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William J. Gutowski Jr.

Abstract

A model designed for studying the interaction between vertical eddy beat fluxes and the vertical temperature structure in midlatitudes is presented. A temperature profile is obtained for the model by computing an equilibrium among heating rates from simplified representations of the large-scale vertical eddy heat flux, moist convection and radiation. In particular, the eddy flux profile is obtained from the quasi-geostrophic, linear baroclinic instability of a single wave, and the eddy amplitude is either specified or else obtained from a closure assumption. Tests using a variety of input conditions indicate that the most appropriate single wave to use is the most unstable mode of the instability problem.

The model's temperature and Brunt-Väisälä frequency profiles am compared with observed profiles. The most unstable mode computations, in particular, reproduce well the observed profiles for both winter and summer conditions. Model runs with various eddy amplitudes show that particular aspects of the observed profiles, such as the strong decrease of Brunt- Väis¨lä frequency with height in the lower troposphere, can be understood in terms of the vertical eddy flux's influence on temperature structure. Also, the eddy flux tends to alter the lapse rate more than the tropopause height as its strength varies. This particular influence is part of a negative feedback between temperature structure and the eddy flux strength: an increase in flux strength causes the lapse rate to decrease, which in turn causes the flux to weaken. The lapse rate alteration occurs primarily in the lower troposphere, indicating that the eddy flux places a significant constraint on how the vertical temperature structure in midlatitudes change when the climate changes.

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William J. Gutowski Jr.

Abstract

An assessment is made of how baroclinically unstable waves might stabilize a zonal mean Bow by adjusting vertical temperature profiles. To this end, temperature profiles for a continuous atmosphere an derived which are baroclinically stable. Features of these profiles of relevance to the atmosphere are discussed, and a requirement is derived for the minimal amount of profile adjustment necessary to stabilize the zonal mean flow. Neither the atmosphere nor realistic models can perform the profile adjustment necessary to attain a rigorously stable state, but a model used here which includes vertical heat fluxes from baroclinically unstable waves can attain an effectively stable state that is very similar to the rigorously stable state. Observed temperature profiles, as well as model profiles that are not effectively stable, also display features of the adjusted profiles, indicating that much of the temperature variation with height in the midlatitude troposphere can be understood in terms of the adjustment. The model results also show that vertical eddy heat fluxes arising from baroclinic instability try to produce the adjustment to a stable state but other processes, most notably those controlling temperature near the surface, prevent the fluxes from being successful. Finally, the model results are used to show similarities between the continuous atmosphere adjustment presented here and the two-layer atmosphere adjustment studied by others.

The work presented here suggests that vertical heat fluxes from baroclinically unstable waves can produce significant variations of midlatitude lapse rates in both height and time; midlatitude models that omit these variations may be missing an important aspect of wave-mean flow interaction.

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Claire Steinweg and William J. Gutowski Jr.

Abstract

A matrix of four GCM–RCM combinations from the NARCCAP project is examined for changes in heat stress between contemporary and future scenario climates in the greater St. Louis region in Missouri. The analysis also compares the contemporary simulations with observation-based results from the North American Regional Reanalysis. The character of heat-stress days in one of the RCMs, the CRCM, tends to be like that of heat-stress days in the North American Regional Reanalysis, with high temperatures accompanied by high humidity. In contrast, heat-stress days in the other RCM, the Weather Research and Forecasting Model with Grell-Devenyi Cumulus Scheme (WRFG), have high temperature, but typically the humidity is similar to or even slightly drier than climatological values.

Although specific magnitudes of change differ between the simulations, all show a marked increase in projected heat stress, from a variety of perspectives. Increases in temperature contribute more to these increases than do increases in humidity, though both are relevant. All simulations agree that the frequency of excessive heat advisories and excessive heat warnings as defined by the National Weather Service could increase by midcentury, with multiple excessive heat advisories occurring every year. The day of first heat stress each summer could occur 3–4 weeks earlier as part of a more prolonged period when the region might experience heat stress each year. Although St. Louis has adopted measures to reduce health threats during heat-stress events, the measures consume human and economic resources; much more frequent and longer-lasting heat-stress events in the future have the potential to impose substantial costs on the region.

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Song Yang and William J. Gutowski Jr.

Abstract

Plumb's formulation of the stationary wave activity flux is used to determine how well versions of the GFDL and NCAR general circulation models simulate the sources, sinks, and horizontal propagation of atmospheric stationary waves, which play an important role in determining regional climate. The wave activity flux provides insight into the simulation of nondynamic as well as dynamic processes in these models. Model performances for current climate simulations are evaluated with respect to NMC analyses averaged over 1978–1990.

The models fare best when the stationary wave forcing is strongest, that is, in the wintertime Northern Hemisphere, where they reproduce the observed three-branch structure of upward wave activity flux. For the Northern Hemisphere summer and the Southern Hemisphere in both summer and winter, the models show less agreement with observations, although they do simulate the generally downward flux observed during Northern Hemisphere summer, which the analysis suggests is caused by convection. C02-doubling changes in the wave activity flux show little consistency between the two models. The analysis suggests that accurate modeling of stationary wave activity flux is strongly dependent on diabatic forcing, especially that occurring in storm tracks. Improving the simulation of stationary wave activity forcing requires a much better understanding of the physics governing storm tracks and latent heat release in the atmosphere, so that improvements in stationary wave simulation in these models will not occur by simply increasing model resolution.

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Sho Kawazoe and William J. Gutowski Jr.

Abstract

The authors analyze the ability of global climate models (GCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel ensemble to simulate very heavy daily precipitation and its supporting processes, comparing them with observations. Their analysis focuses on an upper Mississippi region for winter (December–February), when it is assumed that resolved synoptic circulation governs precipitation. CMIP5 GCMs generally reproduce well the precipitation versus intensity spectrum seen in observations to intensities as strong as 20 mm day−1. Most models do not produce the highest precipitation intensities seen in observations. Models show good agreement at the 95th percentile, while the coarsest resolution models generally show lower precipitation at high-intensity thresholds, such as the 99.5th percentile. There is no dominant month for simulated very heavy events to occur, although observed very heavy events occur most frequently in December. Further analysis focuses on precipitation events exceeding the 99.5th percentile that occur simultaneously at several points in the region, yielding so-called “widespread events.” Examination of additional fields during widespread very heavy events shows that the models produce these events under the same physical conditions seen in the observations. The coarsest models generally produce similar behavior, although features have smoother spatial distributions. However, the resolution in itself could not be identified as a major reason that separates one model from another. The capabilities of the CMIP5 GCMs examined here support using them to assess changes in very heavy precipitation under future climate scenarios.

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William J. Gutowski Jr. and Weidong Jiang

Abstract

The authors examine the role of convection in the dynamics of eddy life cycles through numerical experiments using initial states that are baroclinically and conditionally unstable in midlatitudes. The location of wave-induced convection and its influence on the growing wave depends on how strongly the wave is coupled to the lower boundary through surface fluxes. For all three convective schemes used here (Emanuel, modified Grell, modified Kuo), convective destabilization is favored in the wave’s warm sector when there are no surface fluxes included in the simulation and in the cold sector when there are. Convection is also shallower when it occurs in the cold sector, though still precipitating. For simulations using Emanuel convection, the relatively shallow convection plays a central role in a water cycle wherein 1) evaporation gives moisture to the cold sector’s boundary layer, 2) convection pumps some of the moisture into the lower troposphere above the boundary layer, 3) the large-scale circulation transports the moisture eastward and upward into the wave’s warm sector, and 4) stable precipitation condenses the moisture into precipitation. The additional condensation catalyzes a more energetic life cycle by inducing stronger vertical motion and, hence, a greater conversion of available potential energy to kinetic energy. This enhancement, however, is parameterization dependent, with the key factor being how much lower-tropospheric moistening a convection scheme produces.

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John P. Iselin and William J. Gutowski Jr.

Abstract

The STORM-FEST (Fronts Experiment Systems Test) rawinsonde data were analyzed to determine the abundance and characteristics of moist layers within the troposphere. A moist layer was defined as a local maximum in relative humidity with lower relative humidity air above and below. Moist layers under the criteria occur in over half the soundings with an average location between 600 and 500 mb and an average thickness of approximately 120 mb. The layers also appeared to be more nearly aligned with isentropic, rather than isobaric, surfaces. Compositing of relative humidity profiles with a layer at approximately the same level showed an increase in lapse rate at the top of moist layers indicating that the layers are contained by dynamic mechanisms. In addition, there was no diurnal cycle to the characteristics of the layers. These factors suggest a close relationship between the layers and large-scale dynamics. An examination of spatial continuity suggests a horizontal scale of a few hundred kilometers. Their appearance poses a challenge for numerical modeling of atmospheric water vapor. Furthermore, limitations of the two types of rawinsonde instruments used in STORM-FEST are apparent in some characteristics of the layers, thus indicating instrumentation challenges posed by these structures for observing the atmospheric branch of the hydrological cycle.

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Weidong Jiang and William J. Gutowski Jr.

Abstract

The influence of convective heating on baroclinic instability in the presence of surface sensible heat and moisture fluxes is investigated. Following previous numerical work, a two-dimensional continuous model on an f plane incorporates diabatic heating effects due to cumulus convection and surface sensible heat flux using parameterizations based on a wave-induced unstable boundary layer and associated moist convective destabilization. The temperature-damping effect of surface sensible heat flux is assumed to decrease exponentially with height, and the vertical distribution of convective heating uses a prescribed profile. The atmosphere is assumed to overlie an oceanic surface. In this configuration, convective heating occurs in the wave’s cold sector.

General forms of the dispersion relation and eigenfunction are derived analytically. Results show that the most unstable wave is modified by the effect of convective latent heating. With weak convection, the wave’s structure does not change much, while the wave’s energy generation is hampered by the negative contribution of convection. In the presence of moderate convective heating, although the wave’s energy generation is decreased by convection, the wave adjusts its structure to minimize the negative effect of convection and retain growth. In the region with strong convective heating, convective heating significantly changes the wave’s temperature structure. Above and below the strong heating region, the wave structure still retains some features of the Eady mode. The results have bearing on how the structure of oceanic storms may be altered by convection.

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Richard D. Rosen and William J. Gutowski Jr.

Abstract

The possible impact of doubling C02 on the zonal-mean zonal winds and the angular momentum of the atmosphere is examined using general circulation model output archived by the Goddard Institute for Space Studies, the National Center for Atmospheric Research, and the Geophysical Fluid Dynamics Laboratory. Whereas the emphasis in most previous studies with these models has been placed on the temperature and precipitation changes expected from a doubled-CO2 scenario, the intent here is to investigate some of the dynamical consequences predicted by them models, especially within the tropics where the zonal-wind and temperature changes are less tightly coupled than elsewhere.

Comparisons among the three models of the difference in zonal-mean zonal winds between 2×C02 and 1×C02 simulations indicate a common tendency when C02 is doubled for winds to become more easterly in much of the tropics during June-July-August. Less of a consensus for the tropics emerges for December-January-February, perhaps as a result of differences among the models' basic climatologies for the zonal-wind field. In general, however, changes predicted for the zonal winds in the tropics and elsewhere are comparable to the interannual variability currently observed, suggesting that these changes ought to become detectable eventually.

Largely because of the tropical wind changes, decreases in the troposphere's relative angular momentum accompany a doubling of C02 in all the model runs. The amplitude of the decrease is typically a considerable fraction of a model's seasonal cycle and, in some cases, is large enough that a measurable change in the length of day could result. Although the possibility of an anthropogenic effect on earth's rotation is noteworthy, such a prediction must be regarded as tentative in light of the shortcomings found in the models’ zonal-wind climatologies and the differences in their zonal-mean responses.

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Sho Kawazoe and William J. Gutowski Jr.

Abstract

The authors analyze the ability of the North American Regional Climate Change Assessment Program's ensemble of climate models to simulate very heavy daily precipitation and its supporting processes, comparing simulations that used observation-based boundary conditions with observations. The analysis includes regional climate models and a time-slice global climate model that all used approximately half-degree resolution. Analysis focuses on an upper Mississippi River region for winter (December–February), when it is assumed that resolved synoptic circulation governs precipitation. All models generally reproduce the precipitation-versus-intensity spectrum seen in observations well, with a small tendency toward producing overly strong precipitation at high-intensity thresholds, such as the 95th, 99th, and 99.5th percentiles. Further analysis focuses on precipitation events exceeding the 99.5th percentile that occur simultaneously at several points in the region, yielding so-called “widespread events.” Examination of additional fields shows that the models produce very heavy precipitation events for the same physical conditions seen in the observations.

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