Low-frequency atmospheric variability patterns and synoptic types linked to large floods in the lower Ebro River basin

This study analyzes the atmospheric variability that caused the largest floods affecting the town of Tortosa in the mouth of the Ebro River (northeast Iberian Peninsula). The Tortosa flood database and flood marks in the nearby town of Xerta are used to define the more relevant flooding episodes (discharges > 2900 m3s−1) of the 1600-2005 period. We explore the atmospheric variability based on low-frequency patterns and synoptic types applying a multivariable analysis to grids at sea-level pressure and geopotential at 500 hPa provided by the 20th Century V3 Reanalysis Project for the instrumental period (since 1836). Output from the Last Millennium Ensemble Project was used to analyze the sea-level pressure over the pre-instrumental period (before 1836). Our analysis includes 33 flood episodes. Four synoptic types are related to floods in Tortosa since 1836, characterized by low-pressure systems that interact with the Mediterranean warm air-mass and promote the atmosphere destabilization. Flooding in Tortosa is related to relative high values of solar activity, positive Northern Hemisphere temperature anomalies and NAO in positive phase. This indicates that the major floods are related to zonal atmospheric circulations (west to east cyclone transfer). During winter, the main impact of the floods is located at the western part of the basin, while the Pyrenean sub-basins are affected during autumn. The major finding is that similar flood behavior is detected since 1600, improving our understanding of past climates, enhancing the knowledge base for some aspects and impacts of climate change and reducing uncertainty about future outcomes.

SEA-LEVEL PRESSURE TRENDS IN THE SOUTHERN OCEAN AND ANTARCTICA FROM REANALYSIS AND IN SITU DATA

Trends in the mean sea-level pressure (SLP) in Antarctica in the last four decades (1980– 2020) have been examined using in situ observations and reanalysis data. The analysis involved time series of monthly mean, season-mean and yearly-mean values of the SLP derived from four reanalysis datasets, NCEP/NCAR, ERA5, JRA55, MERRA2, and from surface observations acquired from the Reference Antarctic Data for Environmental Research (READER) dataset. With this data we have evaluated the trends, characterized their seasonal peculiarities and variation across the high-latitude region of the Southern Hemisphere. The results of the analysis confirmed the dominance of decreasing trends in the annual mean SLP in Antarctica. Larger negative trends were found in the Western Antarctica with the most pronounced pressure drop in the South Pacific. The long-term decrease in the annual mean SLP in Antarctica was due to strong negative pressure trends in the austral summer and fall season whereas in winter and in spring the trends turn to mixed and mostly positive. The comparison of multiyear time series of SLP reanalysis data with in situ observations at Antarctic stations revealed a considerable overestimate of negative SLP trends in the NCEP/NCAR dataset. Among the four examined reanalysis datasets, ERA5 provided the best agreement with the station data on the annual mean and monthly mean SLP trend values.

Some climatological characteristics of mean sea level pressure in Bangladesh

On the basis of climatological data of 30 years (1951-1980) for 16 stations a climatological study of mean sea level pressure in Bangladesh has been accomplished. Spatial distribution and actual variation of mean sea level pressure have been studied. Attempt has been made to explain the cause of annual variation of mean sea, level pressure in Bangladesh from the point of view of synoptic meteorology. "Stability" of the meteorological stations of Bangladesh with respect to mean sea level pressure has been quired. The spatial variations of correlation of coefficients with regard to mean sea level pressure have been analysed. Finally, some characteristics of probabilities of mean sea level pressure at different materials for selected stations have been obtained.

Network of Automatic Weather Stations : Pseudo random burst sequence type

A network of 125 Automatic Weather Stations (AWS) has been set up by India Meteorological Department (IMD) during the year 2006-07 across India. Each station is configured to measure air temperature, hourly maximum temperature, hourly minimum temperature, relative humidity, station level pressure, hourly rainfall and cumulative rainfall for the day, Wind speed and Wind direction. In addition to these parameters, 25 stations provide data for global solar radiation and soil temperature. Five stations also provide soil moisture in addition to soil temperature. Each station transmits a data stream at an interval of an hour in a Pseudo Random Burst Sequence (PRBS) manner via UHF transmitter and a dedicated meteorological satellite KALPANA-1/ INSAT-3A to the central AWS data receiving Earth Station facility established at IMD, Pune. Mean sea level pressure, dew point temperature, duration of bright sunshine and daily maximum & minimum temperature are derived at the receiving Earth Station. Data archival in near real time is done at the receiving Earth Station. Data dissemination in WMO code form is also done in near real time through Global Telecommunication System. This paper provides technical description of various sub-systems of PRBS type Indian Automatic Weather Station network including instrument, satellite transmission technique, sensor characteristics, siting and exposure conditions and performance of a representative station.

Real-time mesoscale modeling for short range prediction of weather over Maitri region in Antarctica

The main objective of this paper is to implement Polar WRF model for the Maitri (Lat. 70° 45 S, Long. 11° 44 E) region at the horizontal resolution of 15 km using initial and boundary conditions of the Global Forecast System T-382 operational at the India Meteorological Department (IMD). The study evaluates the performance of the model using the conventional approach of case studies. The results of the case studies illustrated in this paper reveal that the model is capable of capturing synoptic and meso-scale weather systems. Forecast fields are consistent with the corresponding analysis fields. Synoptic charts of mean sea level pressure prepared by the Weather Service of South Africa at Pretoria are used for the model validation. The model derived meteograms of mean sea level pressure are compared against the corresponding observations. The study demonstrates the usefulness of the forecast products for short range forecasting of weather over the Maitri region. The forecast outputs are made available in the real-time mode in the national web site of IMD www.imd.gov.in. The study is expected to benefit weather forecasters at Maitri.

The onset and advance of Indian summer monsoon in relation with the sea level pressure field

The dates of onset of monsoon over Kerala and the advance of monsoon exhibit large interannual and intra seasonal variability. The genesis of Indian summer monsoon is considered to be in the southern hemisphere, over the Mascarene Island. The pressure gradient between the high pressure over Mascarene Island and low pressure over North West India is the fundamental physical cause of the monsoon, which acts as a driving force for the monsoon winds. Thus, the genesis of monsoon and its further advance is closely related to the sea level pressure field. With this backdrop, the variability of the onset and advance of the monsoon in terms of sea level pressure field is examined in this study. The pentad sea level pressure field does show contrast for the years of early and late onset of monsoon over Kerala and also for the years of rapid and sluggish advance of monsoon.

Variability of Saharan Cyclone Tracks, I: Climatology Study

Objectively, Saharan cyclones have been detected for the period from 1967 to 2019 using mean sea level pressure (SLP); their tracks have been specified from nearest neighbor cyclonic positions and classified into long/short tracks depending on the area of influence of the cyclones. Additionally, the detected long tracks have been objectively classified into five main routes directed generally eastward, northeastward and northward, accounting for approximately 41.6%, 19.7% and 30.4% of the total long tracks, respectively. Mainly for long tracks, three cyclogenesis areas, where more than 99% of cyclones are generated, were identified, with more than 61% generated in the Atlas region. Moreover, four far cyclolysis areas were identified, where approximately 74% of these cyclones terminated, with more than 66% of them terminating in the eastern study region. Furthermore, statistical analysis indicated that Saharan cyclones are commonly generated in the spring and summer, with ~35.3% and 46.3%, respectively. However, the highest numbers occur in spring in the northern Saharan and in summer in the southern Saharan, with ~49.1% and 57.7%, respectively. Temporally, the monthly distribution indicates that most of the cyclones moving along the five main routes are generated in warm months, namely, May to August. Approximately 85% of these cyclones have a lifespan of three days, while only 1% span more than five days.