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Michael J. Fox

Abstract

A technique is described to determine 24-hr stream-bed evaporation subject to the constraints that it be inexpensive, uncomplicated, and accurate to within ± 20%. Using maximum thermometers, a totalizing anemometer, and pans filled with dry sand, evaporation E is predicted from E = (b 1 δT + b 2 δU)δT, where δT is the difference between the maximum temperature of the dry sand and the maximum temperature of the stream bed, δU the 24-hr wind passage obtained from the totalizing anemometer, and b 1 and b 2 are constants peculiar to each soil type and are determined by a statistical curve-fitting method on experimental data. The result of statistical evaluation of the prediction equation on the experimental data gives an average percentage error of 13% and a standard error of 0.88 mm. Using this technique, it was determined that evaporation is not important in the annual hydrological balance of a stream bed.

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J. S. Margolis and Kenneth Fox

Abstract

Rotational temperatures and abundances and methane in the Jovian atmosphere have been calculated for Lorentz half-widths γ = 0.15, 0.20 and 0.30 cm−1. The results are within ∼10 to 20% of those previously determined for γ = 0.10 cm−1, and are not far from the values corresponding to no saturation.

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Alan D. Fox and Stephen J. Maskell

Abstract

Two-way interactive nesting of primitive equation ocean models is investigated, with special attention to the problems encountered when oceanic features, for example fronts, intersect the boundaries between the models, and also when topography is present. The model has two interacting components, the coarse and fine grid regions. The fine grid region can have a finer resolution both in the horizontal and in the vertical than the coarse grid model, allowing refinement of topographic features in the vertical as well as the horizontal, although a model with nesting only in the horizontal is considered first, to clarify the effects of the lateral boundaries. An adaptation of the method of Spall and Holland is used. The interaction is two-way: the coarse grid fields are interpolated to provide boundary conditions for the fine grid region, and the variables on the fine grid are suitably averaged onto the coarse grid in order to drive the coarse grid model. Nested calculations with 3:1 grid ratios are presented.

Modeling of frontal features using nested models is addressed and topography is introduced, but without refinement of the grid in the vertical in the fine grid region. It is shown that the generation of noise at the interface of the fine grid and coarse grid regions in the presence of topography can be dealt with by the use of a Newtonian damping scheme. Refinement of the fine grid region resolution in the vertical is implemented, and attention is drawn to problems arising when the fine and coarse grid topographies are not identical.

Care also needs to he taken to ensure that the initial fields are resolved on both the coarse grid and the fine grid in the region of the interface in order to minimize the generation of disturbances.

Comparison of the results of the nested model with a fine grid everywhere reference calculation shows the nesting technique to be working successfully over reasonably short periods of time integration (16 days) such as may be used operationally for ocean forecasting.

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Neil I. Fox and Anthony J. Illingworth

Abstract

The radiative characteristics of stratocumulus clouds are dependent upon their microphysical properties, primarily the liquid water content and effective radius of the drop population. Aircraft observations of droplet spectra in warm stratocumulus over the North Atlantic and around the British Isles by the Hercules C-130 aircraft of the U.K. Meteorological Office Meteorological Research Flight have been used to calculate the radar reflectivity, liquid water content, and effective radius. Empirically derived relationships, found from more than 4000 km of flight data on 11 separate days, that link reflectivity with either liquid water content or effective radius have been derived. These empirical relationships are significantly different from those predicted if the cloud droplet spectrum is modeled as a gamma function. Occasional drizzle-sized drops are frequently present within the cloud, and even though their concentration is very low, they dominate the reflectivity and these empirical relationships fail. However, although the drizzle drops increase the reflectivity, they have a negligible effect on the liquid water content and effective radius of the cloud. As these drops have a significant fall velocity in comparison to the cloud droplets, it is suggested that a ground-based Doppler radar could separate the components of the reflectivity due to bimodal drop spectra and the vertical structure of the cloud properties that determine radiative transfer could be retrieved.

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Steven J. Ghan, Timothy Shippert, and Jared Fox

Abstract

The climate simulated by a global atmosphere–land model with a physically based subgrid orography scheme is evaluated in 10 selected regions. Climate variables simulated for each of multiple elevation classes within each grid cell are mapped according to a high-resolution distribution of surface elevation in each region. Comparison of the simulated annual mean climate with gridded observations leads to the following conclusions. At low to moderate elevations the downscaling scheme correctly simulates increasing precipitation, decreasing temperature, and increasing snow with increasing elevation across distances smaller than 100 km. At high elevations the downscaling scheme correctly simulates decreasing precipitation with increasing elevation. The rain shadow of many mountain ranges is poorly resolved, with too little precipitation simulated on the windward side of mountain ranges and too much on the lee side. The simulated sensitivity of surface air temperature to surface elevation is too strong, particularly in valleys influenced by drainage circulations. Observations show little evidence of a “snow shadow,” so the neglect of the subgrid rain shadow does not produce an unrealistic simulation of the snow distribution. Summertime snow area, which is a proxy for land ice, is much larger than observed, mostly because of excessive snowfall but in some places because of a cold bias. Summertime snow water equivalent is far less than the observed thickness of glaciers because a 1-m upper bound on snow water is applied to the simulations and because snow transport by slides is neglected. The 1-m upper bound on snow water equivalent also causes an underestimate of seasonal snow water during late winter, compared with gridded station measurements. Potential solutions to these problems are discussed.

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Neil I. Fox and Anthony J. Illingworth

Abstract

The radar reflectivity and liquid water content of stratocumulus clouds have been computed from cloud droplet spectra recorded during more than 4000 km of cloud penetrations by an aircraft, and the probability of detecting various values of liquid water content as a function of the radar sensitivity threshold has been derived. The goal of the study is to specify the sensitivity required for any future spaceborne cloud radar. In extensive marine stratocumulus deeper than about 200 m, occasional but ubiquitous drizzle-sized droplets of up to 200 μm dominate the radar return and increase it by between 10 and 20 dB above the cloud droplet contribution to the return, making radar detection easier, although the concentration of the drizzle drops is so low that they have no effect on the liquid water content or effective radius. These occasional drizzle-sized droplets are present throughout the vertical and horizontal extent of such clouds but should evaporate within 200 m of cloud base. On occasion, the drizzle can fall farther and may yield a false measurement of cloud-base altitude, but such cases can be recognized by examining the vertical profile of reflectivity. A radar sensitivity threshold of −30 dBZ would detect 80%, 85%, and 90% of the marine stratocumulus, with a liquid water content above 0.025, 0.05, and 0.075 g m−3, respectively. Because nonprecipitating drizzle droplets are rare in continental stratocumulus, the equivalent figures are reduced to 38%, 33%, and 25%. Improving the sensitivity to −40 dBZ increases detection probability to nearly 100% for both types of cloud. These figures are based on the assumption that the cloud is deep enough to fill the radar pulse volume.

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Joseph Skitka, J. B. Marston, and Baylor Fox-Kemper

Abstract

The combined effectiveness of model reduction and the quasilinear approximation for the reproduction of the low-order statistics of oceanic surface boundary layer turbulence is investigated. Idealized horizontally homogeneous problems of surface-forced thermal convection and Langmuir turbulence are studied in detail. Model reduction is achieved with a Galerkin projection of the governing equations onto a subset of modes determined by proper orthogonal decomposition (POD). When applied to boundary layers that are horizontally homogeneous, POD and a horizontally averaged quasilinear approximation both assume flow features that are horizontally wavelike, making the pairing very efficient. For less than 0.2% of the modes retained, the reduced quasilinear model is able to reproduce vertical profiles of horizontal mean fields as well as certain energetically important second-order turbulent transport statistics and energies to within 30% error. Reduced-basis quasilinear statistics must approach the full-basis statistics as the basis size approaches completion; however, some quasilinear statistics resemble those found in the fully nonlinear simulations at smaller basis truncations. Thus, model reduction could possibly improve upon the accuracy of quasilinear dynamics.

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S. Fox, A. J. Pitman, A. Boone, and F. Habets

Abstract

Six modes of complexity of the Chameleon land surface model (CHASM) are used to explore the relationship between the complexity of the surface energy balance (SEB) formulation and the capacity of the model to explain intermodel variations in results from the Rhône-Aggregation Intercomparison Project (Rhône-AGG). At an annual time scale, differences between models identified in the Rhône-AGG experiments in the partitioning of available energy and water at the spatial scale of the Rhône Basin can be reproduced by CHASM via variations in the SEB complexity. Only two changes in the SEB complexity in the model generate statistically significant differences in the mean latent heat flux. These are the addition of a constant surface resistance to the simplest mode of CHASM and the addition of tiling and temporally and spatially variable surface resistance to produce the most complex model. Further, the only statistically significant differences in runoff occur following the addition of a constant surface resistance to the simplest mode of CHASM. As the time scale is reduced from annual to monthly, specific mechanisms begin to dominate the simulations produced by each Rhône-AGG model and introduce parameterization-specific behavior that depends on the time evolution of processes operating on longer time scales. CHASM cannot capture all this behavior by varying the SEB complexity, demonstrating the contribution to intermodel differences by hydrology and snow-related processes. Despite the increasing role of hydrology and snow in simulating processes at finer time scales, provided the constant surface resistance is included, CHASM's modes perform within the range of uncertainty illustrated by other Rhône-AGG models on seasonal and annual time scales.

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Michael S. Fox-Rabinovitz, Georgiy L. Stenchikov, Max J. Suarez, Lawrence L. Takacs, and Ravi C. Govindaraju

Abstract

The impact of introducing a realistic orographic forcing into a uniform- and variable-resolution stretched-grid GCM dynamical core is investigated by performing long-term and medium-range integrations. Comparisons are made between various stretched-grid simulations and a control that consists of a uniform grid integration at high resolution. These comparisons include those where the orography has and has not been filtered to eliminate small-scale noise. Results from the region of interest with highest resolution show that 1) the stretched-grid GCM provides an efficient downscaling over the area of interest, that is, it properly simulates not only large-scale but also mesoscale features; and 2) the introduction of orography has a greater impact than the effect of stretching. Results presented here suggest that dynamical core integrations with both uniform and stretched grids should consider orographic forcing as an integral part of the model dynamics.

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Michael S. Fox-Rabinovitz, Lawrence L. Takacs, Ravi C. Govindaraju, and Max J. Suarez

Abstract

The development of and results obtained with a variable-resolution stretched-grid GCM for the regional climate simulation mode are presented. A global variable-resolution stretched grid used in the study has enhanced horizontal resolution over the United States as the area of interest. The stretched-grid approach is an ideal tool for representing regional- to global-scale interactions. It is an alternative to the widely used nested-grid approach introduced over a decade ago as a pioneering step in regional climate modeling.

The major results of the study are presented for the successful stretched-grid GCM simulation of the anomalous climate event of the 1988 U.S. summer drought. The straightforward (with no updates) 2-month simulation is performed with 60-km regional resolution. The major drought fields, patterns, and characteristics, such as the time-averaged 500-hPa heights, precipitation, and the low-level jet over the drought area, appear to be close to the verifying analyses for the stretched-grid simulation. In other words, the stretched-grid GCM provides an efficient downscaling over the area of interest with enhanced horizontal resolution, in spite of degradation of skill over the coarser resolution far away from the area of interest. It is also shown that the stretched-grid GCM skill is sustained over the area of interest throughout the simulation extended to 1 yr.

The stretched-grid GCM, developed and tested in a simulation mode, is a viable tool for regional and subregional climate studies and applications.

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