling below the “cloud” threshold over land. By requiring the difference between the 3.7- and the 11.0-μm bands to be greater than 5.0 K, successful discrimination of low clouds and fog is found 85% of the time for 21 cases from 14 September 2005 to 6 March 2006 over the GOES-12 sounder domain. For these 21 clear and cloudy cases the solar zenith angle ranged from 87° to 77°; however, the range of solar zenith angles for cloudy cases was from 85° to 77°.

The success rate further improved to 95% (20 out of 21 cases) by including a difference threshold of 5.0 K between the 3.7- and 4.0-μm bands, requiring that the 11.0-μm band be greater than 260 K, and limiting the test to fields of view where the surface elevation is below 999 m. These final three limitations were needed to more successfully deal with cases involving snow cover and dead vegetation. To ensure that only the time period immediately after sunrise is included the solar zenith angle threshold for application of these tests is between 89° and 70°.

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