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Gary T. Bates

Abstract

A mesoscale model has been used to simulate an observed trough system which crossed the Rocky Mountains between 24 and 27 March 1983. Numerical simulations have been conducted with and without topography to isolate the effects that the mountains have on the cyclone and the subsequent lee cyclogenesis that occurs in eastern Colorado. The applicability of two theories to describe processes occurring in the cyclone as it crosses the mountains have been investigated: 1) superposition or masking of the cyclone by a topographically induced anticyclone, and 2) upper-level forcing coupled with low-level blocking.

In this case study, the low-level absolute vorticity of the cyclone over the region of the Rocky Mountains is less in the simulations with topography than in the simulations without. However, later in the simulations as the cyclone moves away from the mountains, vorticity differences between the simulations decrease markedly. In association with decreased vorticity, higher geopotential heights are found at all tropospheric levels over the mountains in the runs with topography. These height differences are similar in magnitude and character to the anticyclone that develops when the zonally averaged mean flow is allowed to impinge on the topography until a quasi-equilibrium is reached.

An upper-tropospheric jet streak and associated indirect circulation are present in this March 1983 case and are simulated by the model. However, comparison of the mountain and no-mountain simulations indicates the presence of topography does not result in significant blocking of the low-level flow or alter the magnitude of the indirect circulation in the lee region. This lack of sensitivity may be a function of the relatively smooth topography employed in the model.

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Filippo Giorgi and Gary T. Bates

Abstract

As part of an ongoing study of the regional climate and hydrology of the southwestern United States, in this paper we investigate the systematic biases of two versions of the PSU/NCAR mesoscale model (MM4). These are a standard version and one that includes a more detailed treatment of radiative transfer, surface physics, and soil hydrology. We simulated the period 1–30 January 1979, in which nine Pacific storms moved across the western United States. Results from both model versions are compared to the large scale analysis used to provide initial and lateral boundary conditions. Both models show a lower tropospheric cold bias of 1–3 K near the surface over land and an upper tropospheric warm bias of less than 1 K, which suggest high model stability and reduced vertical mixing. The model atmospheres are wetter than that of the analysis, particularly in the lower troposphere and over the ocean. The wind magnitude bias is positive near the surface (∼1.5–3 m s−1), negative in the upper troposphere (∼−1.5 m s−1) and positive above the jet-level (∼3 m s−1). The wind direction bias is small throughout the model atmospheres except at the top model layer near 10 mb. These results indicate that the model evaporation and nighttime land surface sensible heat fluxes are larger compared to the analysis, while the daytime sensible heat fluxes and surface wind drag are smaller. The biases are generally smaller in the midtroposphere than in the lower troposphere and in the stratosphere. In general, both models capture most regional features of the orographic forcing of precipitation by the western United States topography quite well. Compared to station data, precipitation amounts tend to be overpredicted. Daily precipitation threat scores for various precipitation thresholds vary between 0.315 and 0.385. The threat scores for the 30-day precipitation, more indicative of the model's ability to simulate climatological precipitation averages, are higher, ⩾0.8 for light precipitation to ∼0.5 for moderate to heavy precipitation. Snow depths predicted by the augmented model also show realistic regional features. In general, the inclusion of the new physics package did not strongly affect precipitation prediction or the temperature, moisture, and wind midtropospheric biases. In the boundary layer over land, however, the augmented model was significantly colder and drier than the standard model due to larger nighttime surface sensible heat fluxes and lower evaporation rates. The regional hydrologic budgets simulated by the soil hydrology package of the augmented MM4 appear realistic in many respects, although verification is difficult at the present model resolution.

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Gary T. Bates and Donald R. Johnson

Abstract

The mass and angular momentum budgets of extratropical cyclones within three numerically simulated adiabatic linear baroclinic waves are investigated through quasi-Lagrangian diagnostics. The waves analyzed, wavenumbers 5, 11 and 17, were chosen to represent a range of baroclinically active horizontal scales observed in the atmosphere within which cyclonic circulations develop. Results are compared with diagnostics of an observed atmospheric cyclone in order to examine the extent to which linear baroclinic instability theory explains extratropical cyclone development in the real atmosphere.

Diagnostics are computed in both isobaric and isentropic coordinates. In the isobaric framework, inward mass and angular momentum transport occurs in the lower troposphere of the linear cyclones while the transport is outward in the upper troposphere Upward transport of these quantities throughout the troposphere redistributes these quantities vertically. Within isobaric coordinates the mass and angular momentum budgets of the cyclones within linear waves resemble results from actual cyclones.

Analysis of these linear model cyclones in isentropic coordinates shows that the mass and angular momentum transport is inward in upper isentropic layers and outward below. Angular momentum is transferred from upper to lower isentrople layers by pressure and inertial torques within the baroclinic structure. The vertical distribution of transport process and torques within linear baroclinic wave is in general agreement with the results occurring during the dry-baroclinic phase of development in a 1971 Alberta cyclone but differs markedly from the distribution observed during the moist-baroclinic phase. The major dissimilarity between the development of the model cyclones and leeside dry-baroclinic cyclogenesis is the existence of a much stronger eddy component of horizontal angular momentum transport in the model cyclones. A strong eddy transport is required for the cyclones to simultaneously deepen and spin-up over the flat terrain in the linear model, while such a transfer is not necessary for development over sloped topography.

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Melanie A. Wetzel and Gary T. Bates

Abstract

Satellite image datasets and regional climate model results are intercompared for evaluation of model accuracy in the simulation of cloud cover. Both monthly average and individual simulation times are analyzed. To provide a consistent comparison, satellite data are first mapped into the model's geographic projection, grid domain, and resolution. It is found that September 1988 monthly average cloud fraction results from the model simulations correspond to observations, in both spatial pattern and magnitude, with bias less than ±20% cloud fraction over the entire inland West. Agreement in the pattern of cloud fraction also is evident for monthly average cloud fraction in July, but there is a negative bias of 10%–30% cloud fraction in the model diagnosis of cloud cover. Correlations between the spatial distributions of model-derived and observed cloud fractions are found to exceed 0.80 for certain geographic regions of the West, and these correlations are largest over mountainous areas during summer. Case studies of a series of daily cloud cover demonstrate the ability of the model to simulate the effects of frontal passage on cloud distribution. The ability of the RegCM1 to simulate daily cloud fraction and diurnal cloud evolution is somewhat weak for the summer convective season. It is anticipated that a more recent version of the regional climate model may improve the simulation of summer season cloud cover, through changes in cloud parameterization and improvements in model resolution.

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Gary T. Bates and Gerald A. Meehl

Abstract

The effect of a doubling of atmospheric CO2 on the characteristics of the 500 mb height field and persistent height anomalies associated with blocking phenomena are investigated in two experiments with the NCAR Community Climate Model (CCM) coupled to a simple ocean mixed layer. This version of the CCM with a seasonal cycle, computed hydrology and the simple mixed layer ocean produces a somewhat improved simulation compared with earlier model versions in spite of a lack of ocean heat transport and overextensive sea ice. In a control experiment with present amounts of CO2, 500 mb height statistics compared best with observations during winter while summer is not simulated as well. In a second experiment, where CO2 is doubled, the troposphere experiences warming most everywhere and 500 mb heights increase, especially near areas where sea ice has retreated and surface air temperature warming is greatest. In most regions of significant blocking activity, standard deviations of 500 mb height and blocking activity are decreased in all seasons. In the Northern Hemisphere, there are also increases in 500 mb standard deviations and blocking activity in the North Pacific during winter, and an increase in standard deviations at high latitudes over Asia and Alaska during summer. There also is a coincident increase of blocking over Asia at those latitudes, but a decrease of blocking over Alaska in summer, partially due to increased variability on shorter time scales there. Thus, in this hemisphere the incidence of blocking does not seem to change significantly with increased CO2, but the centers of action move geographically. On the other hand, in the Southern Hemisphere, blocking activity is generally reduced.

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Filippo Giorgi, Gary T. Bates, and Steven J. Nieman

Abstract

This paper presents a validation analysis of the climatology of a version of the National Center for Atmospheric Research-Pennsylvania State University limited-area model (MM4) developed for application to regional climate simulation over the western United States. Two continuous multiyear simulations, for the periods 1 January 1982–31 December 1983 and 1 January 1988–25 April 1989, were performed over this region with the MM4 driven by ECMWF analyses of observations and run at a horizontal resolution of 60 km. The model used in these simulations includes horizontal diffusion on terrain-following σ coordinates, a Kuo-type cumulus parameterization, sophisticated radiative transfer and surface physics-soil hydrology packages, and a relaxation boundary- conditions procedure.

Model-produced surface air temperatures, precipitation, and snow depths were compared with observations from about 390 stations distributed throughout the western United States. The base-model run reproduced the seasonal cycle of temperature and precipitation well. Monthly and seasonal temperature biases were generally less than a few degrees. The effects of topography on the regional distribution of precipitation were also well reproduced, although local detail was in several instances poorly captured. When regionally averaged, absolute model-precipitation biases were mostly in the range of 10% –50% of observations. The model generally simulated precipitation better in the cold season than in the warm season, and over coastal regions than in the continental interior. The simulated seasonal cycles of snowpack formation and melting were realistic, although modeled and observed snow-depth values differed significantly locally.

Over the Rocky Mountain regions the model reproduced wintertime precipitation amounts well but overpredicted summertime precipitation. Because of this overprediction of summertime precipitation, a version of the model was also tested in a simulation from 30 May 1988–26 December 1988 in which the horizontal diffusion coefficients were reduced over topographical gradients and an inflow-outflow lateral boundary condition was implemented for water vapor. These modifications were found to provide an improved simulation of summer precipitation while not substantially altering wintertime precipitation.

This work shows that it is feasible to perform good quality, multiyear simulations with current limited-area models and, therefore, that it is feasible to apply such models to climate studies.

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Maurice L. Blackmon, Steven L. Mullen, and Gary T. Bates

Abstract

A variety of statistical comparisons is made between the fluctuations occurring in a 1200-day perpetual January simulation of a spectral general circulation model and those occurring in a 20-winter data set. Attention is focused on the persistent anomalies with lifetimes greater than one week. Over the Atlantic and Pacific oceans, we show that there is generally good agreement between modeled and observed persistent anomalies in their frequency of occurrence and mean lifetime. However, we note a striking deficiency in the simulation of persistent anomalies over the Soviet Union. We focus further on a subset of persistent anomalies blocking highs, and show that the model blocks have vertical structure in agreement with observations. We also show an example of the development of a blocking event which follows an episode of explosive cyclogenesis. Finally, we show an example of the interaction of low-and high-frequency eddies during a period of blocking and at the termination of model blocking.

The results of this study demonstrate that the internal dynamics and physics of the model by themselves are able to generate quite realistic blocking episodes over the wintertime North Atlantic and Pacific oceans. We conclude that blocking is a naturally occurring, internally generated phenomenon of the model and believe that model data can be used as a reasonable proxy for observations to study this phenomenon.

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Gary T. Bates, Filippo Giorgi, and Steven W. Hostetler

Abstract

This paper describes a set of numerical experiments aimed at evaluating the feasibility of applying a version of the National Center for Atmospheric Research-Pennsylvania State University regional model (MM4) to regional climate simulation over the Great Lakes Basin. The objectives of this initial modeling investigation are 1) to examine whether the MM4 can capture the primary forcing exerted by the Great Lakes on the regional climate and 2) to evaluate what model resolution and configuration are needed to simulate such forcing. Simulations over the Great Lakes region are conducted with and without representation of the lakes at four model gridpoint resolutions ranging from 15 to 90 km. One experiment at 60-km resolution is discussed in which a one-dimensional thermal eddy diffusion model is interactively coupled to the MM4 to represent the lakes. Initial and lateral boundary conditions necessary to drive these simulations are provided by European Centre for Medium-Range Weather Forecasts (ECMWF) analyses of observations. All simulations conducted are 10 days in length, from 22 December 1985 to 1 January 1986.

When driven with data from ECMWF analyses of observations, the climate version of the MM4 reproduces the basic characteristics of the distribution of lake-effect precipitation over the Great Lakes Basin. Differences between simulations with and without the lakes represented indicate that the lakes accounted for approximately 25% of the precipitation over the basin during the 10-day period simulated. Over localized areas, identified as the major snowbelts downwind from the lakes, lake effects were responsible for 50%–70% of the precipitation.

Basinwide precipitation did not vary greatly among the simulations with resolutions of 60, 30, and 15 km, although biases between model results and station observations did decrease slightly with increasing model resolution. Basinwide maximum and minimum temperature biases decreased more markedly with finer resolution. In the snowbelt regions downwind from the lakes, precipitation was underforecast at all four model resolutions, but precipitation generally increased with finer resolution. Differences between the results from the simulations at the three finest resolutions were greater over snowbelt regions than over the basin as a whole.

A simulation was conducted with the MM4 coupled to a lake model in an interactive two-way nested configuration. The implementation of this coupling was accomplished in a straightforward manner, with no model tuning required, and added very little to the computation time needed for the MM4 system. This coupled modeling system was found to produce realistic distributions of lake surface temperatures, evaporation rates, and ice thicknesses across the lakes. In climate simulations where the MM4 is nested in a general circulation model (GCM), we believe that the use of this coupled modeling system is preferable to specifying lake parameters by interpolation from GCM output. The next step in this work is to conduct a simulation of at least one annual cycle over the region to more fully test the coupled MM4-take model system.

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Filippo Giorgi, Gary T. Bates, and Steven J. Nieman

As part of the development effort of a regional climate model (RCM) for the southern Great Basin, this paper presents a validation analysis of the climatology generated by a high-resolution RCM driven by observations. The RCM is a version of the National Center for Atmospheric Research/Pennsylvania State University mesoscale model, version 4 (MM4), modified for application to regional climate simulation. Two multiyear simulations, for the periods 1 January 1982 to 31 December 1983 and 1 January 1988 to 25 April 1989, were performed over the western United States with the RCM driven by European Centre for Medium-Range Weather Forecasts analyses of observations. The model resolution is 60 km. This validation analysis is the first phase of a project to produce simulations of future climate scenarios over a region surrounding Yucca Mountain, Nevada, the only location currently being considered as a potential high-level nuclear-waste repository site.

Model-produced surface air temperatures and precipitation were compared with observations from five southern Nevada stations located in the vicinity of Yucca Mountain. The seasonal cycles of temperature and precipitation were simulated well. Monthly and seasonal temperature biases were generally negative and largely explained by differences in elevation between the observing stations and the model topography. The model-simulated precipitation captured the extreme dryness of the Great Basin. Average yearly precipitation was generally within 30% of observed and the range of monthly precipitation amounts was the same as in the observations. Precipitation biases were mostly negative in the summer and positive in the winter. The number of simulated daily precipitation events for various precipitation intervals was within factors of 1.5–3.5 of observed. Overall, the model tended to overestimate the number of light precipitation events and underestimate the number of heavy precipitation events. At Yucca Mountain, simulated precipitation, soil moisture content, and water infiltration below the root zone (top 1 m) were maximized in the winter. Evaporation peaked in the spring after temperatures began to increase.

The conclusion drawn from this validation analysis is that this high-resolution RCM simulates the regional surface climatology of the southern Great Basin reasonably well when driven by meteorological fields derived from observations.

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Gary T. Bates, Martin P. Hoerling, and Arun Kumar

Abstract

Dynamical methods are used to investigate atmospheric teleconnections associated with extreme seasonal precipitation anomalies over the central United States during April–June. The importance of sea surface temperature (SST) anomalies in forcing atmospheric teleconnections is specifically addressed through analyses of atmospheric general circulation model (GCM) simulations forced with the monthly varying SSTs of the years 1950–98. The results from three different models, each run in ensemble mode, are compared with observations of extreme April–June precipitation events in the central United States during the last half of the twentieth century.

Analysis of GCM simulations of April–June 1988 indicates that the atmospheric circulation anomalies associated with the 1988 drought were not forced by SST anomalies and that the coexistence of central U.S. drought and La Niña during that spring was coincidental. Likewise, composite analysis reveals no SST forcing for the teleconnections associated with extreme dry spring seasons over the central United States during the last half of the twentieth century in either observations or GCMs. Nonetheless, this characteristic teleconnection pattern of the composite analysis resembles the circulation anomalies of 1988. The results imply that such drought events and the teleconnections related with them have little SST-based predictability.

A somewhat different conclusion is drawn regarding the role of tropical SSTs in the occurrence of extreme wet spring seasons over the central United States. Simulations of the 1993 flood period exhibit skill in reproducing the seasonal circulation anomalies over the Pacific–North American region, and the ensemble mean precipitation anomalies in one GCM nearly replicate the observed strength and distribution of positive rainfall anomalies over the United States. Further composite analysis of extreme wet spring seasons over the last half of the twentieth century confirms the impression gathered from the 1993 case study, with observations and all three GCMs possessing positive tropical east Pacific SST anomalies in conjunction with extreme wet spring seasons over the central United States. Some SST-based potential predictability of extreme wet springs over the central United States consequently exists.

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