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J. C. King and W. M. Connolley

Abstract

Surface radiation measurements and other climatological data were used to validate the representation of the surface energy balance over the East Antarctic Ice Sheet in the U.K. Meteorological Office Unified Climate Model. Model calculations of incident and reflected shortwave radiation are in good agreement with observations, but the downward component of longwave radiation at the surface appears to be underestimated by up to 20 W m−2 in the model. Over much of the interior of Antarctica this error appears to be compensated for by an overestimate in turbulent transport of heat to the surface, while over the steep coastal slopes the heat flux is in good agreement with observations but the surface temperature is too low. The excessive heat flux over the interior results largely from the use of an inappropriately large bulk transfer coefficient under very stable conditions, suggesting that the surface heat flux scheme in the model is not ideally formulated for the conditions that prevail in the Antarctic boundary layer.

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Fred M. Vukovich and William J. King

Abstract

A three-dimensional primitive model was used to predict the early afternoon wind velocity field over St. Louis, Missouri. Four case studies were then performed where model results at various levels were compared with observed data from the METROMEX network. With proper initialization, the model very closely simulated actual conditions, the main feature of which was the urban heat island circulation. In-consistencies were due mainly to synoptic-scale changes and air mass anomalies, which the model is not designed to handle, and to limitations in the data associated with the network density.

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E. W. Eloranta, J. M. King, and J. A. Weinman

Abstract

Vertical profiles of the horizontal radial wind component in the lowest kilometer of the atmosphere have been measured remotely with lidar. Wind speed determinations were made by observing the motion of naturally occurring aerosol density inhomogeneities. Lidar wind measurements compare favorably with simultaneous pilot balloon observations of the wind.

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A. J. Dyer, B. B. Hicks, and K. M. King

Abstract

As a result of experience gained with the Evapotron in the measurement of eddy fluxes, a new instrument called the Fluxatron has been developed. The computing efficiency has been improved by filtering out slow eddies which do not contribute to the eddy flux.

The Fluxatron employs a propeller anemometer to detect the vertical wind component, and the response time of this device (0.3 sec) is suitable for measurements to be made at a height of 4 m. Only 2 W of battery power are consumed, in contrast to the Evapotron which uses 50N–100 W.

The new instrument is extremely simple to operate in the field, and is thus suitable for use by relatively unskilled personnel.

In its present form, the Fluxatron measures only sensible heat transfer. The measurement of the evaporative flux presents no difficulty in principle, but it is hoped that an alternative humidity sensor may be found other than a fine-wire wet bulb as in the Evapotron.

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Fred M. Vukovich, William J. King, J. W. Dunn III, and J. J. B. Worth

Abstract

The observed surface and upper air temperature and wind field patterns on 8 June 1976 in St. Louis, Missouri, were analyzed and compared with simulation results from a three-dimensional hydrodynamic model. An urban heat island (1–2°C temperature difference between the urban and rural regions) persisted during the day. The daytime temperature differential was relatively weak compared to that at night (∼5°C difference). In contrast, the urban heat island circulation was more intense during the day. This is thought to be due to the heating being distributed through a deeper layer. In the early evening, the heat island circulation dissipated due to the development of a surface-based inversion in the boundary layer.

The highest concentrations of O3 at the surface were found in the zone of convergence associated with the urban heat island circulation immediately downwind of the center of the city. As the heat island circulation dissipated during the early evening, the area of high O3 concentration was displaced further downstream.

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J. M. Rees, J. C. W. Denholm-Price, J. C. King, and P. S. Anderson

Abstract

Internal gravity waves are frequently observed in stably stratified regions of the atmospheric boundary layer. In order to determine the statistical influence of such waves on the dynamics of the boundary layer it is necessary to compile information concerning properties of the waves such as frequency of occurrence, propagation, and spectral characteristics. Gravity wave climatologies have been compiled from relatively few locations. In this paper a climatological study of gravity waves, in the period range 1–20 min, propagating in the stably stratified atmospheric boundary layer overlying an Antarctic ice shelf is presented. An extensive set of boundary layer measurements were compiled throughout 1991. Surface pressure fluctuations were recorded from a spatial array of six sensitive microbarographs. Wind and temperature records from an instrumented mast were also available. A beam-steering technique has been used to determine wave parameters from the surface pressure data. The microbarographs detected the presence of internal gravity waves throughout the observational campaign. Root-mean-square pressure values were typically in the region 16–40 μb, but a significant number of isolated events with amplitudes of up to 180 μb were also found. Wave properties have been studied in conjunction with the mean wind and temperature profiles in the boundary layer. It was found that most of the wave activity did not originate locally, but from shear layers aloft, or, more commonly, from the katabatic flow regime where the ice shelf joins the Antarctic continent.

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D. J. Lea, I. Mirouze, M. J. Martin, R. R. King, A. Hines, D. Walters, and M. Thurlow

Abstract

A new coupled data assimilation (DA) system developed with the aim of improving the initialization of coupled forecasts for various time ranges from short range out to seasonal is introduced. The implementation here is based on a “weakly” coupled data assimilation approach whereby the coupled model is used to provide background information for separate ocean–sea ice and atmosphere–land analyses. The increments generated from these separate analyses are then added back into the coupled model. This is different from the existing Met Office system for initializing coupled forecasts, which uses ocean and atmosphere analyses that have been generated independently using the FOAM ocean data assimilation system and NWP atmosphere assimilation systems, respectively. A set of trials has been run to investigate the impact of the weakly coupled data assimilation on the analysis, and on the coupled forecast skill out to 5–10 days. The analyses and forecasts have been assessed by comparing them to observations and by examining differences in the model fields. Encouragingly for this new system, both ocean and atmospheric assessments show the analyses and coupled forecasts produced using coupled DA to be very similar to those produced using separate ocean–atmosphere data assimilation. This work has the benefit of highlighting some aspects on which to focus to improve the coupled DA results. In particular, improving the modeling and data assimilation of the diurnal SST variation and the river runoff should be examined.

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P. H. Gudiksen, J. M. Leone Jr., C. W. King, D. Ruffieux, and W. D. Neff

Abstract

An experimental and modeling investigation of nocturnal drainage flows within the Mesa Creek valley in western Colorado revealed their wind and temperature characteristics and the effects of the ambient meteorology on their development. The valley, located about 30 km east of Grand Junction, is situated on the north slopes of the Grand Mesa. It is surrounded by ridges on three sides with low terrain toward the north. The terrain at the higher elevations is characterized by steep slopes that become shallower at the lower elevations. A network of seven meteorological towers and a monostatic solar collected data within the study area from December 1988 through November 1989. Analysis of the experimental data indicated that shallow drainage flows generated over the many individual slopes at the higher elevations converge at the lower elevations to form deeper flows that join with those generated within adjacent drainage areas. The characteristics of the flows generally deviated from those displayed by idealized slope flows due to both internal circulations within the valley and external influences. During the summer, the depths of the flows were typically a few tens of meters along the upper slopes and about 100 m over the upper part of the lower slopes while during the winter, the depths decreased to about 10 and 60 m, respectively. Their frequency of occurrence was highest during the summer or fall, about 50%, when the synoptic-scale influences were minimal. The flows along the upper slopes were particularly susceptible to influences by the ambient meteorology due to minimal terrain shielding. When the larger-scale ambient flows over the Grand Mesa were greater than about 5 m s−1, the surface cooling along the slopes was unable to develop and maintain the surface temperature inversion needed to generate strong drainage flows. The radiative cooling rates of the sloped surfaces, as characterized by net radiation measurements, were correlated with the downslope wind speeds observed along the upper slopes. Thus, a decrease in the observed net radiation level will produce a corresponding decrease in the downslope wind speed. Since temporal changes in net radiation levels are primarily governed by variations in atmospheric moisture, the effect of increased atmospheric moisture is to retard the development of the drainage flows.

In order to place the observations in proper perspective, it was necessary to employ numerical models that account for the physical processes governing the dynamics of the flows. The general features of the wind and temperature characteristics of the valley circulations and the influence of strong ambient winds and atmospheric moisture on the drainage flows over the upper slopes could be accounted for by numerical modeling techniques based on solving the equations of momentum, continuity, and energy coupled with a surface energy budget and a radiation module.

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Nicole P. M. van Lipzig, Gareth J. Marshall, Andrew Orr, and John C. King

Abstract

The large regional summer warming on the east coast of the northern Antarctic Peninsula (AP), which has taken place since the mid-1960s, has previously been proposed to be caused by a trend in the Southern Hemisphere Annular Mode (SAM). The authors utilize a high-resolution regional atmospheric model climatology (14-km grid spacing) to study the mechanisms that determine the response of the near-surface temperature to an increase in the SAM (ΔT/ΔSAM). Month-to-month variations in near-surface temperature and surface pressure are well represented by the model. It is found that north of ∼68°S, ΔT/ΔSAM is much larger on the eastern (lee) side than on the western (windward) side of the barrier. This is because of the enhanced westerly flow of relatively warm air over the barrier, which warms (and dries) further as it descends down the lee slope. The downward motion on the eastern side of the barrier causes a decrease in surface-mass balance and cloud cover. South of ∼68°S, vertical deflection across the barrier is greatly reduced and the contrast in ΔT/ΔSAM between the east and west sides of the barrier vanishes. In the northeastern part of the AP, the modeled ΔT/ΔSAM distribution is similar to the distribution derived from satellite infrared radiometer data. The region of strongest modeled temperature sensitivity to the SAM is where ice shelf collapse has recently taken place and does not extend farther south over the Larsen-C Ice Shelf.

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Fred M. Vukovich, Bobby W. Crissman, Mark Bushnell, and W. J. King

Abstract

Satellite infrared data and in situ data were used to study eddies off the east coast of Florida. The surface thermal manifestation of the eddies identified in the infrared data were alternating cold and warm tongues, conforming to the cyclonic spin-off eddies observed by Lee (1975); however, the eddies identified in the satellite data were larger than those observed by Lee. Statistics derived from the satellite data indicated that the eddies had average major and minor axes of 136 and 36 km, respectively. assuming an elliptic shape. They moved northward at an average speed of 30 km day−1, and the average period was 9 days.

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