Sea Level Pressure Gradient Research Articles

An Unusual Summertime Downslope Wind Event in Fort Collins, Colorado, on 3 July 1993

Abstract An unseasonal, severe downslope windstorm along the eastern foothills of the Colorado Rocky Mountains is described. The storm, which occurred on 3 July 1993, produced wind guts in Fort Collins, Colorado, over 40 m s−1 and resulted in extensive tree and roof damage. The synoptic pattern preceding the wind event resembled a pattern typical of that for a Front Range late fall or wintertime wind storm, including a strong south–southwest-oriented height gradient at 700 mb and a strong west to east sea level pressure gradient across the Front Range. A particularly interesting facet of the event was that one small geographical area in and near Fort Collins experienced wind gusts nearly 40% stronger than any other location involved in the event. The mesoscale forecast version of the Regional Atmospheric Modeling System (RAMS) with 16-km grid spacing over Colorado was run for the storm. Consistent severe winds were not predicted by the model in this configuration. Increasing resolution in postanalysis to ...

Short-term climatic fluctuations in North Atlantic Oscillation and frequency of cyclonic disturbances over North Indian Ocean and Northwest Pacific

A relationship between mean sea level pressure gradient between Azores High (AH) and Icelandic Low (IL) here after called North Atlantic Oscillation Index (NAOI) and the frequency of cyclonic disturbances over North Indian Ocean is investigated using 98 years of date (1891–1988). The analysis is carried out on monthly, seasonel, annual and decadal scales. Similar studies are also done for North West Pacific ocean cyclonic disturbances. It is noticed that the number of cyclonic disturbances over North Indian Ocean during monsoon season (June–September) as well as on annual scale is significantly correlated with NAOI. However for pre (April–May) and post (October-November-December) monsoon seasons frequency of cyclonic disturbances do not bear similar relationship with NAOI. The study also shows that decadal scale variability of cyclonic disturbances during the summer monsoon over North Indian Ocean has a remarkable correspondence with the decadal variability of NAOI.

Comments on “a synoptic climatological analysis of air quality in the Grand Canyon National Park”

Comments on “a synoptic climatological analysis of air quality in the Grand Canyon National Park”

Large‐scale circulation associated with westerly wind bursts and deep convection over the western equatorial Pacific

The relationship between deep equatorial convection over the western Pacific and atmospheric circulation during November to February, 1986–1992 is studied using cross correlations between outgoing longwave radiation (OLR) and National Meteorological Center global analyses. We focus on intraseasonal convective events on the 6‐ to 30‐day timescale over two regions: just east of Borneo and over the Tropical Ocean and Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) domain to the northeast of New Guinea. Although comparatively little work has been done on this timescale, we show that the bulk of the intraseasonal variability in both convection and low‐level circulation over the tropical western Pacific occurs in this period range. As a comparison with these higher‐frequency events, the evolution of COARE convection on the 30‐ to 70‐day timescale is also considered. The results of the historical data analysis are compared to a case study of deep convection associated with strong surface westerly winds during November 1989. This analysis is meant to serve as a benchmark for future case studies from the Tropical Ocean‐Global Atmosphere COARE experiment and other periods. In general, 6‐ to 30‐day equatorial convection in the regions studied occurs in conjunction with low‐level equatorial westerly wind anomalies, paired anomalous cyclonic circulations straddling the equator, and a strengthening of the sea level pressure gradient along the equator. The increase in the pressure gradient appears most often tied to the simultaneous equatorward movement of high‐pressure systems originating over southeastern Asia and Australia. A surge of trades over the central Pacific also accompanies the development of convection over both study regions. Following the convective peak, the low‐level westerlies slowly decay, as the northern cyclone moves northwestward away from the equator in a track characteristic of a tropical depression. It is also observed that low‐level westerly anomalies occur frequently without deep convection. The vertical structure during convective events along the equator on the 30‐ to 70‐day timescale and over Indonesia in the 6‐ to 30‐day band is baroclinic, with easterly anomalies at upper levels overlying low‐level westerlies. At 200 mbar a strengthened meridional outflow from the OLR signal into a wave train on the equatorward side of the Asian jet is observed to develop following the convective peak, suggesting forcing of this circulation by the convection. Over the COARE domain there is evidence that 6‐ to 30‐day convection often occurs in conjunction with deep westerly flow. It is suggested that these types of events are frequently characterized by vertical propagation of westerlies from the surface to the upper troposphere, as is shown for a case study from November 1989. This westerly burst episode is shown to have many features in common with the “typical” case as defined from historical data.

A Thermodynamic Equilibrium Climate Model for Monthly Mean Surface Winds and Precipitation over the Tropical Pacific

Abstract Diagnosis of the dynamic and thermodynamic balances using observed climatological monthly mean data reveals that 1) anisotropic, latitude-dependent Rayleigh friction coefficients lead to much improved modeling of the monthly mean surface wind field for a given monthly mean sea level pressure field, and 2) the annual variation of the vertically averaged lapse rate is important for modeling sea level pressure. Based on the aforementioned observations, a thermodynamic equilibrium climate model for the tropical Pacific is proposed. In this model, the sea level pressure is thermodynamically determined from sea surface temperature (SST) through a vertically integrated hydrostatic equation in which the vertical mean lapse rate is a function of SST plus a time-independent correction. The surface winds are then computed from sea level pressure gradients through a linear surface momentum balance with anisotropic, latitude-dependent Rayleigh friction coefficients. The precipitation is finally obtained from ...

Zonal gradient of the winter sea level atmospheric pressure at 50°N: An indicator of atmospheric forcing of North Pacific surface conditions

The Aleutian Low dominates the winter atmospheric circulation of the midlatitude North Pacific. The long‐term average of the lowest value of the winter (December, January, and February) mean sea level pressure, the AL, is located near 50°N, 177°E. Anomalies in the winter atmospheric circulation are primarily reflected in the magnitude and longitude of the lowest sea level pressure and also in the zonal gradient of sea level pressure at 50°N. West of the AL the zonal gradient is negative, indicating a northerly component of the geostrophic wind. East of the AL the zonal gradient is positive, indicating a southerly component of the geostrophic wind. Because of their pertinence to forcing of near‐surface ocean conditions, the zonal gradients at 50°N for the winters of 1947–1990 have been analyzed, and anomalous distributions have been identified. In this series, two distinct gradient patterns can be recognized. In the first pattern, proceeding eastward from the minimum near 140°E, the gradient rises monotonically to its maximum near 140°W. The magnitudes of the northerly and southerly components of the geostrophic winds are at their maximum at 140°E and 140°W longitudes, respectively. In the second group, again proceeding eastward from the minimum near 140°E, the gradient first rises to a secondary maximum, usually east of the AL and then declines to a secondary minimum before rising again to its maximum value near 130°W. In terms of the geostrophic wind, proceeding eastward from the longitude of the AL, the southerly component first increases then decreases and, in extreme cases, may reverse direction to a northerly component before increasing again to its maximum near the eastern boundary of the ocean. Results of model studies point to a possible linkage between patterns of the zonal gradient and tropical disturbances. To gain an appreciation of how an anomalous atmospheric circulation may affect near‐surface ocean conditions, the characteristic features of the zonal pressure gradient are examined and compared to previous studies. These include the effects on surface currents, on the turbulent flux of heat across the sea surface, and on the precipitation over the northwestern United States, British Columbia, and Alaska. The comparisons show that in the absence of direct observations, the zonal gradient of the mean winter (December, January, and February) sea level pressure at 50°N can be used to infer the consequences of an anomalous atmospheric circulation on ocean conditions in the high midlatitudes of the North Pacific.

The Influence of Coastal Orography: The Yakutat Storm

An unforecast windstorm in the vicinity of Yakutat, Alaska, on 14 March 1979 illustrates the importance of ageostrophic dynamics within a coastal zone proximal to significant terrain. Large pressure rises [greater than 4 mb (3 h)−1]were observed along the southeastern Alaska coast after passage of a cold front when the low- level geostrophic flow was directed onshore. These pressure rises did not occur simultaneously along the coast, but rather propagated northward along the coast as a coherent pulse or surge. Strong surface winds (approximately 25–30 m s−1) were observed in the region of laid sea level pressure gradient at the leading edge of the surge and occurred after the passage of the synoptic front. Although the sparseness of the observations prevent definite conclusions, this feature resembles a Kelvin wave more than a density current. Omega dropwindsonde observations collected along the coast of Alaska during two other, less dramatic, situations suggest damming and downslope flow structures important to the interpretation of the Yakutat storm. Coastal semigeostrophic dynamics, that is, an ageostrophic momentum balance in the alongshore direction, occurs when the coastal mountains are hydrodynamically steep. The steep regime is defined by the nondimensional slope (hm/lm)N/f>1, where hm is mountain height, lmis mountain half-width, N is the static stability for the incident flow, and f is the Coriolis parameter. For typical values of N∼10−2 s−1 the coast is wall-like when hm>0.01. Given a wall-like nature of the coast, trapped isolated mesoscale features, with an offshore length scale given by a Rossby radius of o(100 km), propagate alongshore ageostrophically due to a combination of Kelvin waves, density currents, or forced response. To correctly forecast in the coastal zone, numerical weather prediction models must qualitatively resolve terrain slopes so that the modeled dynamics are in the correct semigeostrophic or quasigeostrophic hydrodynamic regime.

Surface Pressure Perturbations Produced by an Isolated Mesoscale Topographic Barrier. Part II: Influence on Regional Circulations

Abstract This paper describes the detailed pressure and surface wind evolution associated with several important mesoscale phenomena in the region surrounding the Olympic Mountains of Washington state. The data used in this study were collected during the Olympex Experiment, during which eleven high-accuracy microbarographs were placed around the Olympics. The diurnal sea level pressure and surface wind variations of western Washington are described: lower pressure is observed over elevated terrain during the day with the opposite configuration at night. Next, this study documents the closed mesoscale lee lows that form in the lee of the Olympics during strong south-southwesterly synoptic flow regimes. Such lee lows create intense sea level pressure gradients and large ageostrophic accelerations along the eastern slopes of the Olympics. Another important feature examined in this paper is the Puget Sound Convergence Zone; it is shown that troughing in the lee of the Olympics plays an important role in dete...

The effect of volcanic aerosols on global climate

The surface temperature decrease after major volcanic eruptions has been demonstrated by a number of groups to be the order of a few tenths of a degree. At first, the stratospheric aerosol is usually restricted to a narrow range of latitudes, and the volcanically induced temperature decrease is found to be nonuniform over the surface of the earth. In fact, because of the differential cooling of land and ocean there should always be volcanically induced temperature gradients. These temperature gradients will induce a redistribution of air mass and anomalous sea level pressure gradients, anomalous winds and anomalous climate on a global scale. In this paper, a hypothesis is presented which assumes that volcanic aerosols, by reducing the solar radiation, induces surface temperature gradients and sea level pressure gradients between the land and the oceans. Since cooling of the land occurs before the oceans, there will be, at first, an increase of sea level pressure over land relative to the oceans. Conservation of total air mass on a global scale requires the sea level pressure over the ocean to be reduced.Because the majority of the land area is in the Northern Hemisphere, the volcanically induced cooling will transfer air mass from the oceanic anticyclones in both the Northern and Southern Hemisphere to the continents and especially to southern Eurasia. The anomalous sea level pressure pattern (SLP) which would be obtained — higher SLP over southern Eurasia and lower SLP over the southern oceans — is very close to the pattern, first named by Sir Gilbert Walker, called the negative phase of the Southern Oscillation. The positive phase of the Southern Oscillation can occur when no volcanic aerosols are present and the stratosphere is clear. Then the additional radiation and heating of the land increases the transfer of air mass from the Northern Hemisphere land mass to the world's oceans (the positive phase of the global Southern Oscillation).In this paper, it will be shown that this hypothesis has the potential to explain the observed association between low-latitude volcanic aerosols and the El Niño/Southern Oscillation. It also has the potential to explain why El Chichón's eruption induced a very strong El Niño compared to the weaker El Niño events induced by the eruptions of Agung and Krakatau. It also explains why almost all monsoon countries experience below-normal precipitation just prior to the maximum of the El Niño warming. The hypothesis also has the potential to explain why low- and high-latitude aerosols will affect the global climate system differently.This hypothesis represents a return to the ideas of Sir Gilbert Walker, who first defined the Southern Oscillation as a global-scale anomaly rather than an index between just two stations.

GCM studies of the African summer monsoon

A 37-year simulation of global climate by a 9-level GCM on an 8°×10° grid showed realistic interannual variation of the computed precipitation over the African Sahel. The model includes an interactive ocean so that interannual variations of sea-surface temperature (SST) also occur. Comparison of an ensemble of five summers that were rainy over the Sahel with five summers of simulated drought showed that insufficient ambient moisture was the immediate cause of the lack of moist convection. The drier conditions are shown to result from weaker moisture advection over the southeast Atlantic Ocean. Weaker southerly winds there and lower sea-level pressure gradients seemed to result from anomalously warm SST. Such SST anomalies have been linked to Sahelian drought in previous observational studies. These regional circulations that were conducive to lower rainfall rates during the north African summer monsoon were not manifestations of the more generalized zonal mean circulation.

Characteristics and frequency of reversals in mean sea level pressure in the north Atlantic sector and their relationship to long‐term temperature trends

AbstractReversals in the usual monthly mean sea level pressure (SLP) gradient over the North Atlantic during winter—an extreme mode of the North Atlantic Oscillation—are documented back to 1867. Such events are typically associated with well‐below‐normal temperatures in north‐west Europe and above‐normal precipitation amounts in the western Mediterranean area. Also noted during these events is the tendency for above‐normal temperatures in western Greenland and below‐normal temperatures in the eastern United States.Temporal changes in the frequency of SLP reversals relate well to trends in the strength of the zonal wind and the frequency of blocking in the north‐east Atlantic area. Part of the fluctuations in temperature in western Europe during the last century can be explained by these changes in circulation. Nevertheless, major trends in winter temperature over this period, particularly the early 20th century rise in temperature, remain when adjustments are made to account for the effects of circulation changes during reversal winters. This suggests that mechanisms other than changes in the frequency of circulation regimes also play an important role in determining hemispheric temperature trends on these time scales.

A Severe Windstorm in the Lee of the Cascade Mountains of Washington State

This paper describes a severe, leeside windstorm that struck parts of western Washington State on 23–25 December 1983. It is shown that strong winds, some exceeding 50 m s−1, were mainly limited to swaths downwind of gaps in the Cascade mountains. An examination of the December 1983 event and previous major windstorms indicates that they are generally associated with strong sea level pressure gradients, a stable layer near crest level east of the Cascades which subsides west of the mountains, and light to moderate easterly flow at crest level. The strongest surface winds during the December 1983 event were associated with the period in which the sea level pressure gradient across the Cascades was increasing rapidly. Furthermore, the variation of the leeside winds appears to be modulated by the flow normal to the mountains. It is shown that the horizontal pressure gradients can explain the strong winds and that lee wave amplification, although possible, is probably not of major importance.

Interannual variations in the half-yearly cycle of pressure gradients and zonal wind at sea level on the Southern Hemisphere

We outline the half-yearly oscillations in sea-level pressure gradients and zonal winds on the Southern Hemisphere by means of daily synoptic maps for several years. The pressure oscillation has amplitude peaks near 50° and the Antarctic coast, separated by a minimum along 60° where the phase changes from maxima in the extreme seasons in the south to maxima in the transitional season in the north. The zonal geostrophic wind consequently has a half-yearly wave with equinoctial maxima in the latitudes near 60°, and one with solstitial maxima north of 50°. We describe the interannual variability of the half-yearly oscillations in the pressure gradients and winds, and give examples of the oscillations during FGGE and the IGY. DOI: 10.1111/j.1600-0870.1984.tb00224.x

An investigation of summer sea surface temperature anomalies in the eastern North Pacific Ocean

A study of summer sea surface temperature anomalies in the eastern North Pacific Ocean was undertaken to examine some of the processes that could affect their evolution and which may be important for their prediction. An empirical approach was utilized. The factors considered include sea surface temperature (SST) persistence (due to the relatively large heat capacity of the water), oceanic thermal advection, and wind mixing (which presumably acts through changes in the mixed layer depth of the ocean). The primary variable studied was the summer SST anomaly for each of 25, 5° latitude by 10° longitude boxes in the region 140°W-170°E, 30–55° N. Thirty years of data were used (1947–76). Diagnostic analyses using data from four Ocean Weather Stations (C, D, P and V) were performed prior to the analyses described above in order to examine wind mixing more closely. Only at the OWS's are high frequency (3 hour), measured wind speeds available at the same location as SST observtions for a long period of time. These analyses indicated a statistically significant lag relationship between average April and May monthly wind speed and subsequent summer SST at Station P. The monthly mean SLP gradient was used as an estimate of average monthly wind speed over the 25 box domain where measured winds are not routinely available. In addition, winds derived from daily sea level pressure (SLP) analyses were averaged to form monthly means. At most boxes, no significant lag relationship between summer SST anomalies and spring (April and May) winds was found using either wind data set. The box-averaged data were employed to specify the summer SST from the components of the total horizontal thermal advection (based on the mean and anomalous components of the surface current and SST fields). The anomalous summer surface current advecting the mean summer SST field was found to be the dominant horizontal advective term affecting the local time rate of change of SST, particularly for 40–50° N. Substitution of derived May anomalous currents for those of summer greatly diminished the strength of the relationship. In the predictive portion of the study, skill relative to persistence was marginal based upon dependent sample testing. Thus, in the predictive mode neither wind mixing (as parameterized here) nor oceanic thermal advection added very much information not contained in the initial SST anomaly field. However, persistence alone exhibited considerable skill, particularly in the northeastern portion of the domain. DOI: 10.1111/j.1600-0870.1983.tb00202.x

On the Use of Analogues to Improve Regression Forecasts

Three methods of objectively forecasting maximum temperature using multiple-stepwise screening are described and compared. The first method is the traditional “perfect prog” approach whereby a historical series of upper air predictors are screened against maximum temperature to establish a prediction equation. The second method employs a stratification of the historical data on the basis of mean sea level pressure gradient analogues prior to the regression step. A new prediction equation is required for each forecast. The third method is similar to the second except that random stratification is used instead of analogue stratification. A comparison of the accuracy of the three methods when tested on independent data indicates that the best forecasts are obtained when analogue stratification is used. Application of the analogue stratification method to “model output statistics” forecasts and the statistical correction of prognoses is discussed.

Arizona Cool Season Surface Wind and Pressure Gradient Study

Abstract The average sea-level pressure gradients that produce sustained surface winds above 8 kt for at least six consecutive hours during the cool season at predetermined key stations in or adjacent to Arizona are investigated. Only wind directions with northerly or easterly components are included in the developmental data sample. Graphs are provided that relate the derived pressure gradients to varying surface wind speeds at each key station.

A Two–Dimensional Global Climatic Model

Abstract A two–dimensional global climatic model has been developed, using a 10° longitude by 10° latitude box grid and a one-month time step. The lapse rates of temperature and the meridional component of the flow in the oceans and atmosphere and of specific humidity in the atmosphere are parameterized in terms of their respective sea-level values. The model then utilizes the vertically-averaged thermodynamic energy equation, the equation of motion for the boundary layer, the thermal wind equation, the hydrostatic equation, the surface water balance equation, and empirical relations between the sea-level temperature and pressure gradient fields and between cloud cover and precipitation to determine the global distribution by months of sea-level pressure, temperature, wind speed and direction, and relative humidity, precipitation, evaporation, runoff, soil mositure content, ice and snow cover and thickness, cloud cover, and poleward energy transport. A simple ocean model is included. Two 100 model-year ru...

Short-Range Prediction in Isentropic Coordinates with Filtered and Unfiltered Numerical Models

Abstract Two limited-area prediction models in isentropic coordinates, one based on potential vorticity conservation and one using the primitive equations of motion, are tested on 50 synoptic cases chosen from winter and spring of 1972/73. Both models disregard diabatic processes, but incorporate variable terrain height. Numerical instabilities caused by overturning or entwining coordinate surfaces appear to be no problem in this approach, and the gravitational noise generated at the lower boundary in the primitive equation model is shown to remain well within acceptable limits. Skill scores based on displacement errors of surface cyclones and on correlations between predicted and observed sea-level pressure gradients indicate that neither model at this stage can compete with the six-level hemispheric model used by the National Weather Service. Of the two isentropic models, the one using primitive equations shows better skill than the potential vorticity model.

ESTIMATING PRECIPITATION AT SAN FRANCISCO FROM CONCURRENT METEOROLOGICAL VARIABLES1

Abstract Variables obtained from synoptic sea level and upper air charts are investigated to determine their significance in the estimation of concurrent rainfall. Eight variables consisting of sea level pressures and pressure gradients, pressure heights and height differences, and the temperature-dew point differences at two upper levels are combined into a graphical procedure to estimate the probability of occurrence of rainfall. With the probability of occurrence rising to above 50 percent, supplementary charts are used to estimate the amount of rainfall to be expected.