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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 A. Browning
and
David Atlas

Abstract

The origin of a severe local storm is traced back to a cluster of three shower cells, each of which produced a fist radar echo close to the −30C level. This level is much higher than that associated with the majority of convective clouds studied by other workers. The great height of the first echoes is attributed to the presence of strong updrafts which carry the cloud particles to high levels in the time taken for them to grow to radar detectable sizes.

Because of their low temperatures, the first echoes were probably due to ice particles. Echo intensification in each cell was fairly rapid during the minute or so after first detection and corresponded to the growth of these particles by gravitational accretion in the presence of a liquid water concentration equal to about half the adiabatic value.

In each cell the reflectivity core due to the growing particles was balanced by the updraft at a constant level for a 5-min period following first detection, after which it descended to the ground with very little further growth. The low reflectivity of the echo core in relation to its rate of descent is interpreted as being due mainly to a smaller-than-usual particle concentration.

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Keith A. Browning
and
David Atlas

Abstract

The nature of the targets responsible for certain clear-air dot angel echoes, and their suitability as wind tracers, are deduced from pulse Doppler radar observations of their velocity characteristics. The angel echoes were observed during a six-hour period in the lowest few thousand feet of the atmosphere. They were all discrete point targets and probably had discrete Doppler velocities as well (i.e., they were coherent). The angel populations showed well-defined swarm velocities with superimposed random deviations. Three types of dot angels were distinguished according to their mean deviation from the swarm velocity and their average vertical motion. Type 1, in the early afternoon, showed mean velocity deviations of 1–15 m sec−1 and average downward motions of up to 4.5 m sec−1. Type 2, in the late afternoon, showed mean deviations of less than 1 m sec−1 and average downward motions of less than 1.8 m sec−1. Type 3, after sunset, showed mean deviations of 1–4 m sec−1 and average vertical motions that were mainly upward at up to 3.7 m sec−1. The small back-scattering cross sections of individual angels (≲ 10−1 CM2), their discreteness in space and velocity, their often quite large mean deviations from a uniform velocity, and the fact that the only major upward velocities occurred after sunset, at a time when the lapse rate was becoming increasingly stable, all suggest insects rather than atmospheric inhomogeneities as the source of the angels. The Type 1 angels had mean swarm velocities differing from the wind by less than 2 m sec−1; Type 2 probably differed by even less than this and are judged to have been good tracers of the wind. The Type 3 angels, on the other hand, had swarm velocities of up to 5 m sec−1 relative to Type 2 and hence to the wind as well, and thus they were poor tracers of the wind.

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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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David W. Keith
and
James G. Anderson

Abstract

The character of data required to measure decade-to-century–scale climatic change is distinctly different from that required for weather prediction or for studies of meteorological processes. The data ought to possess the accuracy to detect the small secular climate changes of interest. To be useful to future investigators, the data must include convincing proof that a given level of accuracy was in fact attained.

Spectrally resolved infrared radiance is one of the most important quantities to measure accurately from space—it contains much of the fingerprint of climate response and of the forcing that causes it. The authors describe the physics of infrared radiance measurements, and demonstrate that trade-offs exist between instrument accuracy (required for climate data) and sensitivity (required for weather prediction). No such simple trade-off exists between spectral resolution and accuracy; in fact, spectral resolution can improve accuracy. The authors analyze the implications of these trade-offs for the design of climate-observing systems based on observation of infrared radiance. It is argued that convincing demonstrations of sensor accuracy requires a measurement approach founded on the overdetermination of instrument calibration, an approach that aims to reveal rather than conceal instrumental error. It is argued that the required accuracy can by achieved in simple instruments that provide spectral resolution if high sensitivity is not simultaneously demanded. Laboratory data are presented to illustrate the means by which radiometric calibration with the accuracy required for climate observation—about 0.1 K in the midinfrared—might be achieved in a practical instrument.

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Daniel B. Kirk-Davidoff
and
David W. Keith

Abstract

Large-scale deployment of wind power may alter climate through alteration of surface roughness. Previous research using GCMs has shown large-scale impacts of surface roughness perturbations but failed to elucidate the dynamic mechanisms that drove the observed responses in surface temperature. Using the NCAR Community Atmosphere Model in both its standard and aquaplanet forms, the authors have explored the impact of isolated surface roughness anomalies on the model climate. A consistent Rossby wave response in the mean winds to roughness anomalies across a range of model implementations is found. This response generates appreciable wind, temperature, and cloudiness anomalies. The interrelationship of these responses is discussed, and it is shown that the magnitude of the responses scales with the horizontal length scale of the roughened region, as well as with the magnitude of the roughness anomaly. These results are further elucidated through comparison with results of a series of shallow-water model experiments.

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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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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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Keith P. Shine
,
David A. Robinson
,
Ann Henderson-Sellers
, and
George Kukla

Abstract

Recent work has emphasized the potential importance of atmospheric aerosols in the Arctic. This paper presents results indicating the large-scale presence of arctic aerosols during late spring. Their screening effect may be sufficient to alter significantly the shortwave radiation budget. The ratios of brightness over sea and snow covered ice surfaces are shown to be considerably lower, using DMSP shortwave imagery, than those calculated for clear skies using a radiative transfer scheme. Our analysis shows that aerosols are the most likely cause of the discrepancy. With additional calibration the method offers the potential for remote sensing of the aerosol distribution and concentration over the Arctic.

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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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