Synoptic Activity Research Articles

A numerical study of the diurnal variation of the Adelie Land katabatic wind regime

Katabatic winds occur with great frequency over the Adelie Land sector of Antarctica. Data collected by automatic weather stations (AWSs) along a string of stations from the coast of Adelie Land to the high interior plateau show a marked diurnal cycle of the wind and temperature regimes of the lower atmosphere during the short austral summer period. Numerical simulations of the katabatic wind regime have been conducted for a variety of solar forcing and synoptic conditions assuming a clear sky environment. Results suggest that the katabatic wind is a robust feature of the Antarctic boundary layer. Significant disruption of the low‐level drainage features occurs only during the midsummer months owing to the solar heating of the ice slopes and from synoptic activity along the Adelie Land coastline. During midsummer, modeled ground temperatures undergo a diurnal cycle with an amplitude of 15°C near the coast, in good agreement with the AWS observations. The katabatic wind decreases considerably during the middle of the day; wind speeds at coastal sites become reduced by more than one half of the early morning values. The numerical simulations depict a rapid summer to winter transition of katabatic wind conditions over the 1‐month period from mid‐February to mid‐March. The surface energy budget becomes negative for the entire 24‐hour period after mid‐February, corresponding to the rapid onset of the wintertime katabatic wind regime.

Modeled Variations of Precipitation over the Greenland Ice Sheet

Abstract A parameterization of the synoptic activity at 500 hPa and a simple orographic scheme are used to model the spatial and temporal variations of precipitation over the Greenland Ice Sheet for 1963–88 from analyzed geopotential height fields produced by the National Meteorological Center (NMC). Model coefficients are fitted to observed accumulation data, primarily from the summit area of the ice sheet. All major spatial characteristics of the observed accumulation distribution are reproduced apart from the orographic accumulation maximum over the northwestern coastal slopes. The modeled time-averaged total precipitation amount over Greenland is within the range of values determined by other investigators from surface-based observations. A realistic degree of interannual variability in precipitation is also simulated. A downward trend in simulated ice sheet precipitation over the 26 years is found. This is supported by a number of lines of evidence. It matches the accumulation trends during this peri...

Characteristics of Arctic synoptic activity, 1952?1989

Synoptic activity for the Arctic is examined for the period 1952–1989 using the National Meteorological Center sea level pressure data set. Winter cyclone activity is most common near Iceland, between Svalbard and Scandinavia, the Norwegian and Kara seas, Baffin Bay and the eastern Canadian Arctic Archipelago; the strongest systems are found in the Iceland and Norwegian seas. Mean cyclone tracks, prepared for 1975–1989, confirm that winter cyclones most frequently enter the Arctic from the Norwegian and Barents seas. Winter anticyclones are most frequent and strongest over Siberia and Alaska/Yukon, with additional frequency maxima of weaker systems found over the central Arctic Ocean and Greenland.

Tropospheric low‐level temperature inversions in the Canadian Arctic

Abstract Climatological characteristics of the low‐level tropospheric temperature inversion in the Canadian Arctic are examined using 10–40 year records of upper‐air meteorological data. Inversions at the northern sites are primarily surface‐based in winter, and elevated from mid‐spring through summer. At the southern sites, a bimodal pattern is observed with surface‐based inversions occurring during late summer, as well as during winter. From comparisons of our results with other published climatologies, it appears that this bimodal pattern reflects interactions between short‐ and long‐wave radiation, synoptic activity and snowmelt. Maxima in inversion depth and temperature difference across the inversion layer occur in February and March; minima occur in August and September. The annual progression of inversion characteristics closely follows the annual pattern of clear‐sky percentages, reflecting the controlling influence of cloud and clear‐sky radiative forcings on the inversion layer.

The Energetics and Propagation Dynamics of Tropical Summertime Synoptic-Scale Disturbances

Periods of enhanced synoptic activity in the tropical western Pacific, Bay of Bengal-northeastern India, and African-Atlantic regions are identified by extended empirical orthogonal function analysis. Composite mete-orological fields for such active periods at various sites are constructed using European Centre for Medium-Range Weather Forecasts (ECMWF) analyses for the northern summers of 1980–1987. These composite data form the basis for evaluating the contributions of different dynamical processes to local balances of beat, moisture, vorticity, enstrophy, and energy, so that the propagation dynamics and principal energy sources of the tropical disturbances may be studied in detail. In all three tropical regions considered here, the westward propagation of the synoptic-scale disturbances is attributed mostly to vorticity advection by both the time-mean flow and the transient fluctuations. In the western Pacific and Indian sectors, condensation heating associated with cumulus convection is seen to be the most significant energy source for the tropical disturbances. The stretching effect associated with large-scale convective activity is the most important mechanism for the generation of eddy enstrophy in these maritime disturbances. There is substantial barotropic conversion of enstrophy and kinetic energy from the time-mean flow to the transient fluctuations. In the African-Atlantic sector, the disturbances along the more prominent northern track at approximately 20°N are accompanied by dry desert-type convection. Vortex stretching associated with the dry convection is still the most important process for the generation of eddy enstrophy in these disturbances. However, the main source of available potential energy for these North African disturbances is the baroclinic conversion from the time-mean flow to the transient fluctuations along the zone of strong temperature gradients south of the Sahara. The dynamics and energetics of the weaker southern disturbances along 10°N in the African-Atlantic sector are similar to the moist disturbances found in the western Pacific and Indian sectors.

Low-Level Temperature Inversions of the Eurasian Arctic and Comparisons with Soviet Drifting Station Data

Seasonal and regional variations in characteristics of the Arctic low-level temperature inversion are examined using up to 12 years of twice-daily rawinsonde data from 31 inland and coastal sites of the Eurasian Arctic and a total of nearly six station years of data from three Soviet drifting stations near the North Pole. The frequency of inversions, the median inversion depth, and the temperature difference across the inversion layer increase from the Norwegian Sea eastward toward the Laptev and East Siberian seas. This effect is most pronounced in winter and autumn, and reflects proximity to oceanic influences and synoptic activity, possibly enhanced by a gradient in cloud cover. East of Novaya Zemlya during winter, inversions are found in over 95% of all soundings and tend to be surface based. For all locations, however, inversions tend to he most intense during winter due to the large deficit in surface net radiation. The strongest inversions are found over eastern Siberia, and reflect the effects of local topography. The frequency of inversions is lowest during summer, but is still >50% at all locations. Most summer inversions are elevated, and are much weaker than their winter counterparts. Data from the drifting stations reveal an inversion in every sounding from December to April. The minimum frequency of 85% occurs during August. While the median inversion depth is over 1200 m during March, it decreases to approximately 400 m during August, with median temperature differences across the inversion layer of 12.6° and 2.8°C, respectively. The median depth of the summertime mixed layer below inversions at the drifting stations ranges from 300 to 400 m. Seasonal changes in these inversion characteristics show a strong relationship with seasonal changes in cloud cover.

Winter atmospheric circulation in the Arctic Basin and possible relationships to the great salinity anomaly in the northern North Atlantic

Variations in synoptic activity are assessed in terms of potential influences on ice transport and indicators of shifts in the Arctic atmospheric circulation related to the “Great Salinity Anomaly” of the northern North Atlantic. Winter pressure anomalies suggest an increased contribution to the Fram Strait sea ice flux by multiyear ice from along the northern coasts of Greenland and Ellesmere Island. This may be related to a teleconnection involving changes in anticyclonicity over the central Arctic Ocean and Canadian Arctic Archipelago.

The lower atmosphere over the Gulf of California

Automated meteorological observations at coastal and island stations, instrumented aircraft flights, and coastal soundings were made between July 1983 and June 1985 to define the variations of the lower atmosphere over the northern half of the Gulf of California during the two important climatic seasons, the mid‐latitude winter and the subtropical summer. A marine layer is well defined over water during both seasons but dissipates within a few kilometers inland. The winter large‐scale pressure field is dominated by the Great Basin high over the southwestern United States. Modulated by upper level synoptic activity, it causes 3 to 6 days events of northwesterly winds (8–12 m s−1) directed along the gulf's axis, which are coherent over basin scales, and bring cool, dry desert air over the gulf. The vertical scale of these winds is 700–1000 m over land on the western side of the gulf, close to the height of the Baja California mountains, but only 100–400 m over water, defined by the wintertime inversion. Cross‐gulf winds, related to topographic effects during upper level trough passages over Baja California, are particularly intense in the northwestern portion of the gulf. The winter marine layer is defined by a 1° to 4.5°C temperature inversion and a dew point temperature of 6°–11°C (a moisture content of 6–8 g kg−1); weak subsidence reduces dew point values to −4°C above the inversion. Winter winds above the marine layer are coherent (0.8) across the width of the gulf. The summer large‐scale pressure field is dominated by a thermal low over the southwestern United States, and drives weak (2–5 m s−1) southeasterly winds, also directed primarily along the gulf, which are less spatially uniform than winter winds. Air temperature and moisture content are rather constant, and topographic effects are absent. The summer marine layer is on the order of 200–300 m thick, with dew point temperatures of 26°–28°C (21–24 g kg−1), and capped by a weak temperature inversion (1°–2°C) over water. Aloft, the dew point temperature is 17°–21°C, and winds are weak and uncorrelated across the width of the Gulf. During June and early July 1984, four week‐long pulses of southeasterly winds (10 m s−1) support the existence, reported previously, of moisture surges from the gulf into the Sonoran desert. A dynamical analysis of subdiurnal motions shows that the cross‐gulf momentum balance is geostrophic, the along‐gulf balance is ageostrophic, with the along‐gulf pressure gradient opposed by friction at the surface. These results are consistent with a scale analysis of the equations of motion in the marine layer over the Gulf of California.

The Dynamics of Low-Frequency Variability in a Simple Model of the Global Atmosphere

Abstract We examine the dynamics of low-frequency variability in a simple global model with zonally homogeneous boundary conditions. Low-frequency structure comprises trains of nearly equivalent barotropic Rossby waves with weak eastward phase velocities and strong eastward group velocities. These low-frequency eddies are strongly coupled to synoptic activity. The flux of vorticity by synoptic waves acts both to reinforce the low-frequency eddies and to retard their eastward propagation. Mechanistic experiments with an imposed forcing show that the synoptic-scale vorticity fluxes are organized by the low-frequency flow. The behavior is generally similar to that found in previous studies of blocks; the present work shows that strong mutual interactions between the low and synoptic frequencies also characterize less dramatic, low-frequency flows.

Characteristics of the low‐level temperature inversion along the Alaskan Arctic coast

AbstractThe climatological characteristics of low‐level tropospheric temperature inversion along the Alaskan Arctic coast are examined from a 10‐year record of surface and upper‐air meteorological data at Barrow and Barter Island, Alaska. The meteorology at the two stations is found to be remarkably similar; surface‐based inversions occur nearly half of the time, but even in the winter months inversions are frequently based up to 200 m above the surface. Median inversion depths range from 250m to 850m, with median temperature differences across the inversion ranging from 2°C to 11°C. In March and April when the inversion is strongest, the lowest levels frequently erode. This indicates that the transient character of the inversion reduces its effectiveness as a barrier to vertical mixing. The inversion depth closely follows the annual cloud‐cover cycle, demonstrating that the development and maintenance of the inversion is a result of complex interactions between radiative forcing, synoptic activity, and sea‐ice dynamics.

Synoptic Activity in the Arctic Basin, 1979–85

Abstract Synoptic activity in the Arctic basin from 70°–907deg;N is examined for the period 1979–85, using improved pressure analyses incorporating data from a network of drifting buoys. Geographical and seasonal variations in cyclone and anticyclone frequencies, mean cyclone pressures and other cyclone characteristics are determined. Results, in general, compare favorably with those from earlier studies. The atmospheric circulation of the Arctic is characterized by strong seasonality, but with considerable year-to-year variability, evidenced through large-scale seasonal shifts in the position and intensity of cyclone and anticyclone frequency maxima, the types of systems comprising these pattern, and major cyclone tracks. In winter and spring, cyclonic activity is largely restricted to the eastern Arctic. Local frequency maxima are found near Svalbard, the northern tip of Novaya Zemlya, and, in winter, also near the Pole at about 90°E. The systems comprising these patterns are migratory, the majority ent...

An Estimate of Mountain Drag during ALPEX for Comparison with Numerical Models

Abstract The pressure drag vector for a limited domain including the Alps is estimated for the ALPEX period of March and April 1982. All of the reported, three hourly, surface pressure data in the domain are used and maps of analyzed deviation surface pressure are obtained for the mountainous area. The very important drag values previously reported are confirmed and shown to occur when tight pressure gradients are localized above the steep orography. The peak values are found to be sensitive to the resolution of the analysis. A composite of the large drag variations occurring in relation to frontal passage and cyclogenesis is constructed. The diurnal cycle observed during periods of quiet synoptic activity is discussed. The observed drag compares favorably with the drag in the simulation of a case of cyclogenesis using a numerical model with comparable horizontal resolution.

Hurricane Debby—An Illustration of the Complementary Nature of VAS Soundings and Cloud and Water Vapor Motion Winds

The utility of VISSR Atmospheric Sounder (VAS) temperature and moisture soundings and cloud and water vapor motion winds in defining a storm and its surroundings at subsynoptic scales has been examined using a numerical analysis and prognosis system. It is shown that the VAS temperature and moisture data, which specify temperature and moisture well in cloud-free areas, are complemented by cloud and water vapor motion data generated in the cloudy areas. The cloud and water vapor 'winds' provide thermal gradient information for interpolating the soundings across cloudy regions. The loss of analysis integrity due to the reduction of VAS sounding density in the cloudy regions associated with synoptic activity is ameliorated by using cloud and water vapor motion winds. The improvement in numerical forecasts resulting from the addition of these data to the numerical analysis is recorded.

The role of local heat sources in synoptic activity within the polar basin

Abstract A quasi‐geostrophic model of the atmosphere is used to determine the significance of the surface enthalpy flux in synoptic activity within the Polar Basin. Of primary interest is whether the enthalpy flux from open water in the seasonal sea‐ice zone is the predominant contributing mechanism or whether the advective fields of vorticity and thickness are controlling factors. This is of importance in discussions of the feedback processes between the atmosphere, cryosphere and ocean. For a case selected in the Laptev Sea near the end of the fall period of ice growth, the surface enthalpy flux is as significant a contribution to synoptic activity as the vorticity advection is. The enthalpy flux is a relatively insignificant factor at this time in the Beaufort Sea, however, because of the smaller area of open water and the lower wind speeds associated with the weaker synoptic systems in this region. It is also relatively insignificant at both locations at the beginning of the fall freeze‐up interval an...

The dynamic climatology of the beaufort to Laptev sea sector of the polar basin for the summers of 1975 and 1976

AbstractFor the summer months of 1975 and 1976, the total vertical velocity at 850 millibars is computed from a diagnostic atmospheric model as an index of synoptic activity in the Beaufort to Laptev Sea sector of the Polar Basin. The objective is to identify the major modes of synoptic activity in the region and to determine the major formative mechanisms for significant synoptic vortices. This will lead to an understanding of the dynamics involved in the evolution and maintenance of synoptic patterns in the Arctic.Cluster analysis is used to stratify the 850 mb geopotential height data into synoptic modes. The mean 850 mb total vertical velocity field for each cluster is calculated, and indices related to each of the formative mechanisms are related to these individual fields through regression analysis and pattern comparison. These mechanisms are: vorticity and thickness advection, diabatic effects of latent heat release and surface heating, and frictional and orographic influences at the surface. Explanations for the dynamics of the major regional vortices are discussed.

Wind forcing due to synoptic storm activity over the North Pacific Ocean

Abstract Six‐hourly surface wind analyses over the North Pacific Ocean covering the 10‐year period 1969–78 are used to describe synoptic storm activity in terms of parameters that are directly related to the atmospheric forcing of the ocean. The cube of the atmospheric friction velocity, u3 * and the curl of the surface wind stress, curl τ, are used because of their relationship to turbulent vertical mixing and Ekman pumping in the ocean, respectively. In an attempt to isolate synoptic disturbances from mean fields, the time series of surface wind components at each individual grid point are partitioned into “high‐pass” (periods shorter than 10 days) and “low‐pass” (periods longer than 10 days) components by means of conventional filtering procedures. The two quantities u3 * and curl τ are then calculated from (a) the high‐pass filtered wind components only, (b) a combination of the filtered wind components that include the interaction between the high‐ and low‐pass fields, and (c) the unfiltered wind com...

Eigenvector Analysis of the Vertical Velocity Field Over the Eastern Canadian Arctic

Abstract To identify the predominant physical controls on the synoptic activity in the eastern Canadian Arctic, a limited area model has been applied to NMC grid data to compute the total vertical velocity and the component fields individually attributable to the differential advection of vorticity, the thickness advection, the release of latent heat, the effects of the surface enthalpy flux, and the influence of friction and orography at the surface. For a 234-day sample from the winter and summer of 1973, the major synoptic systems are identified by pattern (eigenvector) classification of the 85 cb total vertical velocity fields. For each synoptic pattern, the average physical processes responsible for the vertical circulation are determined by empirical comparison of the patterns in each component field of vertical velocity with the pattern of total vertical velocity. The physical linkages proposed are verified by subjective analysis of individual cases.

Some Effects of Surface Anomalies in a Global General Circulation Model1

The Mintz-Arakawa two-level general circulation model has been used in a series of experiments to compute the response of the atmosphere to certain persistent sea-surface temperature anomalies and to changes in the position of the continental Northern Hemisphere snow line over periods up to 90 days. Results are shown in terms of differences between anomaly and control histories are revealed by global, 30-day mean sea-level pressure maps, and time series of three regional indices of synoptic activity. The experiments show significant interhemispheric effects after about 1 month, phase shifts of 1 to 2 weeks in major cyclone developments, stronger reactions to sea-temperature anomalies in winter than in summer, and marked influence of the snow line on the winter monsoonal pressure difference between the continents and the North Atlantic Ocean.

ARCTIC SYNOPTIC ACTIVITY IN WINTER

Abstract The wintertime distribution, structure and motion of cyclones and anticyclones in the Arctic are analyzed through the use of 15 winter months of data. Histories of the most important types of pressure systems are described, and note is made of the interaction between the Arctic and middle latitudes. The effect of the receipt of weather reports from the central Arctic on previous concepts of the circulation is discussed. This study shows that the Arctic is a region of great synoptic activity with cyclones outnumbering anticyclones about 2 to 1. The central Arctic Basin is a region of cyclonic frequency equivalent to any place in the Northern Hemisphere outside of the semipermanent centers of action. In the mean, cyclones are warm in the troposphere because most of them enter the Arctic from over the warm waters of the Atlantic Ocean. Cyclones which originate in the Arctic are cold-core. Anticyclones, in the mean, are colder than cyclones. However, their centers are usually capped by slightly warme...

HEMISPHERIC NUMERICAL FORECASTING WITH THE BAROTROPIC MODEL, AND SOME REMARKS ON BOUNDARY-CONDITION ERROR

Abstract After further experiments with a pilot case reported earlier, the results are summarized for a selected ten-day series of predictions obtained over the northern hemisphere by conventional numerical methods with the barotropic model. The overall accuracy of the forecasts is found to decay steadily with increasing forecast period up to 72 hours. In comparison with a series of barotropic forecasts prepared earlier for North America only, the hemispheric predictions are shown to be free of serious boundary-condition error in the middle-latitude regions of major synoptic activity. Outstanding among the errors remaining in the hemispheric integrations, however, are those due to the variation of the density of observational data (especially serious over the Pacific and Asiatic regions), those caused by excessive anticyclogenesis, those due to truncation error and the lack of smoothing, and those inherent in the model's neglect of baroclinic development. Research is in progress on these and other errors,...