Weather & weather systems at Schirmacher Oasis (Maitri) during recent two decades - A review

Indian Antarctic station Maitri experiences varying external influences from interior of east Antarctica as well as moving depressions and cyclones along the coast. The relative position of circumpolar trough and strengthening of high pressure centre near pole influences variation of atmospheric pressure at Maitri. The diurnal, daily and seasonal variation of temperature mainly depend upon moving pressure systems, katabatic winds, change of solar insulation with change of seasons, reflectivity from clouds and snow surface. The katabatic winds prevail over Maitri which is highly directional from South- East sector due to increase of slope towards south. The blizzards are main weather at Maitri, fog and white out are occasional phenomena. The precipitation is mostly in form of snowfall but rain is very rare at Maitri. Heavy or moderate snowfall indicative of active front leading edge of warm air masses being transported southwards. Strong temperature variant near Schirmacher oasis give precipitation in form of snow. Fog occurred due to slow movement of relatively warm air from lower latitude over the colder surface. Winter season witnessed more snowfall accumulation at Maitri than other season. During summer rise of temperature accompanied with absorption of latent heat by ice pellets in low level of atmosphere results precipitation in form of water droplets. Highest number of blizzards occurs during winter season whereas lowest number of blizzards occurs during summer season. Normally due to cyclonic activities, warm air masses transported towards the Schirmacher oasis which causes rise of temperature at Maitri. Longer duration of the blizzards over the station depends upon strength of slow moving blocking anticyclone situated east of Maitri at lower latitude. Tremendous fluctuation of atmospheric electric field observed before onset is a pre-indication of commencement of blizzards.

Impacts of atmospheric rivers on the hydrometeorology of the Euphrates-Tigris Basin in the snowmelt season

<p>Atmospheric rivers (ARs) are important components of the global water cycle as they are responsible for over 90% of the poleward moisture transport at middle to high latitudes. ARs travelling thousands of kilometers over arid North Africa could interact with the highlands of the Mesopotamia and thus affect the hydrometeorology and water resources of the Euphrates-Tigris Basin. Here, we use a state-of-the-art AR tracking database, and reanalysis and observational datasets to investigate the climatology (1979-2017) and influences of these ARs in snowmelt season (March-April). The Red Sea and northeast Africa are found to be the major source regions of these ARs, which are typically associated with the eastern Mediterranean trough positioned over the Balkan Peninsula and a blocking anticyclone over the Near East-Caspian region, triggering southwesterly air flow towards the highlands of the Euphrates-Tigris Basin. AR days exhibit enhanced precipitation over the crescent-shaped orography of the Euphrates-Tigris Basin. Mean AR days indicate wetter (up to +2 mm day<sup>-1</sup>) and warmer (up to +1.5<sup>o</sup>C) conditions than all-day climatology. On AR days, while snowpack tends to decrease (up to 30%) in the Zagros Mountains, it can show decreases or increases in the Taurus Mountains depending largely on elevation. A further analysis with the aid of observations and reanalysis for the three extreme AR events indicates that ARs coinciding with large scale sensible heat transport can have notable impacts on the surface hydrometeorological conditions such as snowmelt, rain-on-snow precipitation and increasing daily discharges of the Euphrates and Tigris rivers. These results suggest that ARs can have notable impacts on the hydrometeorology and water resources of the basin, particularly of lowland Mesopotamia, a region that is famous with great floods in the ancient narratives.</p>

A new detection scheme of wave-breaking events with blocking flow configurations

Blocking flow configurations, which tend to accompany strong circulation anomalies and therefore can cause extreme weather conditions, have recently been studied in relation to large-scale wave breaking (WB). Although WB events have been detected often from an instantaneous morphology perspective, the present study proposes a new approach for the detection from a wave-activity perspective in focusing on its accumulation, saturation and release. This evolution of wave activity is theoretically equivalent to anomalous potential vorticity (PV) flux with its sign changing from negative to positive, which is utilized in this study to detect WB events that accompany high-amplitude height anomalies and blocking flow configurations. As in previous studies, a given WB event is classified into a high-pressure type or low-pressure type depending upon the sign of the primary PV anomaly center and further into an eastward or westward type depending upon the longitudinal movement of that center. The new method applied to the wintertime Northern Hemisphere shows that a WB event with a blocking anticyclone is likely to accompany an eastward moving PV anomaly center, occurring mostly under anticyclonic westerly shear. By contrast, a WB event with a strong cyclonic anomaly mostly accompanies the eastward-moving PV anomaly center under cyclonic westerly shear. Composite analysis confirms the consistency between the sign-changing anomalous PV flux and convergence/divergence of wave-activity flux of quasi-stationary Rossby wave trains around the WB region.

The role of large-scale dynamics in an exceptional sequence of severe thunderstorms in Europe May–June 2018

Over 3 weeks in May and June 2018, an exceptionally large number of thunderstorms hit vast parts of western and central Europe, causing precipitation accumulations of up to 80 mm within 1 h and several flash floods. This study examines the conditions and processes that made this particular thunderstorm episode exceptional, with a particular focus on the interaction of processes across scales. During the episode, a blocking situation persisted over northern Europe. Initially, the southwesterly flow on the western flank of the blocking anticyclone induced the advection of warm, moist, and unstably stratified air masses. Due to the low-pressure gradient associated with the blocking anticyclone, these air masses were trapped in western and central Europe, remained almost stationary, and prevented a significant air mass exchange. In addition, the weak geopotential height gradients led to predominantly weak flow conditions in the mid-troposphere and thus to low vertical wind shear that prevented thunderstorms from developing into severe organized systems. Due to a weak propagation speed in combination with high rain rates, several thunderstorms were able to accumulate enormous amounts of precipitation that affected local-scale areas and triggered several torrential flash floods. Atmospheric blocking also increased the upper-level cut-off low frequency on its upstream regions, which was up to 10 times higher than the climatological mean. Together with filaments of positive potential vorticity (PV), the cut-offs provided the mesoscale setting for the development of a large number of thunderstorms. During the 22 d study period, more than 50 % of lightning strikes can be linked to a nearby cut-off low or PV filament. The exceptionally persistent low stability over 3 weeks combined with a weak wind speed in the mid-troposphere has not been observed during the past 30 years.

The role of large-scale dynamics in an exceptional sequence of severe thunderstorms in Europe May/June 2018

Over three weeks in May and June 2018, an exceptionally large number of thunderstorms hit vast parts of western and central Europe, causing precipitation of up to 80 mm and several flash floods. During this time, the large-scale atmospheric circulation, which was characterized by a blocking situation over northern Europe, influenced atmospheric conditions relevant for thunderstorm development. Initially, the southwesterly flow on the western flank of the blocking anticyclone induced the advection of warm, moist, and unstably stratified air masses. Due to a low-pressure gradient associated with the blocking anticyclone, these air masses were trapped in western and central Europe, remained almost stationary and prevented a significant air mass exchange. In addition, the low-pressure gradient led to weak flow conditions in the mid-troposphere and thus to low vertical wind shear that prevented thunderstorms from developing into severe organized systems. Most of the storms formed as local-scale, relatively slow-moving single cells. However, due to the related weak propagation speed, several thunderstorms were able to produce torrential heavy rain that affected local-scale areas and triggered several flash floods. Atmospheric blocking also increased the upper-level cut-off low frequency on its upstream regions, which was up to 10 times higher than the climatological mean. Together with filaments of positive potential vorticity (PV), the cut-offs served as trigger mechanisms for a majority of the thunderstorms. For the 22-day study period, we found that more than 50 % of lightning strikes can be linked to a nearby cut-off low or PV filament. The exceptional persistence of low stability combined with weak wind speed in the mid-troposphere over three weeks has not been observed during the past 30 years.

First data set of H₂O/HDO columns from the Tropospheric Monitoring Instrument (TROPOMI)

Global measurements of atmospheric water vapour isotopologues aid to better understand the hydrological cycle and improve global circulation models. This paper presents a new data set of vertical column densities of H2O and HDO retrieved from short-wave infrared (2.3 µm) reflectance measurements by the Tropospheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor satellite. TROPOMI features daily global coverage with a spatial resolution of up to 7 km×7 km. The retrieval utilises a profile-scaling approach. The forward model neglects scattering, and strict cloud filtering is therefore necessary. For validation, recent ground-based water vapour isotopologue measurements by the Total Carbon Column Observing Network (TCCON) are employed. A comparison of TCCON δD with ground-based measurements by the Multi-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water (MUSICA) project for data prior to 2014 (where MUSICA data are available) shows a bias in TCCON δD estimates. As TCCON HDO is currently not validated, an overall correction of recent TCCON HDO data is derived based on this finding. The agreement between the corrected TCCON measurements and co-located TROPOMI observations is good with an average bias of (-0.2±3)×1021 molec cm−2 ((1.1±7.2) %) in H2O and (-2±7)×1017 molec cm−2 ((-1.1±7.3) %) in HDO, which corresponds to a mean bias of (-14±17) ‰ in a posteriori δD. The bias is lower at low- and mid-latitude stations and higher at high-latitude stations. The use of the data set is demonstrated with a case study of a blocking anticyclone in northwestern Europe in July 2018 using single-overpass data.

First data set of H₂O/HDO columns from TROPOMI

This paper presents a new data set of vertical column densities of the water vapour isotopologues H2O and HDO retrieved from short-wave infrared (2.3 μm) reflectance measurements by the Tropospheric Monitoring Instrument (TROPOMI) aboard the Sentinel-5 Precursor satellite. TROPOMI features daily global coverage with a spatial resolution of up to 7 km × 7 km. The retrieval utilises a profile-scaling approach. The forward model neglects scattering, thus strict cloud filtering is necessary. For validation, recent ground-based water vapour isotopologue measurements by the Total Carbon Column Observing Network (TCCON) are employed. A comparison of TCCON δD with measurements by the project Multi-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water (MUSICA) for data prior to 2014 (where MUSICA data is available) shows a bias in TCCON δD estimates. As TCCON HDO is currently not validated, an overall correction of recent TCCON HDO data is derived based on this finding. The agreement between the corrected TCCON measurements and collocated TROPOMI observations is good with an average bias of (0.02 ± 2) · 1021 molec cm−2 in H2O and (−0.3 ± 7) · 1017 molec cm−2 in HDO, which corresponds to a bias of (−12 ± 17) ‰ in a posteriori δD. The use of the data set is demonstrated with a case study of a blocking anticyclone in northwestern Europe in July 2018 using single overpass data.