Search Results

You are looking at 1 - 10 of 22 items for

  • Author or Editor: Richard Wilson x
  • Refine by Access: All Content x
Clear All Modify Search
Chris Wilson and Richard G. Williams

Abstract

The mechanisms controlling the direction of eddy tracer fluxes are examined using eddy-resolving isopycnic experiments for a cyclic zonal channel. Eddy fluxes are directed downgradient on average when either (i) there is a Lagrangian increase in tracer variance or (ii) there is strong dissipation of tracer variance. The effect of the eddies on the mean tracer evolution can be described through an ensemble of eddies that each have a particular life cycle. Local examination of the eddy behavior, such as fluxes, eddy kinetic energy, and tracer variance appears complex, although the cumulative time-mean picture has coherence: eddies are preferentially formed in localized regions with downstream growth and increase in tracer variance concomitant with downgradient eddy tracer fluxes, while eventually the eddies decay with a decrease in tracer variance and upgradient eddy tracer fluxes. During spinup, tracer deformation through flow instability leads to an area-average increase in tracer variance (although locally it is increasing and decreasing with the individual eddy life cycles) and therefore an implied area-average, downgradient tracer flux. At a steady state, part of the pattern in eddy fluxes simply reflects advection of background tracer variance by the time-mean and eddy flows. The eddy flux becomes biased to being directed downgradient if there is a strong sink in the tracer, which is likely to be the case for eddy heat fluxes along isopycnals outcropping in the mixed layer or for eddy nitrate fluxes along isopycnals intersecting the euphotic zone.

Full access
Chris Wilson and Richard G. Williams

Abstract

Eddy fluxes systematically affect the larger-scale, time-mean state, but their local behavior is difficult to parameterize. To understand how eddy fluxes of potential vorticity (PV) are controlled, the enstrophy budget is diagnosed for a five-layer, 1/16°, eddy-resolving, isopycnic model of a wind-driven, flat-bottom basin. The direction of the eddy flux across the mean PV contours is controlled by the Lagrangian evolution of enstrophy, including contributions from the temporal change and mean and eddy advection, as well as dissipation of enstrophy. During the spinup, an overall increase in enstrophy is consistent with eddy fluxes being directed downgradient on average and homogenization of PV within intermediate layers. Enstrophy becomes largest along the flanks of the gyre, where PV gradients are large, and becomes smallest in the interior. At a statistically steady state, there is a reversing pattern of up- and downgradient eddy PV fluxes, which are locally controlled by the advection of enstrophy. A downgradient eddy PV flux occurs only on the larger scale over the gyre flanks and part of the western boundary. These larger-scale patterns are controlled by the eddy advection of enstrophy, which becomes significant in regions of high eddy enstrophy. As a consequence, at a statistically steady state, the eddy PV fluxes are not simply related to the mean fields, and their local, finescale pattern is difficult to parameterize.

Full access
Richard G. Wilson and Wayne R. Rouse

Abstract

Energy balance measurements of evapotranspiration from a developing corn crop are compared with daily equilibrium evapotranspiration estimates to examine the accuracy of the model and the environmental conditions under which it can be applied. Equilibrium estimates compared closely (a standard error of 6%) with the measured values when the surface was moderately dry, a condition which applied to 14 of the 24 days of the experiment. The ratio of actual evapotranspiration to available energy and the Bowen ratio are used to establish moisture and temperature limits for the model. The success of the model was related to a typical diurnal pattern of the difference between actual and equilibrium evapotranspiration which reflects expected variations of moisture stress during daytime hours. The performance of the model was nearly independent of the physical condition of the surface and the height of the required air temperature measurement. An equation is presented which permits easy calculation of equilibrium evapotranspiration from air temperature, net radiation, and soil heat flux data.

Full access
Richard E. Thomson and Robert E. Wilson

Abstract

Cape St. James is an extensive triangular-shaped promontory located in a tidally energetic region at the southern tip of the Queen Charlotte Islands approximately 150 km off the mainland coast or British Columbia. Several years of oceanographic data collected in vicinity of the cape reveal a regional circulation characterized by a strong (0.50 m s−1) coastal current along the western continental margin and respective clockwise and counterclockwise rotating mesoscale baroclinic eddies to the west and south of the cape. The coastal current flows counter to the prevailing winds while the anticyclonic eddy to the west of the cape is a particularly intense feature that appears consistently in AVHRR imagery of the region. The structure of the mean flow, combined with the marked O(0.1 0 m s−1) low-frequency current variability at fortnightly and monthly tidal periods plus significant coherence at fortnightly periods between low-frequency currents and demodulated tidal flow, suggests that rectification of the strong diurnal and semidiurnal tidal currents is the principal cause of the residual circulation in the vicinity of the cape.

Results from an analytical model indicate that generation of the mean residual circulation is due primarily to the M2 tidal current constituent and that maximum countercurrent velocities occur over the inner portion of the continental shelf. The fortnightly modulation of the mean flow is effected by both diurnal and semidiurnal currents but with a tendency for semidiurnal contributions to dominate in regions of greatest counterflow. Generic depth-dependent numerical simulations for nondimensional frictional parameters typical of the region verify that the asymmetry in the observed location and intensity of the eddy field, together with the presence of the strong coastal countercurrent on the west side of the cape and a narrow jet to the south of the cape, are associated with tidal rectification. These models also suggest that residual vertical motion due to topographic lifting and Ekman suction are responsible for the observed tilting of the isopycnals and thereby the development of baroclinicity in the residual horizontal motion.

Full access
Chris Wilson, Bablu Sinha, and Richard G. Williams

Abstract

The control of atmospheric storm tracks by ocean dynamics, orography, and their interaction is investigated using idealized experiments with a simplified coupled atmosphere–ocean climate model. The study focuses on the quasi–steady state for the storm tracks in the Northern Hemisphere winter mean. The experiments start with a background state without mountains and ocean dynamics, and in separate stages include orography and a dynamic ocean to obtain a more realistic control simulation. The separate effects of ocean dynamics, orography, and their nonlinear interaction are identified for the storm tracks and the surface thermodynamic forcing over the ocean.

The model study suggests that atmospheric storm tracks are a robust feature of the climate system, occurring at midlatitudes even if there is no orographic forcing or ocean heat transport. Ocean dynamics generally lead to a poleward shift in both the storm track and the maximum in atmospheric northward heat transport and induce a northeastward tilt over the Atlantic. This poleward shift is linked to the extra heat transport by the ocean and the tightening of sea surface temperature gradients on the western side of ocean basins. Orographic forcing causes along-track variability with a weakening of the storm track over the continents and induces a northeastward tilt over the western Pacific, which is associated with a stationary planetary wave train generated by the Tibetan Plateau. The interaction between ocean dynamics and orographic forcing plays a localized role, enhancing the contrast between the Atlantic and Pacific. Much of the response to the forcing is eddy mediated and transient eddies help to spread the influence of orographic and ocean forcing.

Full access
Richard Wilson, Francis Dalaudier, and Francois Bertin

Abstract

Small-scale turbulence in the free atmosphere is known to be intermittent in space and time. The turbulence fraction of the atmosphere is a key parameter in order to evaluate the transport properties of small-scale motions and to interpret clear-air radar measurements as well.

Mesosphere–stratosphere–troposphere (MST)/stratosphere–troposphere (ST) radars provide two independent methods for the estimation of energetic parameters of turbulence. First, the Doppler spectral width σ 2 is related to the dissipation rate of kinetic energy εk. Second, the radar reflectivity, or C 2 n, relates to the dissipation rate of available potential energy εp. However, these two measures yield estimates that differ with respect to an important point. The Doppler width measurements, and related εk, are reflectivity-weighted averages. On the other hand, the reflectivity estimate is a volume-averaged quantity. The values of εp depend on both the turbulence intensity and the turbulent fraction within the radar sampling volume.

Now, the two dissipation rates εp and εk are related quantities as shown by various measurements within stratified fluids (atmosphere, ocean, lakes, or laboratory). Therefore, by assuming a “canonical” value for the ratio of dissipation rates, an indirect method is proposed to infer the turbulent fraction from simultaneous radar measurements of reflectivity and Doppler broadening within a sampling volume. This method is checked by using very high resolution radar measurements (30 m and 51 s), obtained by the PROUST radar during a field campaign. The method is found to provide an unbiased estimation of the turbulent fraction, within a factor of 2 or less.

Full access
Kevin Hamilton, R. John Wilson, and Richard S. Hemler

Abstract

The tropical stratospheric mean flow behavior in a series of integrations with high vertical resolution versions of the Geophysical Fluid Dynamics Laboratory (GFDL) “SKYHI” model is examined. At sufficiently high vertical and horizontal model resolution, the simulated stratospheric zonal winds exhibit a strong equatorially centered oscillation with downward propagation of the wind reversals and with formation of strong vertical shear layers. This appears to be a spontaneous internally generated oscillation and closely resembles the observed quasi-biennial oscillation (QBO) in many respects, although the simulated oscillation has a period less than half that of the real QBO. The same basic mean flow oscillation appears in both seasonally varying and perpetual equinox versions of the model, and most of the analysis in this paper is focused on the perpetual equinox cases. The mean flow oscillation is shown to be largely driven by eddy momentum fluxes associated with a broad spectrum of vertically propagating waves generated spontaneously in the tropical troposphere of the model. Several experiments are performed with the model parameters perturbed in various ways. The period of the simulated tropical stratospheric mean flow oscillation is found to change in response to large alterations in the sea surface temperatures (SSTs) employed. This is a fairly direct demonstration of the link between the stratospheric mean flow behavior and tropical convection that is inherent in current theories of the QBO. It is also shown in another series of experiments that the oscillation is affected by the coefficients used for the subgrid-scale diffusion parameterization. These experiments demonstrate that at least one key reason why reasonably fine horizontal resolution is needed for the model to simulate a mean flow oscillation is the smaller horizontal diffusion that can be used at high resolution.

Full access
James Wilson, Richard Carbone, Harold Baynton, and Robert Serafin

Single Doppler weather radar velocity and reflectivity fields have been obtained with the National Center for Atmospheric Research (NCAR) 5 cm radars for a wide variety of weather situations. Among those weather features that can be identified by means of color displays are the vertical variation of wind with height in widespread precipitation, frontal boundaries, gust fronts, “downbursts,” tornadoes, hurricane winds, wind shears dangerous to aircraft, and winds in the boundary layer in clear air.

It is concluded that, even though a Doppler radar observes only the radial component of the wind, a wide variety of weather features of great importance to weather forecasters can easily be identified with a single radar. For operational applications a national network of Doppler radars seems justified. It is recommended, particularly in regions of the country where severe storms or high rainfall rates are relatively frequent, that these be 10 cm wavelength radars with a beam width of 1° and that automatic procedures for removing velocity and range ambiguities be incorporated.

Full access
Kevin Hamilton, R. John Wilson, and Richard S. Hemler

Abstract

The large-scale circulation in the Geophysical Fluid Dynamics Laboratory “SKYHI” troposphere–stratosphere–mesosphere finite-difference general circulation model is examined as a function of vertical and horizontal resolution. The experiments examined include one with horizontal grid spacing of ∼35 km and another with ∼100 km horizontal grid spacing but very high vertical resolution (160 levels between the ground and about 85 km). The simulation of the middle-atmospheric zonal-mean winds and temperatures in the extratropics is found to be very sensitive to horizontal resolution. For example, in the early Southern Hemisphere winter the South Pole near 1 mb in the model is colder than observed, but the bias is reduced with improved horizontal resolution (from ∼70°C in a version with ∼300 km grid spacing to less than 10°C in the ∼35 km version). The extratropical simulation is found to be only slightly affected by enhancements of the vertical resolution. By contrast, the tropical middle-atmospheric simulation is extremely dependent on the vertical resolution employed. With level spacing in the lower stratosphere ∼1.5 km, the lower stratospheric zonal-mean zonal winds in the equatorial region are nearly constant in time. When the vertical resolution is doubled, the simulated stratospheric zonal winds exhibit a strong equatorially centered oscillation with downward propagation of the wind reversals and with formation of strong vertical shear layers. This appears to be a spontaneous internally generated oscillation and closely resembles the observed QBO in many respects, although the simulated oscillation has a period less than half that of the real QBO.

Full access
Laurence J. Wilson, Stephane Beauregard, Adrian E. Raftery, and Richard Verret

Abstract

Bayesian model averaging (BMA) has recently been proposed as a way of correcting underdispersion in ensemble forecasts. BMA is a standard statistical procedure for combining predictive distributions from different sources. The output of BMA is a probability density function (pdf), which is a weighted average of pdfs centered on the bias-corrected forecasts. The BMA weights reflect the relative contributions of the component models to the predictive skill over a training sample. The variance of the BMA pdf is made up of two components, the between-model variance, and the within-model error variance, both estimated from the training sample. This paper describes the results of experiments with BMA to calibrate surface temperature forecasts from the 16-member Canadian ensemble system. Using one year of ensemble forecasts, BMA was applied for different training periods ranging from 25 to 80 days. The method was trained on the most recent forecast period, then applied to the next day’s forecasts as an independent sample. This process was repeated through the year, and forecast quality was evaluated using rank histograms, the continuous rank probability score, and the continuous rank probability skill score. An examination of the BMA weights provided a useful comparative evaluation of the component models, both for the ensemble itself and for the ensemble augmented with the unperturbed control forecast and the higher-resolution deterministic forecast. Training periods around 40 days provided a good calibration of the ensemble dispersion. Both full regression and simple bias-correction methods worked well to correct the bias, except that the full regression failed to completely remove seasonal trend biases in spring and fall. Simple correction of the bias was sufficient to produce positive forecast skill out to 10 days with respect to climatology, which was improved by the BMA. The addition of the control forecast and the full-resolution model forecast to the ensemble produced modest improvement in the forecasts for ranges out to about 7 days. Finally, BMA produced significantly narrower 90% prediction intervals compared to a simple Gaussian bias correction, while achieving similar overall accuracy.

Full access