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Michael J. Dickinson and David J. Knight

Abstract

A two-dimensional, hydrostatic, nearly adiabatic primitive equation model is used to study the evolution of a front passing across topography. Frontogenesis is forced by shearing deformation associated with the nonlinear evolution of an Eady wave. This study extends previous work by including an upper-level potential vorticity (PV) anomaly and a growing baroclinic wave in a baroclinically unstable basic state.

Results for the Eady wave simulations show that the mountain retards and blocks the approaching front at the surface while the upper-level PV anomaly associated with the front moves across the domain unaffected. Warm advection ahead of the lee trough forces convergence and cyclonic vorticity growth near the base of the lee slope. This vorticity growth is further encouraged by the approach of the upper-level PV anomaly. The upper-level PV anomaly then couples with this new surface vorticity center and propagates downstream. The original surface front remains trapped on the windward slope. Thus when the upstream blocking is strong, frontal propagation is discontinuous across the ridge. This evolution occurs for tall mountains and narrow mountains, as well as weak fronts. For low mountains, wide mountains, and strong fronts, only weak retardation is observed on the windward slope. The surface front remains coupled with the upper-level PV anomaly. The front moves continuously across the mountain.

The net result, regardless of mountain size and shape, is that the front reaches the base of the lee slope stronger, sooner, and with a decreased cross-front scale compared to the “no-mountain” case. Well downstream of the mountain, no position change of the surface front is observed.

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David J. Knight and Peter V. Hobbs

Abstract

A two-dimensional, hydrostatic, primitive-equation model is used to investigate the dynamics of frontogenesis in a moist atmosphere. The development of a cold front is simulated through shear-deformation associated with the non-linear evolution of an Eady wave. Simulations are performed with 5, 10, 40 and 80 km horizontal resolutions and 14 levels in the vertical (four in the boundary layer).

Compared to the dry case, the inclusion of moisture in the model produces a stronger low-level jet ahead of the front and a stronger upper-level jet. Moisture also produces a stronger ageostrophic circulation across the front and a more concentrated updraft just ahead of the surface front. The updraft develops a banded structure above and behind the surface front, with a wavelength of about 70 km. Bands form near the back edge of the cloud shield and move toward the surface front with a relative velocity of ∼1 m s−1. These characteristics agree with observations of wide cold-frontal rainbands.

The banded structures form in a convectively stable region. The first band that appears in the numerical simulation forms and intensifies in a region of negative equivalent potential vorticity. Subsequent bands form behind the first and intensify as they move into the region of negative equivalent potential vorticity, indicating that conditional symmetric instability (CSI) may play an important role in their formation and intensification. Many of the characteristics of the bands agree with the theory of CSI. The bands disappear when equivalent potential vorticity is everywhere positive. The bands are poorly resolved when the horizontal resolution (Δx) of the model is 40 km, and they are absent with Δx = 80 km. However, the strength and horizontal scale of the bands is about the same with Δx = 5 km and Δx = 10 km. This indicates that the banded structure is not an artifact of the model.

Frictional convergence in the boundary layer forces a narrow cold-frontal rainband (NCFR) just above the surface front. The horizontal dimension of this band is greater than that for observed NCFR, presumably because of limited resolution in the model.

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John Molinari, David Knight, Michael Dickinson, David Vollaro, and Steven Skubis

Abstract

A significant sign reversal in the meridional potential vorticity gradient was found during the summer of 1991 on the 310-K isentropic surface (near 700 mb) over the Caribbean Sea. The Charney–Stern necessary condition for instability of the mean flow is met in this region. It is speculated that the sign reversal permits either invigoration of African waves or actual generation of easterly waves in the Caribbean.

During the same season, a correlation existed between the strength of the negative potential vorticity gradient in the Caribbean and subsequent cyclogenesis in the eastern Pacific. The meridional PV gradient, convective heating measured by outgoing longwave radiation data, and eastern Pacific cyclogenesis all varied on the timescale of the Madden–Julian oscillation (MJO). It is hypothesized that upstream wave growth in the dynamically unstable region provides the connection between the MJO (or any other convective forcing) and the associated enhanced downstream tropical cyclogenesis.

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Philip N. Schumacher, David J. Knight, and Lance F. Bosart

Abstract

A comparison between the climatological structure of retarded and unretarded fronts aligned parallel to the Appalachian Mountains is investigated. With the average height of the Appalachians being 1 km, retarded and unretarded fronts are common occurrences during the cold season. Because of the narrow half-width of 100 km and the 1000-km length of the mountain chain, a comparison to two- and three-dimensional numerical studies can be performed. Of the 142 cases of frontal passages over the Appalachians during the winters between October 1984 and April 1990, over 55% of all cold fronts were retarded by the mountains. Statistical analysis showed that retarded fronts have a stronger cross-front temperature gradient and a weaker cross-front pressure gradient. Composite fields of sea level pressure, 850-, 500-, and 200-mb heights; quasigeostrophic potential vorticity and its advection, and potential height (U/N) were computed for all retarded and unretarded fronts. Unretarded fronts were associated with stronger cyclones, larger potential vorticity anomalies, larger positive potential vorticity advection, and more amplified flow at all levels. There was no significant difference between the potential height fields of the two types of fronts. In addition the average potential height, for both groups of fronts, easily met the criteria for retardation. Rather than depending upon the Froude number of the flow, it is hypothesized that the strength of the synoptic-scale circulations in the middle and upper troposphere primarily determines whether or not a front will be retarded by the Appalachian Mountains.

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David Fereday, Robin Chadwick, Jeff Knight, and Adam A. Scaife

Abstract

The IPCC Fifth Assessment Report highlighted large uncertainty in European precipitation changes in the coming century. This paper investigates the sources of intermodel differences using CMIP5 model European precipitation data. The contribution of atmospheric circulation to differences in precipitation trends is investigated by applying cluster analysis to daily mean sea level pressure (MSLP) data. The resulting classification is used to reconstruct monthly precipitation time series, thereby isolating the component of precipitation variability directly related to atmospheric circulation. Reconstructed observed precipitation and reconstructions of simulated historical and projection data are well correlated with the original precipitation series, showing that circulation variability accounts for a substantial fraction of European precipitation variability. Removing the reconstructed precipitation from the original precipitation leaves a residual component related to noncirculation effects (and any small remaining circulation effects). Intermodel spread in residual future European precipitation trends is substantially reduced compared to the spread of the original precipitation trends. Uncertainty in future atmospheric circulation accounts for more than half of the intermodel variance in twenty-first-century precipitation trends for winter months for both northern and southern Europe. Furthermore, a substantial part of this variance is related to different forced dynamical responses in different models and is therefore potentially reducible. These results highlight the importance of understanding future changes in atmospheric dynamics in achieving more robust projections of regional climate change. Finally, the possible dynamical mechanisms that may drive the future differences in regional circulation and precipitation are illustrated by examining simulated teleconnections with tropical precipitation.

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Anthony R. Lupo, Joseph J. Nocera, Lance F. Bosart, Eric G. Hoffman, and David J. Knight

Abstract

This paper examines the climatological, large-scale, and synoptic-scale aspects of South American cold surges using NCEP–NCAR gridded reanalyses for the 1992–96 period. Three common cold surge types are identified on the basis of a thickness (1000–850 hPa) criteria: type 1—a transient surge associated with weak anticyclone development east of the Andes in the absence of ridging aloft, type 2—a strong and persistent surge associated with dynamic anticyclogenesis aloft and strong surface anticyclone development east of the Andes, and type 3—a surge east of the Brazilian coastal mountains. Cold surges are most common during the winter and spring (Jun–Nov), accounting for 189 of the 256 events (74%).

Case studies of two events (19–22 Jul 1992 and 12–14 Apr 1993) are conducted from both a conventional isobaric and a potential vorticity (PV) perspective. The upper-air flow pattern in the July 1992 type 2 case is characterized by the presence of a strong ridge–trough couplet, which amplifies and becomes quasi-stationary, allowing for a deep layer of equatorward flow over South America. Dynamically, this flow pattern favors the development of a very strong surface anticyclone to the east of the Andes in response to a combination of differential anticyclonic vorticity advection, low-level cold advection, and, equivalently, positive PV advection. Because of the associated cold air damming east of the Andes, modified cool air is transported into the western part of Amazonia. Cold air damming east of the Brazilian coastal mountains is associated with the transition of the July 1992 type 2 surge into a type 3 surge.

The cold surge of April 1993 is examined as a rare event that does not fit the above classification. It is characterized by explosive cyclogenesis close to the coast of Argentina. Unlike the representative type 2 cold surge of July 1992, which tends to occur in association with southwesterly flow aloft, the April 1993 cold surge occurs beneath westerly and northwesterly flow aloft. Cold air penetration into lower latitudes is restricted because the geostrophic wind has a component directed away from the Andes equatorward of the cyclone. The dynamical forcing mechanisms associated with the April 1993 event are of smaller scale than those of the much more common surges typified by the July 1992 event.

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Thomas R. Karl, Wei-Chyung Wang, Michael E. Schlesinger, Richard W. Knight, and David Portman

Abstract

Important surface observations such as the daily maximum and minimum temperature, daily precipitation, and cloud ceilings often have localized characteristics that are difficult to reproduce with the current resolution and the physical parameterizations in state-of-the-art General Circulation climate Models (GCMs). Many of the difficulties can be partially attributed to mismatches in scale, local topography. regional geography and boundary conditions between models and surface-based observations. Here, we present a method, called climatological projection by model statistics (CPMS), to relate GCM grid-point flee-atmosphere statistics, the predictors, to these important local surface observations. The method can be viewed as a generalization of the model output statistics (MOS) and perfect prog (PP) procedures used in numerical weather prediction (NWP) models. It consists of the application of three statistical methods: 1) principle component analysis (FICA), 2) canonical correlation, and 3) inflated regression analysis. The PCA reduces the redundancy of the predictors The canonical correlation is used to develop simultaneous relationships between linear combinations of the predictors, the canonical variables, and the surface-based observations. Finally, inflated regression is used to relate the important canonical variables to each of the surface-based observed variables.

We demonstrate that even an early version of the Oregon State University two-level atmospheric GCM (with prescribed sea surface temperature) produces free-atmosphere statistics than can, when standardized using the model's internal means and variances (the MOS-like version of CPMS), closely approximate the observed local climate. When the model data are standardized by the observed free-atmosphere means and variances (the PP version of CPMS), however, the model does not reproduce the observed surface climate as well. Our results indicate that in the MOS-like version of CPMS the differences between the output of a ten-year GCM control run and the surface-based observations are often smaller than the differences between the observations of two ten-year periods. Such positive results suggest that GCMs may already contain important climatological information that can be used to infer the local climate.

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Chris K. Folland, Jeff Knight, Hans W. Linderholm, David Fereday, Sarah Ineson, and James W. Hurrell

Abstract

Summer climate in the North Atlantic–European sector possesses a principal pattern of year-to-year variability that is the parallel to the well-known North Atlantic Oscillation in winter. This summer North Atlantic Oscillation (SNAO) is defined here as the first empirical orthogonal function (EOF) of observed summertime extratropical North Atlantic pressure at mean sea level. It is shown to be characterized by a more northerly location and smaller spatial scale than its winter counterpart. The SNAO is also detected by cluster analysis and has a near-equivalent barotropic structure on daily and monthly time scales. Although of lesser amplitude than its wintertime counterpart, the SNAO exerts a strong influence on northern European rainfall, temperature, and cloudiness through changes in the position of the North Atlantic storm track. It is, therefore, of key importance in generating summer climate extremes, including flooding, drought, and heat stress in northwestern Europe. The El Niño–Southern Oscillation (ENSO) phenomenon is known to influence summertime European climate; however, interannual variations of the SNAO are only weakly influenced by ENSO. On interdecadal time scales, both modeling and observational results indicate that SNAO variations are partly related to the Atlantic multidecadal oscillation. It is shown that SNAO variations extend far back in time, as evidenced by reconstructions of SNAO variations back to 1706 using tree-ring records. Very long instrumental records, such as central England temperature, are used to validate the reconstruction. Finally, two climate models are shown to simulate the present-day SNAO and predict a trend toward a more positive index phase in the future under increasing greenhouse gas concentrations. This implies the long-term likelihood of increased summer drought for northwestern Europe.

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Warren M. Washington, Albert J. Semtner Jr., Gerald A. Meehl, David J. Knight, and Thomas A. Mayer

Abstract

This paper describes the construction and results of a comprehensive, three-dimensional general circulation model (GCM) of the earth's climate. The model, developed at the National Center for Atmospheric Research (NCAR), links separate existing models of the atmosphere, ocean and sea ice. The atmospheric model is a version of the third-generation NCAR GCM which has a relatively complete treatment of physical processes. It uses a generalized vertical coordinate with eight layers (∼3 km thick) and 5° horizontal grid spacing over the entire globe. The ocean model, using the primitive equations and the hydrostatic and Boussinesq approximations, was changed to the world domain from an earlier model developed by Bryan (1969) and reprogrammed by Semtner (1974). The model has four unequally spaced vertical layers and 5° horizontal grid structure. The sea ice model is a simple thermodynamic model using a simplified calculation of heat flux through sea ice (Semtner, 1976).

The method of coupling the atmosphere and ocean models is an attempt to deal with the two different time scales of the atmosphere and ocean in a computationally efficient fashion. By means of four relatively short integrations, the atmospheric model provides samples (10–30 days in length) of four seasonal months—January, April, July and October. The data from the four atmospheric model months are fitted to annual and semiannual harmonics and are used to drive the ocean model for five years. The process is iterated for a number of cycles to achieve an approximate equilibrium.

The atmospheric circulation in the coupled model is similar to that obtained previously by Washington et al. (1979) with climatological ocean forcing. The simulated ocean surface temperature pattern is reasonably similar to the observed pattern, but the calculated ocean temperatures tend to be as much as 3°C too cold locally in the tropics and up to 4°C too warm in the midlatitudes. Possible reasons for these discrepancies are discussed. The major mean ocean current gyre systems are reproduced in the ocean model second layer where effects of non-geostrophic Ekman drift and short-term wind-stress averaging bias are not felt. These effects, however, tend to complicate somewhat the computed surface current pattern. The computed horizontal oceanic heat flux compares favorably with the observed of Oort and Vonder Haar (1976) in phase and amplitude. Vertical velocities at the bottom of the 50 m surface layer, which can be considered a simple mixed layer, have the same general pattern as those calculated using observed wind stress. The simulation of sea ice thickness and seasonal geographical extent is closer to the observed in the Arctic than in the Antarctic region.

The experiment described here must be regarded as preliminary; even though many first-order aspects of the climate system are simulated, improvements are still needed.

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Thomas R. Karl, Richard W. Knight, David R. Easterling, and Robert G. Quayle

A framework is presented to quantify observed changes in climate within the contiguous United States through the development and analysis of two indices of climate change, a Climate Extremes Index (CEI) and a U.S. Greenhouse Climate Response Index (GCRI). The CEI is based on an aggregate set of conventional climate extreme indicators, and the GCRI is composed of indicators that measure changes in the climate of the United States that have been projected to occur as a result of increased emissions of greenhouse gases.

The CEI supports the notion that the climate of the United States has become more extreme in recent decades, yet the magnitude and persistence of the changes are not large enough at this point to conclude that the increase in extremes reflects a nonstationary climate. Nonetheless, if impacts due to extreme events rise exponentially with the index, then the increase may be quite significant in a practical sense. Similarly, the positive trend of the U. S. GCRI during the twentieth century is consistent with an enhanced greenhouse effect. The increase is unlikely to have arisen due to chance alone (there is about a 5% chance). Still, the increase of the GCRI is not large enough to unequivocally reject the possibility that the increase in the GCRI may be the result of other factors, including natural climate variability, and the similarity between the change in the GCRI and anticipated changes says little about the sensitivity of the climate system to the greenhouse effect. Both indices increased rather abruptly during the 1970s, a time of major circulation changes over the Pacific Ocean and North America.

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