Keith M. Hines

Artificial surface pressure trends in the NCEP–NCAR reanalysis over the Southern Ocean and Antarctica

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 458S. The largest trend in surface pressure is near 658S where an approximately steady long-term pressure reduction of about 0.20 hPa yr21 (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 8S continues well into the 1990s although a reasonable number of stations between 658 and 708S 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 408 and 608S, 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.

Mesoscale Modeling of Katabatic Winds over Greenland with the Polar MM5

Abstract Verification of two months, April and May 1997, of 48-h mesoscale model simulations of the atmospheric state around Greenland are presented. The simulations are performed with a modified version of The Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5), referred to as the Polar MM5. Global atmospheric analyses as well as automatic weather station and instrumented aircraft observations from Greenland are used to verify the forecast atmospheric state. The model is found to reproduce the observed atmospheric state with a high degree of realism. Monthly mean values of the near-surface temperature and wind speed predicted by the Polar MM5 differ from the observations by less than 1 K and 1 m s−1, respectively, at most sites considered. In addition, the model is able to simulate a realistic diurnal cycle for the surface variables, as well as capturing the large-scale, synoptically forced changes in these variables. Comparisons of modeled profil...

Development and Testing of Polar Weather Research and Forecasting (WRF) Model. Part I: Greenland Ice Sheet Meteorology*

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–Janjic ´ 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.

Development and testing of Polar Weather Research and Forecasting model: 2. Arctic Ocean

[1] A version of the state-of-the-art Weather Research and Forecasting model (WRF) has been developed for polar applications. The model known as ‘‘Polar WRF’’ is tested over the Arctic Ocean with a western Arctic grid using 25-km resolution. The model is based upon WRF version 2.2, with improvements to the Noah land surface model and the snowpack treatment. The ocean surface treatment is modified to include fractional sea ice. Simulations consist of a series of 48-h integrations initialized daily at 0000 UTC. The initial 24 h are taken as model spin-up time for the atmospheric hydrology and boundary layer processes. Arctic conditions are simulated for the selected months: January 1998, June 1998, and August 1998 representing midwinter, early summer, and late summer conditions, respectively, from the Surface Heat Budget of the Arctic (SHEBA) study. The albedo of sea ice is specified as a function of time and latitude for June and as a function of time for August. Simulation results are compared with observations of the drifting ice station SHEBA in the Arctic ice pack. Polar WRF simulations show good agreement with observations for all three months. Some differences between the simulations and observation occur owing to apparent errors in the synoptic forecasts and the representation of clouds. Nevertheless, the biases in the simulated fields appear to be small, and Polar WRF appears to be a very good tool for studies of Arctic Ocean meteorology.

Development and testing of polar WRF. Part III: Arctic land

A version of the state-of-the-art Weather Research and Forecasting model (WRF) has been developed for use in polar climates. The model known as ‘‘Polar WRF’’ is tested for land areas with a western Arctic grid thathas25-kmresolution.Thisworkservesaspreparationforthehigh-resolutionArcticSystemReanalysisof the years 2000‐10. The model is based upon WRF version 3.0.1.1, with improvements to the Noah land surface model and snow/ice treatment. Simulations consist of a series of 48-h integrations initialized daily at 0000 UTC, with the initial 24 h taken as spinup for atmospheric hydrology and boundary layer processes. Soil

Comprehensive evaluation of polar weather research and forecasting model performance in the Antarctic

Received 22 May 2012; revised 7 November 2012; accepted 8 November 2012; published 17 January 2013. [1] Recent versions of the Polar Weather Research and Forecasting model are evaluated over the Antarctic to assess the impact of model improvements, resolution, large-scale circulation variability, and uncertainty in initial and lateral boundary conditions. The model skill differs more between forecasts using different sources of lateral boundary data than between forecasts from different model versions or simulated years. Using the ERA-Interim reanalysis for initial and lateral boundary conditions produces the best skill. The forecasts have a cold summer and a warm winter bias in 2m air temperatures, with similar but smaller bias in dew point temperatures. Upper air temperature biases are small and remain less than 1 C except at the tropopause in summer. Geopotential height biases increase with height in both seasons. Deficient downward longwave radiation in all seasons and an under representation of clouds enhance radiative loss, leading to the cold summer bias. Excess summer surface incident shortwave radiation plays a secondary role, because 80% of it is reflected, leading to greater skill for clear compared with cloudy skies. The positive wind speed bias produces a warm surface bias in winter resulting from anomalously large downward flux of sensible heat toward the surface. Low temperatures on the continent limit sublimation and hence the precipitable water amounts over the ice sheet. ERA-Interim experiments with higher precipitable water showed reduced biases in downwelling shortwave and longwave radiation. Increasing horizontal resolution from 60 to 15km improves the skill of surface wind forecasts.

Arctic System Reanalysis: Call for Community Involvement

Arctic climate encompasses multiple feedbacks, the most important of which is the ice-albedo feedback. Enhanced Arctic changes, first recognized in the nineteenth century, increasingly are being observed across terrestrial, oceanic, atmospheric, and human systems, inspiring interdisciplinary research efforts, including the Study of Environmental Arctic Change (SEARCH) program, to understand the nature and future development of the Arctic system. In response to the need for enhanced understanding outlined in the 2005 SEARCH Implementation Plan [Arctic Research Consortium of the United States, 2005], an ongoing Arctic System Reanalysis (ASR) project builds on previous programs to observe the Arctic climate. The ASR is a multi-institutional, interdisciplinary collaboration that optimally merges measurements and modeling to provide a high-resolution description of the regions atmosphere/sea ice/land system by assimilating a diverse suite of observations into a regional model. The project builds upon lessons learned from past reanalyses by optimizing model physics parameterizations and methods of data assimilation for Arctic conditions. The ASR, which is a partnership with the broader Arctic research community, represents a synthesis tool for assessing and monitoring variability and change in the Arctic system.

The impact of Antarctic cloud radiative properties on a GCM climate simulation

A sensitivity study to evaluate the impact upon regional and hemispheric climate caused by changing the optical properties of clouds over the Antarctic continent is conducted with the NCAR Community Model version 2 (CCM2). Sensitivity runs are performed in which radiation interacts with ice clouds with particle sizes of 10 and 40 mm rather than with the standard 10-mm water clouds. The experiments are carried out for perpetual January conditions with the diurnal cycle considered. The effects of these cloud changes on the Antarctic radiation budget are examined by considering cloud forcing at the top of the atmosphere and net radiation at the surface. Changes of the cloud radiative properties to those of 10-mm ice clouds over Antarctica have significant impacts on regional climate: temperature increases throughout the Antarctic troposphere by 18‐28C and total cloud fraction over Antarctica is smaller than that of the control at low levels but is larger than that of the control in the midto upper troposphere. As a result of Antarctic warming and changes in the north‐south temperature gradient, the drainage flows at the surface as well as the meridional mass circulation are weakened. Similarly, the circumpolar trough weakens significantly by 4‐8 hPa and moves northward by about 48‐58 latitude. This regional mass field adjustment halves the strength of the simulated surface westerly winds. As a result of indirect thermodynamic and dynamic effects, significant changes are observed in the zonal mean circulation and eddies in the middle latitudes. In fact, the simulated impacts of the Antarctic cloud radiative alteration are not confined to the Southern Hemisphere. The meridional mean mass flux, zonal wind, and latent heat release exhibit statistically significant changes in the Tropics and even extratropics of the Northern Hemisphere. The simulation with radiative properties of 40-mm ice clouds produces colder surface temperatures over Antarctica by up to 38C compared to the control. Otherwise, the results of the 40-mm ice cloud simulation are similar to those of the 10-mm ice cloud simulation.

Wintertime Surface Winds over the Greenland Ice Sheet

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 kat...

Influence of Surface Drag on the Evolution of Fronts

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 win...