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Steven J. Ghan

Abstract

The interaction between trace shortwave radiative absorbers and the dynamical circulation is shown to be linearly unstable for horizontally uniform basic states with a vertical gradient in the basic-state absorber mixing ratio. Two types of instability are identified, described as the advective mode and the propagating mode. The advective mode is unstable when the basic-state absorber mixing ratio decreases with height. Upward motion, high absorber concentration and warm temperature are typically in phase for this mode. Growth rates, which can be competitive with those associated with baroclinic instability, are largest for perturbations that are much shorter than the internal deformation radius. Thus, the requirement that the basic state be horizontally uniform is often satisfied for the advective mode. The propagating mode is unstable when the basic-state absorber mixing ratio increases with altitude. Propagating waves such as Rossby and inertia-gravity waves are amplified by the feedback with absorber transport and radiative heating. Growth rates for the propagating mode are usually bounded by the frequency of oscillation of the ambient wave, an important limitation for slowly propagating waves such as Rossby waves. Vertical transport of the absorber by the amplifying mode is down the basic-state absorber mixing ratio gradient in each case.

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Steven J. Ghan

Abstract

The basic theory of unstable radiative-dynamical interactions developed in a companion paid is extended to account for several complicating effects. Treating the effect of absorber perturbations on the shortwave radiative heating rate at all levels, rather than simply locally, is shown to either enhance or suppress the radiative-dynamical instability, depending on the perturbation vertical wavelength. Dissipative processes generally reduce or eliminate the instability, although in one case mechanical dissipation can actually enhance the instability. Vertical shear generally suppresses the instability for radiative-dynamical feedback rates greater than the adiabatic growth rate associated with baroclinic instability, and enhances the instability for feedback rates less than the adiabatic growth rate. The radiative-dynamical interaction generally increases the growth ate beyond that associated with baroclinic instability, but the growth rate never exceeds the sum of the feedback rate and the adiabatic growth rate. Scattering of sunlight by the radiative constituent can either suppress or enhance the instability, depending particularly on the solar zenith angle. The effect of vertical variations in the radiative-dynamical feedback parameter is also assessed.

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Steven J. Ghan

Abstract

The relationship between the eddy heat flux and vertical shear in the extratropical atmosphere is studied by developing various linear stochastic models fitted to the observed January and July Northern Hemispheric data. Models are univariate or bivariate, continuous or discrete. An objective procedure selects the second-order bivariate model as most appropriate in midlatitudes. The first-order continuous bivariate model indicates that feedback within the flux-shear system is comparable to damping on short time scales (days), but is somewhat weaker on intermediate time scales (weeks).

Observational errors are found to influence several results. When these errors are not accounted for, dissipation is found to be quite strong, with a damping time for the shear of 1 to 3 days and, in apparent contradiction to the results of viscid finite amplitude models of baroclinic instability, damping of the eddy heat flux is somewhat stronger in July than in January. When observational errors are considered, the damping time for the shear is about five days and damping of the flux in midlatitudes is nearly equal for January and July.

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Steven J. Ghan and Timothy Shippert

Abstract

A global atmosphere–land model with an embedded subgrid orography scheme is used to simulate the period 1977–2100 using ocean surface conditions and radiative constituent concentrations for a climate change scenario. Climate variables simulated for multiple elevation classes are mapped according to a high-resolution elevation dataset in 10 regions with complex terrain. Analysis of changes in the simulated climate leads to the following conclusions. Changes in surface air temperature and precipitation differ from region to region in a manner similar to simulations without the subgrid scheme. Subgrid elevation contributes little to spatial variability of the change in temperature and the relative change in precipitation. In some regions somewhat greater warming occurs at higher elevations because of the same tendency in the free troposphere, but in others greater warming occurs near the melting level where snow albedo feedback amplifies the warming. Changes in snow water are highly dependent on altitude because of its nonlinear dependence on changes in the melting level. Absolute changes usually increase with altitude because more snow is currently available for depletion, but for extremely cold conditions the simulated warming is insufficient to increase melting. Relative changes in snow water always decrease with altitude as the likelihood that a warming will enhance melting or change the phase of precipitation decreases with decreasing temperature at higher altitudes. In places where snow accumulates, an artificial upper bound on snow water (which is required in any climate model that does not treat lateral snow transport) limits the sensitivity of snow water to climate change considerably. The simulated impact of climate change on regional mean snow water varies widely, with little impact in regions in which the upper bound on snow water is the dominant snow-water sink, moderate impact in regions with a mixture of seasonal and pemanent snow, and profound relative impacts on regions with little permanent snow.

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Steven J. Ghan and Donald R. Collins

Abstract

A method of retrieving vertical profiles of cloud condensation nuclei (CCN) concentration from surface measurements is described. Surface measurements of the CCN concentration are scaled by the ratio of the backscatter (or extinction) vertical profile to the backscatter (or extinction) at or near the surface. The backscatter (or extinction) profile is measured by Raman lidar and is corrected to dry conditions using the vertical profile of relative humidity (also measured by Raman lidar) and surface measurements of the dependence of backscatter (or extinction) on relative humidity. The method assumes that the aerosol composition and the shape of the aerosol size distribution at the surface are representative of the vertical column. Aircraft measurements of aerosol size distribution are used to test the dependence of the retrieval on the uniformity of the shape of the aerosol size distribution. The retrieval is found to be robust for supersaturations less than 0.02% but breaks down at higher supersaturations if the vertical profile of the shape of the aerosol size distribution differs markedly from the shape of the distribution at the surface. Such conditions can be detected from the extinction/backscatter ratio.

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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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Dennis L. Hartmann and Steven J. Ghan

Abstract

An objective method is used to isolate instances of blocking ridges from instances of transient ridges for 10 winters of Northern Hemisphere data. The vorticity budgets and heat budgets of the blocking ridge cases are compared statistically with those of transient ridges for both the Pacific and Atlantic oceanic regions. It is demonstrated that there are statistically significant differences in the vorticity and heat budgets of blocking compared to transient ridges, which may be related primarily to changes in zonal advection. While the dominant scale of the vorticity pattern associated with blocking is represented by a zonal scale of about 60° of longitude, the local zonal wind reductions associated with blocks are contributed primarily by wavenumber 1 (∼40%) with smaller contributions from the zonal mean (∼20%). The most important mechanisms for sustaining blocking ridges are found to be different in the Atlantic and Pacific regions. In the Pacific the major difference between blocking and transient ridges is reduced eastward advection of vorticity, whereas in the Atlantic the blocking ridges are more baroclinic than the transient ridges.

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Steven J. Ghan and Richard C. Easter

Abstract

Bulk cloud microphysics parameterizations typically employ time steps of a few tens of seconds. Although the computational burden of these parameterizations is acceptable for the 1-day mesoscale cloud simulations for which they were designed, the time steps are unacceptably short for direct application of them parameterizations to global-climate simulation. To increase the computational efficiency of bulk cloud microphysics parameterizations, we introduce two approximations that are appropriate for stratiform clouds. By diagnosing rather than predicting rain and snow concentrations and by assuming instantaneous melting of snow, we have found that the permissible time step is increased tenfold (to 2–6 min) with little loss in accuracy for vertical motions and time scales characteristic of those resolved by general circulation models (GCMs). Such time steps are sufficiently long to permit application of bulk cloud microphysical parameterizations to GCMs for multiyear global simulations. However, we also find that the vertical resolution must be considerably finer (100–200 m) than that currently employed in GCMs.

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Steven J. Ghan, Xindi Bian, and Lisa Corsetti

Abstract

The low-level jet frequently observed in the Great Plains of the United States forms preferentially at night and apparently influences the timing of thunderstorms in the region. The authors have found that both the European Centre for Medium-Range Weather Forecasts general circulation model and the National Center for Atmospheric Research Community Climate Model simulate the low-level jet rather well, although the spatial distribution of the jet frequency simulated by the two GCMs differs considerably. Sensitivity experiments have demonstrated that the simulated low-level jet is surprisingly robust, with similar simulations at much coarser horizontal and vertical resolutions. However, both GCMs fail to simulate the observed relationship between clouds and the low-level jet. The pronounced nocturnal maximum in thunderstorm frequency associated with the low-level jet is not simulated well by either GCM, with only weak evidence of a nocturnal maximum in the Great Plains.

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Karl E. Taylor and Steven J. Ghan

Abstract

A set of general circulation model simulations is analyzed to determine how cloud distribution and cloud radiative properties might change as climate warms and to isolate and quantify the various feedback effects of clouds on climate sensitivity. For this study the NCAR Community Climate Model (CCM1) was modified so that the cloud radiative properties (albodo, emissivity, and absorptivity) are no longer prescribed, but are functions of the cloud liquid water content. Following the Cess and Potter approach for estimating climate sensitivity, we consider results from two sets of simulations. In one set, cloud liquid water is diagnosed from the simulated condensation rate and thus is free to vary with condensation, while in the other set, the cloud liquid water content is a fixed field (dependent only on altitude and latitude) that is obtained by averaging the results of the first set of experiments. The experiments make it possible to isolate the effects of cloud liquid water feedback.

We find that changes in cloud amount, cloud liquid water content, and cloud distribution (especially in the vertical) are all of comparable importance, but some of these changes provide a positive feedback while others provide a negative feedback. Separation of cloud feedback into individual components makes it clear that in this model as climate warms the general increase in the liquid water content of each cloud layer is partially offset by an upward shift in cloud altitude. The effects of clouds on longwave radiation also generally tend to cancel the effects on shortwave radiation. Consequently, the net cloud feedback represents a residual of several offsetting effects, which nevertheless is large enough to nearly double the sensitivity of the simulated climate. Another important conclusion is that it is impossible to parameterize cloud albedo in terms of average cloud liquid water content because the average liquid water is dominated by the thicker clouds, whereas the average albedo depends on clouds with relatively little liquid water as well.

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