Thomas R. Parish

Continental-scale simulation of the Antarctic katabatic wind regime

Abstract Katabatic winds are a common feature of the lower Antarctic atmosphere. Although these drainage flows are quite shallow, there is increasing evidence that the low-level circulations are an important component in establishing large-scale tropospheric motions in the high southern latitudes. Three-dimensional numerical simulations of the Antarctic katabatic wind regime and attendant tropospheric circulations have been conducted over the entire continent to depict the topographically forced drainage patterns in the near-surface layer of the atmosphere. Results of the simulation enable a mapping of katabatic wind potential and identification of coastal regions which may experience anomalously intense katabatic winds. A large upper-level cyclonic circulation forms rapidly in response to the evolving katabatic wind structure in the lower atmosphere, suggesting that the drainage circulations are an important component in prescribing the resulting resulting circumpolar vortex. These results imply that som...

Barrier Winds Along the Sierra Nevada Mountains

Abstract Observational evidence from instrumented aircraft, Doppler radar and rawinsondes suggest low-level, mountain-parallel jets are a common wintertime feature along the western slope of the Sierra Nevada Range and extending into the California Valley. It is proposed that the formation and maintenance of the low-level jet is a result of the pressure field created by the damming of stable air as it is forced up against the steep mountain barrier. Numerical experiments, using a two-dimensional (x, z) primitive equation model incorporating terrain representative of the Sierra Nevada Mountains, are carried out to test this assertion.

Reexamination of the Near-Surface Airflow over the Antarctic Continent and Implications on Atmospheric Circulations at High Southern Latitudes*

Abstract Previous work has shown that winds in the lower atmosphere over the Antarctic continent are among the most persistent on earth with directions coupled to the underlying ice topography. In 1987, Parish and Bromwich used a diagnostic model to depict details of the Antarctic near-surface airflow. A radially outward drainage pattern off the highest elevations of the ice sheets was displayed with wind speeds that generally increase from the high interior to the coast. These winds are often referred to as “katabatic,” with the implication that they are driven by radiational cooling of near-surface air over the sloping ice terrain. It has been shown that the Antarctic orography constrains the low-level wind regime through other forcing mechanisms as well. Dynamics of the lower atmosphere have been investigated increasingly by the use of numerical models since the observational network over the Antarctic remains quite sparse. Real-time numerical weather prediction for the U.S. Antarctic Program has been ...

The Forcing of Antarctic Katabatic Winds

Abstract The temporal and spatial development of katabatic winds along an idealized slope representative of Anmtarctic terrain is examined using a hydrostatic, two-dimensional primitive equation model with explicit longwave radiation parameterization. A detailed diagnosis is made of a simulation in which katabatic flow develops from rest due to the strong radiational cooling of the underlying surface. Significant thermodynamic and dynamic differences are seen between the gravity-driven flows over the gently-sloping interior and over the steep ice slopes near the coast. The strongest temperature inversions and largest static stabilities are found over the interior of the continent, although the net cooling of the katabatic layer and magnitude of the downslope-directed horizontal pressure gradient force are greatest at the coast. The interior is characterized by low Rossby number, quasi-geostrophic type flows, while more intense, near-antitriptic winds occur at the coast. Model results are in reasonable agr...

Forcing of the Summertime Low-Level Jet along the California Coast

Abstract Coast-parallel low-level jets are commonplace in the offshore environment along the west coast of the United States during summer. The jet often has wind speeds in excess of 30 m s−1 and is typically situated near the top of the marine boundary layer. A field study was conducted in early summer of 1997 to study the kinematics and dynamics of the low-level jet off the California coast. The University of Wyoming King Air research aircraft was the primary observation platform. Measurement of the horizontal pressure gradient force was fundamental to understanding the dynamics of the jet. By flying at constant pressure, the height of an isobaric surface could be determined by the radar altimeter. The slope of a constant pressure surface is proportional to the pressure gradient force and hence provides an estimate of the geostrophic wind. Data are presented for two episodes of the low-level jet. In both cases wind speed maxima extending in excess of 100 km from the coast were observed. In contrast to p...

The Role of Katabatic Winds on the Antarctic Surface Wind Regime

Abstract Antarctica is known for its strong and persistent surface winds that are directed along topographic pathways. Surface winds are especially strong during the winter period. The high directional constancy of the wind and the close relationship of the wind direction to the underlying terrain can be interpreted as evidence of katabatic wind activity. Observations show that the directional constancy of the Antarctic surface wind displays little seasonal variation. Summertime winds cannot be expected to contain a significant katabatic component, owing to enhanced solar heating of the ice slopes. Observations also show that the coastal environs are subjected to wide variation in atmospheric pressure associated with frequent cyclone activity. The robust unidirectional nature of the Antarctic surface wind throughout the year implies that significant topographic influences other than those from katabatic forcing must be acting. Idealized numerical simulations have been performed to illustrate the potential...

A Case Study of Antarctic Katabatic Wind Interaction with Large-Scale Forcing*

Surface pressure decreases of up to 20 hPa occurred over much of the Antarctic continent during a 4-day midwinter period of 1988. The widespread change in the pressure field accompanied intense cyclonic activity to the north of the ice sheet. The equatorward mass transport across the Antarctic coastline resulted in a redistribution of atmospheric pressure that extended to the subtropics of the Southern Hemisphere. Most of the mass flux from Antarctica was the result of low-level processes and appears tied to the katabatic wind circulation. The observed surface pressure decrease over the continent reflects a perturbation of the mean meridional circulation between Antarctica and the subpolar latitudes by synoptic-scale processes. Zonally averaged circulations over Antarctica were examined using output from the European Centre for Medium-Range Weather Forecasts model. Results suggest that only a poorly defined return branch of the meridional circulation exists in the middle and upper troposphere. This southward-directed flow does not compensate for the northward mass transport provided by the katabatic wind outflow in the lower atmosphere. Isallobaric contours over the Antarctic ice sheet roughly match the area of the large-scale drainage catchment associated with katabatic wind transport through the Ross Sea sector. An intense extratropical cyclone was present in the circumpolar oceanic belt to the north of the continent. The horizontal pressure gradient associated with the cyclone prompted enhanced drainage off the high interior plateau. The resulting katabatic flow issued from the continent through a narrow corridor across the Ross Ice Shelf and out over the Southern Ocean.

Surface Airflow Over East Antarctica

Abstract Surface winds over the Antarctic interior occur mainly due to the strong radiational cooling of the ice slopes. As a consequence, such winds exhibit a high degree of persistence with a predominant direction closely related to the terrain orientation. Using detailed contour maps of the interior ice topography and representative values of the mean wintertime strength of the temperature inversion, it is possible to infer the terrain-induced accelerations. A simple diagnostic equation system is formulated, from which a time-averaged surface airflow pattern of East Antarctica is generated. The results appear consistent with observations. The occurrence of localized, anomalously strong katabatic winds is explained as a result of typographically forced patterns of cold-air convergence depicted in the airflow analysis.

A Case Study of the Summertime Great Plains Low Level Jet

Abstract A case study of the kinematical and dynamical evolution of the summertime Great Plains low level jet (LLJ) is presented. Airborne radar altimetry was used to discern the x and y components of the geostrophic wind at three levels in the lower atmosphere throughout the LLJ episode. Results appear to confirm previous theoretical and numerical studies regarding the importance of the diurnal cycle of heating over sloping terrain in producing an oscillating horizontal pressure gradient force. Inertial turning of the LLJ as a result of frictional decoupling was also documented. It is concluded that the inertial oscillation resulting from the sudden decrease in friction in the lower atmosphere during the early evening is the dominant mechanism in forcing this example of a summertime Great Plains LLJ.

A Numerical Study of Strong Katabatic Winds over Antarctica

Abstract Certain coastal sections of Antarctica, most notably Adelie Land and Terra Nova Bay, experience anomalously intense, persistent katabatic winds. The forcing of such katabatic outflow is believed to originate several hundred kilometers upslope in the interior of the continent where cold air drainage currents from a large area converge into a relatively narrow zone focused on the steeply-sloping ice terrain near the coastline. Numerical simulations with a three-dimensional hydrostatic model incorporating terrain features representative of Adelie Land reveal a significant topographical channeling of the surface airflow. Katabatic wind speeds as depicted by the model are greatly enhanced downslope of the convergence channel. These results emphasize the importance of topography in the continental interior in shaping the character of coastal katabatic flow.