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Keith M. Hines and Carlos R. Mechoso

Abstract

Basic issues regarding upper-level frontogenesis addressed in this paper are: (i) simulated frontogenesis influenced by the initial flow, (ii) upper-level frontogenesis as essentially a two-dimensional process, and (iii) frontal-scale positive feedback between vertical advection of momentum and vorticity advection by the ageostrophic wind, which is important for the intensification of upper-level frontal zones. The methodology for investigation is based on analysis of simulated upper-level frontogenesis with a three-dimensional primitive-equation model. The model is a simplified version of the UCLA GCM, with 21 layers in the vertical, horizontal resolution of 1.2° lat × 1.5° long; a 60° sector of one hemisphere as periodic domain, and physics reduced to horizontal diffusion and dry convective adjustment. Simulations initialized with jet streams symmetric about the latitude of maximum wind at each pressure level produce—in the middle troposphere—the strongest frontal zones downstream of the trough of growing baroclinic waves. Strongest upper-level frontal zones originating upstream of the wave trough, as observed, are produced when initial jet streams and perturbations are chosen so that the growing waves have small meridional phase tilt in the initial stages.

In the simulations, tilting associated with divergence of the across-jet ageostrophic flow is the dominant frontogenetical process upstream of the wave trough. Further, tilting associated with divergence of the ageostrophic wind along the jet also contributes to frontogenesis, but to a lesser extent. The former result is similar to that obtained with two-dimensional models in which frontogenetical vertical motions are associated with divergence of the ageostrophic wind across the front.

No definitive evidence is found proving that the simulated frontogenesis is enhanced by a positive-feedback process involving vertical advection of momentum and vorticity advection by the ageostrophic wind. It is found, however, that both of these processes are nonnegligible contributors to the frontal intensification.

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Keith M. Hines and Carlos R. Mechoso

Abstract

Surface frontal structure during cyclogenesis, and the sensitivity of this structure to surface friction, is examined. The approach is based on the analyses of simulations using a primitive equation model, with the domain restricted to a sector of one hemisphere, and the physics reduced to surface drag, horizontal diffusion, and dry convective adjustment. The model horizontal resolution is 1.2° latitude × 1.5° longitude, and there are 21 layers in the vertical. The drag coefficient is varied in simulations with midlatitude jet streams as initial conditions. The extent to which simulations in the adiabatic framework or with highly simplified representations of physical processes succeed in producing features of cyclone evolution emphasized by recent observational analyses is evaluated.

Shallow bent-back warm fronts develop in simulations with surface drag coefficients that are zero or representative of ocean surfaces. Horizontal advection, first in strong easterly and later in strong northerly winds, is primarily responsible for the resulting bent-back structure of the warm front.

The effect of surface drag on simulated lower-tropospheric wind speeds and frontogenesis is nonuniform. Warm frontogenesis is enhanced in simulations with relatively low surface drag through a feedback process involving vorticity, deformation, convergence, and warm-air advection. Surface drag tends to inhibit warm frontogenesis by decreasing the low-level wind speed and reducing the contribution of warm advection to the feedback. Consistently, a distinct warm front does not develop in the simulation with a surface drag coefficient representative of continental surfaces. Cold frontogenesis, on the other hand, is not very sensitive to surface drag.

Further simulations with doubled horizontal resolution (0.6° latitude × 0.75° longitude), slightly higher baroclinity at lower levels in the initial conditions, and small surface drag produce bent-back fronts that spiral around the surface pressure minimum. These results suggest that there are important differences in the structure of surface fronts associated with marine and continental cyclogenesis.

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Keith M. Hines and David H. Bromwich

Abstract

A polar-optimized version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) was developed to fill climate and synoptic needs of the polar science community and to achieve an improved regional performance. To continue the goal of enhanced polar mesoscale modeling, polar optimization should now be applied toward the state-of-the-art Weather Research and Forecasting (WRF) Model. Evaluations and optimizations are especially needed for the boundary layer parameterization, cloud physics, snow surface physics, and sea ice treatment. Testing and development work for Polar WRF begins with simulations for ice sheet surface conditions using a Greenland-area domain with 24-km resolution. The winter month December 2002 and the summer month June 2001 are simulated with WRF, version 2.1.1, in a series of 48-h integrations initialized daily at 0000 UTC. The results motivated several improvements to Polar WRF, especially to the Noah land surface model (LSM) and the snowpack treatment. Different physics packages for WRF are evaluated with December 2002 simulations that show variable forecast skill when verified with the automatic weather station observations. The WRF simulation with the combination of the modified Noah LSM, the Mellor–Yamada–Janjić boundary layer parameterization, and the WRF single-moment microphysics produced results that reach or exceed the success standards of a Polar MM5 simulation for December 2002. For summer simulations of June 2001, WRF simulates an improved surface energy balance, and shows forecast skill nearly equal to that of Polar MM5.

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Keith M. Hines and David H. Bromwich

Abstract

Low-level clouds are extensive in the Arctic and contribute to inadequately understood feedbacks within the changing regional climate. The simulation of low-level clouds, including mixed-phase clouds, over the Arctic Ocean during summer and autumn remains a challenge for both real-time weather forecasts and climate models. Here, improved cloud representations are sought with high-resolution mesoscale simulations of the August–September 2008 Arctic Summer Cloud Ocean Study (ASCOS) with the latest polar-optimized version (3.7.1) of the Weather Research and Forecasting (Polar WRF) Model with the advanced two-moment Morrison microphysics scheme. Simulations across several synoptic regimes for 10 August–3 September 2008 are performed with three domains including an outer domain at 27-km grid spacing and nested domains at 9- and 3-km spacing. These are realistic horizontal grid spacings for common mesoscale applications. The control simulation produces excessive cloud liquid water in low clouds resulting in a large deficit in modeled incident shortwave radiation at the surface. Incident longwave radiation is less sensitive. A change in the sea ice albedo toward the larger observed values during ASCOS resulted in somewhat more realistic simulations. More importantly, sensitivity tests show that a reduction in specified liquid cloud droplet number to very pristine conditions increases liquid precipitation, greatly reduces the excess in simulated low-level cloud liquid water, and improves the simulated incident shortwave and longwave radiation at the surface.

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Keith M. Hines, David H. Bromwich, and Thomas R. Parish

Abstract

The meteorology of high Southern latitudes during winter is simulated using a cloud-free version of The Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model version 4 (MM4) with a 100-km horizontal resolution. Comparisons between idealized simulations of Antarctica with MM4 and with the mesoscale model of Parish and Waight reveal that both models produce similarly realistic velocity fields in the boundary layer. The latter model tends to produce slightly faster drainage winds over East Antarctica. The intensity of the katabatic winds produced by MM4 is sensitive to parameterizations of boundary layer fluxes. Two simulation are performed with MM4 using analyses from the European Centre for Medium-Range Weather Forecasts for June 1988 as initial and boundary conditions. A simulation of the period from 0000 UTC 2 June to 0000 UTC 8 June produces realistic synoptic phenomena including ridge development over East Antarctica, frontogenesis over the Amundsen Sea, and a katabatic surge over the Ross Ice Shelf. The simulated two-averaged fields for June 1988, particularly that of a 500-hPa height, are in good agreement with time-averaged fields analyzed by the European Centre for Medium-Range Weather Forecasts. The results of the simulations provide detailed features of the Antarctic winter boundary layer along the steeply sloping terrain. Highest boundary layer wind speeds averaged over the month-long simulation are approximately 20 m s−1. The lack of latent heating in the simulations apparently results in some bias in the results. In particular, the cloud-free version of MM4 underpredicts the intensity of lows in the sea level pressure field.

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Keith M. Hines, David H. Bromwich, and Gareth J. Marshall

Abstract

An examination of 50 years of the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis from 1949 to 1998 reveals that significant spurious trends occur in the surface pressure field. Long-term surface pressure reductions are apparent south of 45°S. The largest trend in surface pressure is near 65°S where an approximately steady long-term pressure reduction of about 0.20 hPa yr−1 (10 hPa in 50 yr) is located. The negative pressure trend represents a gradual reduction in a positive bias for the reanalysis. Observations at Antarctic stations do not support this long-term trend, although short-term interannual variations are reasonably well captured after about 1970. The negative pressure tendency near 65°S continues well into the 1990s although a reasonable number of stations between 65° and 70°S began taking observations along the coast of east Antarctica during the 1950s and 1960s. Few Antarctic observations, however, are used by the reanalysis until about 1968, and the quality of the pressure field for the reanalysis appears poor in high southern latitudes prior to then. The trend in high southern latitudes appears to be a component of global temporal variations in the reanalysis, some of which are supported by observations but others are not.

In the Southern Hemisphere, the sea level pressure difference between 40° and 60°S, an indicator of westerly wind intensity, increases approximately from 20 hPa in the early 1950s to 25 hPa in the early 1970s and 28 hPa in recent years. The relatively high density of observing stations along the Antarctic Peninsula, however, results in an approximately steady local surface pressure after the pressure fell about 4 hPa during the late 1950s. Based upon these findings, researchers should account for jumps and long-term trends when making use of the NCEP–NCAR reanalysis.

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Zhichang Guo, David H. Bromwich, and Keith M. Hines

Abstract

The impacts of the El Niño–Southern Oscillation (ENSO) on the Antarctic region are of special importance in evaluating the variability and change of the climate system in high southern latitudes. In this study, the ENSO signal in modeled precipitation over West Antarctica since 1979 is evaluated using forecast precipitation from several meteorological analyses and reanalyses. Additionally, a dynamical retrieval method (DRM) for precipitation is applied. Over the last two decades, the Southern Oscillation index (SOI) has an overall anticorrelation with precipitation over the West Antarctic sector bounded by 75°–90°S, 120°W–180° while it is positively correlated with precipitation over the South Atlantic sector bounded by 65°–75°S, 30°–60°W.

Decadal variations are found as the relationship between the SOI and West Antarctic precipitation is stronger in the 1990s than that in the 1980s. The polar front jet stream, West Antarctic precipitation, and the SOI show a well-ordered correspondence during the 1990s as the jet zonal speed is negatively correlated to the SOI and positively correlated to West Antarctic precipitation. These relationships are weaker during the 1980s, consistent with the change in sign of the correlation between the SOI and West Antarctic precipitation. The decadal variations are apparently related to changes in the quasi-stationary eddies that determine the local onshore and offshore flow over West Antarctica.

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David H. Bromwich, Yang Du, and Keith M. Hines

Abstract

Clear-sky, wintertime surface winds over the Greenland Ice Sheet are simulated with a three-dimensional mesoscale numerical model. It is shown that the simulated winds blow from the broad gently sloped interior to the steep coastal margins. This general wind pattern is similar to that found over Antarctica due to the same governing dynamics. The longwave radiational cooling of the sloping ice terrain is the key driving force of this cold airflow. In some coastal areas the downslope winds converge into large fjords, such as Kangerlussuaq and Sermilik. This is consistent with the frequent presence in these areas of warm signatures on cloud-free thermal infrared satellite images that are generated by katabatic winds. The shape of the Greenland Ice Sheet plays an important role in directing the flow of the surface winds. The study demonstrates that the surface wind pattern is only moderately affected by climatological flow around and over the ice sheet. The mass redistribution associated with the katabatic wind circulation plays an important role in generating prominent features of the time-averaged sea level pressure and upper-level circulation fields near Greenland.

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Aaron B. Wilson, David H. Bromwich, and Keith M. Hines

Abstract

Numerical simulations using the National Center for Atmospheric Research Community Atmosphere Model (CAM) are conducted based on tropical forcing of El Niño flavors. Though these events occur on a continuum, two general types are simulated based on sea surface temperature anomalies located in the central (CP) or eastern (EP) tropical Pacific. The goal is to assess whether CAM adequately represents the transient eddy dynamics associated with each of these El Niño flavors under different southern annular mode (SAM) regimes. CAM captures well the wide spatial and temporal variability associated with the SAM but only accurately simulates the impacts on atmospheric circulation in the high southern latitudes when the observed SAM phase is matched by the model. Composites of in-phase (El Niño–SAM−) and out-of-phase (El Niño–SAM+) events confirm a seasonal preference for in-phase (out of phase) events during December–February (DJF) [June–August (JJA)]. Modeled in-phase events for both EP (during DJF) and CP (during JJA) conditions support observations of anomalous equatorward momentum flux on the equatorward side of the eddy-driven jet, shifting this jet equatorward and consistent with the low phase of the SAM. Out-of-phase composites show that the El Niño–associated teleconnection to the high southern latitudes is strongly modulated by the SAM, as a strong eddy-driven jet is well maintained by high-latitude transient eddy convergence despite the tropical forcing. A regional perspective confirms that this interaction takes place primarily over the Pacific Ocean, with high-latitude circulation variability being a product of both tropical and high-latitude forcing.

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Keith M. Hines, David H. Bromwich, Philip J. Rasch, and Michael J. Iacono

Abstract

To evaluate and improve the treatment of clouds and radiation by the climate models of the National Center for Atmospheric Research (NCAR), simulations by the NCAR Community Climate Model version 3 (CCM3), as well as the recently released Community Atmosphere Model version 2 (CAM2), are examined. The Rasch and Kristjánsson prognostic cloud condensate scheme, which is now the standard scheme for CAM2, is included in a version of CCM3 and evaluated. Furthermore, the Rapid Radiative Transfer Model (RRTM), which alleviates the deficit in downward clear-sky longwave radiation, is also included in a version of CCM3. The new radiation scheme in CAM2 also alleviates the clear-sky longwave bias, although RRTM is not included. The impact of the changes is especially large over the interior of Antarctica. The changes induced by the introduction of the prognostic cloud scheme are found to have a much larger impact on the CCM3 simulations than do those from the introduction of RRTM. The introduction of the prognostic cloud scheme increases cloud emissivity in the upper troposphere, reduces cloud emissivity in the lower troposphere, and results in a better vertical distribution of cloud radiative properties over interior Antarctica. The climate simulations have a very large cold bias in the stratosphere, especially during summer. There are significant deficiencies in the simulation of Antarctic cloud radiative effects. The optical thickness of Antarctic clouds appears to be excessive. This contributes to a warm bias in surface temperature during winter and a deficit in downward shortwave radiation during summer. Some biases for Antarctica are larger for CCM3 with the prognostic cloud condensate scheme than with the standard diagnostic clouds. When the mixing ratio threshold for autoconversion from suspended ice cloud to falling precipitation is reduced toward a more realistic value, the Antarctic clouds are thinned and some of the biases are reduced. To improve the surface energy balance, not only must the radiative effects of clouds be improved, it is also necessary to improve the representation of sensible heat flux. Insufficient vertical resolution of the frequently very shallow, very stable surface boundary layer apparently contributes to an excessive heat flux from the atmosphere to the surface during winter. The representations of Antarctic clouds and radiation by the new NCAR CAM2 are not clearly improved compared to those of the earlier CCM3. For example, the surface albedo over Antarctica is descreased in CAM2 and Community Climate System Model version 2 (CCSM2) simulations in comparison to CCM3 simulations, contributing to a summer warm bias in tropospheric temperature for the former.

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