Thunderstorm climatology over northeast and adjoining east India

In this study an attempt has been made to develop climatology of thunderstorm, hail and squall over the northeast and adjoining east India region based on data of 26 India Meteorological Department (IMD) observatories for the months of April and May during 1981-2008. The data of 8 Indian Air Force (IAF) observatories in these regions which are available for the period of 1991-2008 have also been considered. The interannual & intraseasonal variation, climate change aspects and diurnal variation of occurrence of thunderstorms have been analysed and discussed. The frequency and time of occurrence of thunderstorm, hail and squall days over the region show large spatial variation. The orographically dominant regions as well as the coastal areas are more prone for such activity. There is increasing trend in number of thunderstorm days over some parts of south coastal Orissa and coastal West Bengal and decreasing trend over some parts of north coastal Orissa, Manipur, Mizoram and Tripura in recent years.

Assessing the Impact of Land Cover, Soil, and Climate on the Storage Potential of Dryland Sand Dams

Sand dams, a water-harvesting structure employed by rural communities in drylands have an inconsistent record of effectiveness. While many sand dams are highly functioning, improper siting, siltation, seepage, and high rates of evaporation from shallow sand reservoirs inhibit the water storage capacity of some sand dams. This study examines large-scale drivers of sand dam storage potential through analysis of an integrated surface and subsurface flow model. Multiple simulations were run, and comparative simulation analyses consider the effect of geomorphological factors, intraseasonal rainfall variability, and future climate conditions on sand dam performance criteria. The analyses revealed that a watershed highly cultivated with low water crops actually reduces evapotranspiration below that of natural vegetation and supports higher groundwater recharge. Additionally, intraseasonal variation and volume of rainfall impact sand dam performance less than the prevailing pattern and duration of dry and rainy seasons. Sand dams constructed in watersheds with sandier soils may experience greater connectivity with the stream margins and thus provide additional groundwater recharge. Lastly, climate change may improve some conditions desirable for sand dam performance, such as extending the duration of the rainy season and reducing overall evapotranspiration. However, the interactions between the expected climate change conditions and other geomorphological factors may result in a net decline in sand dam performance. The results of this study may help identify watersheds that are likely to support a sand dam with high potential for capturing and storing water throughout the dry season.

Distinct Western North Pacific Tropical Cyclogenesis During Inactive Intraseasonal Phase of August 1996 and 2014

While intraseasonal oscillation was in the inactive phase over the western North Pacific (WNP) during August of 1996 and 2014, no tropical cyclone (TC) genesis occurred in August of 2014, whereas 9 TCs (average 5.7 TCs) formed in August of 1996 with 5 TCs in the northeastern part (the largest number since 1979) and 4 TCs in the southwestern part. The present analysis reveals an obvious southwest-northeast-oriented lower-level wave train over the WNP associated with anomalous convection around the Maritime Continent in August 1996. This wave train induced anomalous cyclone and enhanced convection over the northeastern WNP, which provided a favorable background for TC genesis. Over the southwestern WNP, although monthly mean anomalies were unfavorable, the intraseasonal variation contributed to positive vorticity anomalies at the time and location of TC genesis. In contrast, both monthly anomalies and daily variations of environment factors were hostile to TC genesis during August 2014.

Intraseasonal variation and future projection of atmospheric diffusion conditions conducive to extreme haze formation over eastern China

<p>    Future projection of diffusion conditions associated with extreme haze events over eastern China is of great importance to government emission regulations and public human health. Here, the diffusion conditions and their changes under future warming scenarios are examined. The relative strength of haze events in the Northern China Plain region increase from 150% during 2006–15 to 190% during 2090–99 under RCP8.5 scenarios, induced by a stronger and longer-lasting anticyclone anomaly in eastern China. The strengthened anticyclone anomaly is mainly induced by increased northern wave train convergence emanating from the Barents–Kara Sea, and the longer duration of the anticyclone anomaly is mainly induced by stronger local feedback that can extract more energy from the basic state to maintain the anticyclone anomaly in eastern China. Aerosol reduction is found to play a dominant role in strengthening the upstream wave train near the Barents–Kara Sea and the downstream anticyclone in eastern China, while the effects from increased greenhouse gases are small. The results of this study indicate that future aerosol emissions reduction can induce deteriorating diffusion conditions, suggesting more stringent regulations on aerosol emissions in China are needed to meet air quality standards.</p>

Characteristics of Orographic Rain Drop-Size Distribution at Cherrapunji, Northeast India

The rain drop size distribution (DSD) at Cherrapunji, Northeast India was observed by a laser optical disdrometer Parsivel 2 from May to October 2017; this town is known for the world’s heaviest orographic rainfall recorded. The disdrometer showed a 30% underestimation of the rainfall amount, compared with a collocated rain gauge. The observed DSD had a number of drops with a mean normalized intercept log 10 N w > 4.0 for all rain rate categories, ranging from <5 to >80 mm h − 1 , comparable to tropical oceanic DSDs. These results differ from those of tropical oceanic DSDs, in that data with a larger N w were confined to the stratiform side of a stratiform/convective separation line proposed by Bringi et al. (2009). A large number of small drops is important for quantitative precipitation estimates by in-situ radar and satellites, because it tends to miss or underestimate precipitation amounts. The large number of small drops, as defined by the second principal component (>+1.5) while using the principal component analysis approach of Dolan et al. (2018), was rare for the pre-monsoon season, but was prevalent during the monsoon season, accounting for 16% (19%) of the accumulated rainfall (precipitation period); it tended to appear over weak active spells or the beginning of active spells of intraseasonal variation during the monsoon season.