MSL Pressure Research Articles

On the relationship between Indian monsoon withdrawal and Iran’s fall precipitation onset

Indian monsoon is the most prominent of the world’s monsoon systems which primarily affects synoptic patterns of India and adjacent countries such as Iran in interaction with large-scale weather systems. In this article, the relationship between the withdrawal date of the Indian monsoon and the onset of fall precipitation in Iran has been studied. Data included annual time series of withdrawal dates of the Indian monsoon prepared by the Indian Institute for Tropical Meteorology, and time series of the first date of 25 mm accumulated precipitation over Iran’s synoptic weather stations in a 10-day period which is the basis for the cultivation date. Both time series were considered in Julian calendar with the starting date on August 1. The studied period is 1960–2014 which covers 55 years of data from 36 meteorological stations in Iran. By classifying the withdrawal dates of the Indian monsoon in three stages of late, normal, and early withdrawals, its relation with the onset of fall precipitation in western, southwestern, southern, eastern, central, and northern regions of Iran was studied. Results demonstrated that in four out of the six mentioned regions, the late withdrawal of the Indian monsoon postpones the onset of fall precipitation over Iran. No significant relation was found between the onset of fall precipitation in central region of Iran and the monsoon’s withdrawal date. In the western, southwestern, southern, and eastern regions of Iran, the late monsoon delays the onset of fall’s precipitation; while in the south Caspian Sea coastal area, it causes the early onset of autumnal precipitation. The lag in onset of fall precipitation in Iran which is coordinated with the late withdrawal of monsoon is accompanied with prolonged subtropical high settling over Iran’s plateau that prevents the southward movement of polar jet frontal systems. Such conditions enhance northerly wind currents over the Caspian Sea which, in turn, increase the precipitation in Caspian coastal provinces, which has a different behavior from the overall response of Iran’s climate to the late withdrawal of monsoon. In the phase of early monsoon withdrawal, the subtropical jet is located at the 200 hPa level in 32.5° north latitude; compared with the late withdrawal date, it shows a 2° southward movement. Additionally, the 500 hPa trough is also located in the Eastern Mediterranean, and the MSL pressure anomaly is between − 4 to − 7 hPa. The Mediterranean trough in the late withdrawal phase is located in its central zones. It seems that the lack of significant correlation between late withdrawal date of Indian monsoon and late fall’s precipitation onset in the central region of Iran depends on three reasons: 1. Lack of adequate weather stations in central region of Iran. 2. Precipitation standard deviations over arid and warm regions are high. 3. Central flat region of Iran without any source of humidity is located to the lee side of Zagros mountain range. So intensification or development of frontal systems is almost prohibited over there.

Variability of Some Meteorological Elements During the Southwest Monsoon Season in Bangladesh

Statistical computation and analysis of the co-efficient of variation of different meteorological parameter such as mean sea level pressure, rainfall, dew-point depression, minimum temperature and maximum temperature over Bangladesh during the onset, on-going and withdrawal phases of southwest monsoon, season of 1981- 2010 was dealt with. The study reveals that the coefficient of variation of the mean sea level (msl) pressure is practically small over Bangladesh during the onset and ongoing phases of southwest monsoon. During the onset phase, the CV of msl pressure is relatively higher and decreases during the on-going phase. The variation of msl pressure increases slightly in the withdrawal phase of monsoon. The ranges of CV of msl pressure are 0.064 - 0.197%, 0.079 - 0.809%, 0.091 - 0.793%, 0.069 - 0.272%, 0.0695 - 0.229% and 0.127 - 0.32% in May, June, July, August, September and October, respectively. The variation of dew-point depression decreases in the eastern part during the onset phase of southwest monsoon and becomes lower all over Bangladesh during the on-going phase of monsoon. It again increases during the withdrawal phase. The maximum co-efficient of variation of rainfall exists over the southeastern part of Bangladesh during the on-going phase of southwest monsoon and decreases in June and July over Bangladesh i.e. during the early on-going phase of monsoon. The co-efficient of variation of minimum and maximum temperatures is found to decrease during the onset and on-going phases of southwest monsoon and increases during the withdrawal of monsoon from Bangladesh.Journal of Bangladesh Academy of Sciences, Vol. 40, No. 2, 155-167, 2016

Mass balance of Mars’ residual south polar cap from CTX images and other data

Erosion of pits in the residual south polar cap (RSPC) of Mars concurrent with deposition and fluctuating cap boundaries raises questions about the mass balance and long term stability of the cap. Determining a mass balance by measurement of a net gain or loss of atmospheric CO2 by direct pressure measurements (Haberle, R.M. et al. [2014]. Secular climate change on Mars: An update using one Mars year of MSL pressure data. American Geophysical Union (Fall). Abstract 3947), although perhaps the most direct method, has so far given ambiguous results. Estimating volume changes from imaging data faces challenges, and has previously been attempted only in isolated areas of the cap. In this study we use 6m/pixel Context Imager (CTX) data from Mars year 31 to map all the morphologic units of the RSPC, expand the measurement record of pit erosion rates, and use high resolution images to place limits on vertical changes in the surface of the residual cap. We find the mass balance in Mars years 9–31 to be −6 to +4km3/♂y, or roughly −0.039% to +0.026% of the mean atmospheric CO2 mass/♂y. The indeterminate sign results chiefly from uncertainty in the amounts of deposition or erosion on the upper surfaces of deposits (as opposed to scarp retreat). Erosion and net deposition in this period appear to be controlled by summertime planetary scale dust events, the largest occurring in MY 9, another, smaller one in MY 28. The rates of erosion and the deposition observed since MY 9 appear to be consistent with the types of deposits and erosional behavior found in most of the residual cap. However, small areas (<10%) of the cap are distinguished by their greater thickness, polygonal troughs, and embayed contacts with thinner units. These deposits may require extended periods (>100♂y) of depositional and/or erosional conditions different from those occurring in the period since MY 9, although these environmental differences could be subtle.

An analysis of selected cases of derecho in Poland

The paper analyses six cases of the derecho phenomena, which occurred in Poland between 2007 and 2012. The input data included reports on dangerous meteorological phenomena, SYNOP and METAR reports, MSL pressure maps, upper air maps at 500hPa and 850hPa, radar depictions and satellite images, upper air sounding plots and data from a system locating atmospheric discharges. Derechos are caused directly by the activity of mesoscale convective systems linked up with either, in winter, a cold front of a deep low-pressure system, or, in summer, with an area of wind convergence in a warm sector of a cyclone or with an articulated cold front which, moving within a low-pressure embayment, develops a very active secondary depression. It was found that southern and central Poland were the regions most frequently affected by derechos. Mid-level and high-level jet streams, augmented by high thermodynamic instability of air masses, were found to be conducive to the development of derechos.

The main characteristics of atmospheric circulation associated with fog in Greece

Abstract. The characteristics of the atmospheric circulation over Europe and the Mediterranean associated with the formation and the dissipation of fog in Greece are examined. The data used consists of: i) 3-hourly meteorological observations recorded at 16 meteorological stations in Greece and ii) daily (00:00 UTC) 2.5×2.5 grid point values of mean sea-level pressure, 500 hPa geopotential height, 850 hPa and 500 hPa air temperatures and 1000–500 hPa thickness over Europe for the period 1957–2002. 1055 fog events are extracted from the 3-hourly meteorological observations. A specific methodology scheme including S-mode Factor Analysis and k-means Cluster Analysis is applied to the grid point data sets for the first day of a fog event (D day), the day prior to D day (D-1 day) and the day that follows the last day of a fog event (END day) and the 1055 evolutions of the atmospheric circulation associated with fog events in Greece are classified into 10 clusters. The mean patterns of MSL Pressure, 850 hPa and 500 hPa air temperatures, 1000–500 hPa thickness and 500 hPa geopotential height show that in most of the clusters, the presence of anticyclonic conditions over the Balkans, a warm front passage, or a weak, humid southerly flow induced by the presence of a shallow depression over the western Mediterranean favor fog formation in Greece, while the dissipation of fog occurs when drier air masses are transferred over the Balkans. The main differences among the 10 clusters refer to the exact position, the intensity and the specific evolution of the surface and the upper air systems, the season of their predominance and the area of the Greek territory that mainly refer to.

The atmospheric conditions over Europe and the Mediterranean, favoring snow events in Athens, Greece

Abstract. The 3-dimensional structure and the evolution of atmospheric circulation favoring snowfall in Athens are examined. The study refers to 61 snow events, which occurred during the period 1958–2001. For each one of the events, the patterns of MSL pressure, 850 hPa and 500 hPa air temperatures, 500 hPa geopotential height and 1000–500 hPa thickness are constructed for the European region, for the day before (D-1), the first day (D) and the day after the end of the event (END). A statistical methodology involving Factor Analysis and Cluster Analysis is applied to the above data sets and the 61 cases are finally classified into five clusters. These clusters are generally characterized by a north-easterly flow in the lower troposphere over the Athens area. This flow is associated with the presence of a low pressure system around Cyprus and an anticyclone over Europe. The position, the intensity and the trajectories of the surface and the upper air systems during D-1, D and END days are generally different among the five clusters.

Polar Mesoscale Cyclones in the Northeast Atlantic: Comparing Climatologies from ERA-40 and Satellite Imagery

Abstract Polar mesoscale cyclones over the subarctic are thought to be an important component of the coupled atmosphere–ocean climate system. However, the relatively small scale of these features presents some concern as to their representation in the meteorological reanalysis datasets that are commonly used to drive ocean models. Here polar mesocyclones are detected in the 40-Year European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis dataset (ERA-40) in mean sea level pressure and 500-hPa geopotential height, using an automated cyclone detection algorithm. The results are compared to polar mesocyclones detected in satellite imagery over the northeast Atlantic, for the period October 1993–September 1995. Similar trends in monthly cyclone numbers and a similar spatial distribution are found. However, there is a bias in the size of cyclones detected in the reanalysis. Up to 80% of cyclones larger than 500 km are detected in MSL pressure, but this hit rate decreases, approximately linearly, to ∼40% for 250-km-scale cyclones and to ∼20% for 100-km-scale cyclones. Consequently a substantial component of the associated air–sea fluxes may be missing from the reanalysis, presenting a serious shortcoming when using such reanalysis data for ocean modeling simulations. Eight maxima in cyclone density are apparent in the mean sea level pressure, clustered around synoptic observing stations in the northeast Atlantic. They are likely spurious, and a result of unidentified shortcomings in the ERA-40 data assimilation procedure.

Warm Season Lightning Probability Prediction for Canada and the Northern United States

Abstract Statistical models valid May–September were developed to predict the probability of lightning in 3-h intervals using observations from the North American Lightning Detection Network and predictors derived from Global Environmental Multiscale (GEM) model output at the Canadian Meteorological Centre. Models were built with pooled data from the years 2000–01 using tree-structured regression. Error reduction by most models was about 0.4–0.7 of initial predictand variance. Many predictors were required to model lightning occurrence for this large area. Highest ranked overall were the Showalter index, mean sea level pressure, and troposphere precipitable water. Three-hour changes of 500-hPa geopotential height, 500–1000-hPa thickness, and MSL pressure were highly ranked in most areas. The 3-h average of most predictors was more important than the mean or maximum (minimum where appropriate). Several predictors outranked CAPE, indicating it must appear with other predictors for successful statistical lightning prediction models. Results presented herein demonstrate that tree-structured regression is a viable method for building statistical models to forecast lightning probability. Real-time forecasts in 3-h intervals to 45–48 h were made in 2003 and 2004. The 2003 verification suggests a hybrid forecast based on a mixture of maximum and mean forecast probabilities in a radius around a grid point and on monthly climatology will improve accuracy. The 2004 verification shows that the hybrid forecasts had positive skill with respect to a reference forecast and performed better than forecasts defined by either the mean or maximum probability at most times. This was achieved even though an increase of resolution and change of convective parameterization scheme were made to the GEM model in May 2004.

Northeast Atlantic winter storminess 1875-1995 re-analysed

Northeast Atlantic winter storminess is analysed for the period 1875–1995 using a new dataset consisting of multi-daily mean sea level pressure observations from a selected set of stations in the northeast Atlantic. An analysis of storminess is presented, based on the high-pass filtered signal from these observations, from which selected percentiles are calculated for each winter. This method avoids potential inhomogeneity problems (artificial trends or jumps). Our finding is an increase, however not a dramatic one, during the past 2–3 decades in the northeasternmost part of the storm track, but the dominant features are inter-annual and decadal variations. Furthermore, the variations in storminess are found to be statistically linked to low-frequency circulation variations represented by the winter average of the MSL pressure field over the North Atlantic. It is argued that the existence of this link could have dynamical consequences for the response of the atmosphere to external forcing.

New regression model for Indian summer monsoon rainfall

Based on the study of 45 years (1948–1992) data, the average lowest MSL pressure of heat low over central Pakistan and adjoining northwest India of the month of May is found to have potential as a parameter for predicting all India Summer monsoon seasonal rainfall. This new parameter is seen to have stable and significant correlation with monsoon rainfall. Its correlation coefficients for different periods are found significant at 0.1% to 1% level of significance. The stability of the correlation coefficients was tested using 10, 20 and 30 year sliding windows. This test revealed that it is the most dependable parameter in comparison with 7 of the well known parameters analysed in this study. Regression models have been developed considering this new parameter along with other circulation parameters. The regression models developed are seen to perform very well for the independent data. The Root Mean Square Error (RMSE) values of some of these models, for independent data, are smaller than those of similar regression models reported in literature.

Evidence of Secular Variations in Indian Monsoon Rainfall–Circulation Relationships

Detailed correlation analysis of the all-India monsoon rainfall and mean sea-level seasonal pressure at Bombay (19°N, 73°E) up to three lags on either side of the monsoon wren during the last 30 years (1951–80) indicates a systematic relationship. The winter-to-premonsoon (March, April, May–Deceinber, January, February; MAM–DJF) seasonal pressure tendency at Bombay shows a correlation coefficient (CC) of −0.70 (significant at 0.1% level) with the Indian monsoon rainfall. Further examination of this relationship over a long period of 144 years (1847–1990), using sliding correlation analysis, reveals some interesting features. The sliding CCs were positive before 1870, negative during 1871–1900, positive in the years 1901–40, and again negative later on, showing systematic turning points around the years 1870, 1900, and 1940. In light of other corroborative evidence, these climatic regimes can be identified as “meridional monsoon” periods during 1871–1900 and after 1940, and as “zonal monsoon” periods before 1870 and during 1901–40, similar to the observation of Fu and Fletcher. It is also observed that the relationship between Bombay pressure and Indian monsoon rainfall becomes dominant when the ENSO variance in Bombay pressure is high and falls apart when the ENSO variance is small. The paper contains a listing of the long homogeneous data series on all-India monsoon rainfall and monthly MSL pressure at Bombay for the period 1847–1990.

Arctic cloudiness in spring from satellite imagery

AbstractMaps of cloud cover over the Arctic have been prepared from manual interpretation of Defense Meteorological Satellite Program images at 3‐day intervals for April, May and June, 1979,1980. Additional maps were prepared for selected dates to give a 10‐day sample for each of eight synoptic MSL pressure pattern types over the same area. Synoptic and monthly cloud distributions are compared with previous estimates. A large bias in our low cloud estimates for April may be caused by springtime Arctic have layers. The identification of substantial geographical and interannual variability of cloud amounts suggests that synoptic controls are important at this season.

Winter depression tracks and climatological jet streams in the Southern Hemisphere during the FGGE year

AbstractA band‐pass filter transparent to fluctuations in the 2£5‐6 day period range is applied to a set of twice‐daily analyses covering the 1979 (FGGE year) winter of the Southern Hemisphere. Fields analysed include MSL pressure, 500mb height, 300mb wind, and the meridional heat flux due to transient eddies at 850mb. Variance maxima in these filtered fields indicate regions of high cyclonic activity, with favoured cyclolysis areas in the vicinity of the Antarctic coastline also showing up in the MSL pressure statistics. Using these results, supplemented by data on the distribution of cyclogenesis, we are able to identify the major depression tracks. In this analysis, extensive use is made of studies relating satellite‐observed cloud vortices to the various stages of cyclonic development. In general, it appears there was significantly more activity in the eastern hemisphere than in the western during the 1979 winter.By evaluating the terms in the time‐averaged zonal momentum equation (using the unfiltered data set), it is possible to assess the role of transient eddies in the maintenance of the time‐mean zonal flow. This analysis shows that Coriolis acceleration associated with poleward time‐mean ageostrophic flow at the tropopause level dominates the forcing of the jet centred around 25°S, but that convergence of eddy momentum flux maintains the higher latitude jet around 50°S. However, a significant contribution to the latter jet comes from poleward ageostrophic flow south of Africa.

Statistical Correction of the Australian Region primitive Equation Model

Abstract The S1, skill score of the operational Australian Region Primitive Equation Model may be, reduced by a statistical correction scheme, in which the model prognosis of MSL pressure is used to predict errors in the MSL pressure differences. The predicted errors in the differences may be integrated and then added to the model prognosis to produce substantially improved MSL charts. Tests on independent data sets show that the scheme is reliable.

Eigenvector Filtering of Three-Dimensional Pressure Field Data

Abstract Grid-point data on the MSL pressure and 500 mb height fields over western North America are filtered with the eigenvectors of their covariance matrix. The filtering allows virtually all the information in 134 grid-point data for each day of the 8-year sample to be represented by 15 eigenvector coefficients. The three-dimensional eigenvector patterns are meteorologically coherent, and the filtering process can be used as a screen for bad data or for anomalous circulation patterns.

The Mass, Absolute Angular Momentum and Kinetic Energy Budgets of Model-Generated Extratropical Cyclones and Anticyclones

Abstract The budgets of mass, absolute angular momentum and kinetic energy for two model-generated cyclones and one anticyclone are examined using a sigma-coordinate framework which moves with the center of the MSL pressure extremum. The mass budgets for all three cases show a concentration of lateral mass transport in the surface boundary layer and at a level near 200 mb. The spin up of the low troposphere during cyclogenesis results from the dominance of the mean mode of lateral transport of absolute angular momentum. The spin up of the upper troposphere results from the combined influence of an inward eddy mode of lateral transport and vertical transport of absolute angular momentum. The eddy mode of lateral transport is determined by the configuration of the upper level flow (particularly jet streaks) and is enhanced by frontogenesis in the low and mid-troposphere as these regions spin up. The increase of kinetic energy in the low troposphere during cyclogenesis results from the dominance of local gen...

Conditions Favoring Glacierization and Deglacierization in North America from A Climatological Viewpoint

Approaches to paleoclimatic reconstruction on a local-regional scale through synoptic climatology and on a global scale through numerical modeling are discussed. Synoptic climatological considerations indicate the importance of southeasterly flow components for glacierization in Baffin Island, Labrador-Ungava, and Keewatin. It appears that no individual synoptic pattern is responsible for most of the ablation of ice bodies in eastern Baffin Island so that a synoptic climatological approach to deglacierization is likely to be indeterminate. The NCAR global circulation model with modified surface boundary conditions has been used to simulate the circulation at the last glacial maximum in January and July. The input data include the vertical and horizontal extent of ice, albedos relating to snow cover and vegetation distributions, and ocean temperature. Comparisons with control cases for present conditions show weaker jet maxima, but rather stronger low to mid-tropospheric westerly flow in the Northern Hemisphere, than at present for January and stronger upper westerlies in July. The MSL pressure maps, which must be interpreted with care in the vicinity of the ice sheets, are also discussed. The reconstructions provide a starting point on which hypotheses relating to the Wisconsin deglaciation must be based.

Conditions Favoring Glacierization and Deglacierization in North America from a Climatological Viewpoint

Approaches to paleoclimatic reconstruction on a local-regional scale through synoptic climatology and on a global scale through numerical modeling are discussed. Synoptic climatological considerations indicate the importance of southeasterly flow components for glacierization in Baffin Island, Labrador-Ungava, and Keewatin. It appears that no individual synoptic pattern is responsible for most of the ablation of ice bodies in eastern Baffin Island so that a synoptic climatological approach to deglacierization is likely to be indeterminate.The NCAR global circulation model with modified surface boundary conditions has been used to simulate the circulation at the last glacial maximum in January and July. The input data include the vertical and horizontal extent of ice, albedos relating to snow cover and vegetation distributions, and ocean temperature. Comparisons with control cases for present conditions show weaker jet maxima, but rather stronger low to mid-tropospheric westerly flow in the Northern Hemisphere, than at present for January and stronger upper westerlies in July. The MSL pressure maps, which must be interpreted with care in the vicinity of the ice sheets, are also discussed. The reconstructions provide a starting point on which hypotheses relating to the Wisconsin deglaciation must be based.