Why does convection weaken over Sumatra Island in an active phase of the MJO?

This study investigated the diurnal cycle of convection over Sumatra Island in an active phase of the Madden-Julian Oscillation (MJO) during the Pre-Years of the Maritime Continent (YMC) observation campaign in December 2015 based on in-situ and satellite observations and a convection-permitting numerical model. Observations suggest that before the active phase of the MJO in early December, convection occurred frequently over the island during the afternoon and at midnight. By contrast, during the active phase of the MJO in mid-December, afternoon convection over the island was delayed and suppressed, and midnight convection was suppressed. Numerical experiments also successfully replicated the main features of the observed modulations. In general, during the active phase of the MJO, the troposphere became drier in the Sumatra region. While the clouds reduced the solar radiation over the land, the sea breeze was also found to be delayed and weakened. As a result, the afternoon convection initiation was delayed and weakened. Further analyses suggested that the sea breeze was weakened mainly due to the orographic stagnation effect rather than the slightly reduced land-sea temperature contrast. On the other hand, the increased stratiform-anvil clouds induced the anomalous evaporative cooling in the mid-troposphere and generated island-scale subsidence during the nighttime, which finally led to the suppression of inland convection. Overall, our study reveals the modulation of diurnal convection over Sumatra Island by an active phase of the MJO and also shows the potential role of land-sea interaction in convection initiation and maintenance.

Climatological characteristics of sea breeze parameters at Chennai

Several sea breeze parameters such as time of onset, withdrawal, duration, depth, variation with height, direction etc. have been derived and studied for Chennai city and Chennai AP observatories in this study, which has been based on a large data base for the period March-October,1969-83. The monthly and sub monthly values of several sea breeze parameters have been derived. By invoking the concept of superposed epoch analysis the important role played by sea breeze in modulating diurnal variation of surface temperature and relative humidity has been established. The sea breeze at Chennai has been shown to be shallow with a depth of under 1 km. Modal directions of sea breeze and its normal speed have been derived.

Structure Analysis of the Sea Breeze Based on Doppler Lidar and Its Impact on Pollutants

The Doppler lidar system can accurately obtain wind profiles with high spatiotemporal resolution, which plays an increasingly important role in the research of atmospheric boundary layers and sea–land breeze. In September 2019, Doppler lidars were used to carry out observation experiments of the atmospheric wind field and pollutants in Shenzhen. Weather Research and Forecasting showed that the topography of Hongkong affected the sea breeze to produce the circumfluence flow at low altitudes. Two sea breezes from the Pearl River Estuary and the northeast of Hong Kong arrived at the observation site in succession, changing the wind direction from northeast to southeast. Based on the wind profiles, the structural and turbulent characteristics of the sea breeze were analyzed. The sea breeze front was accurately captured by the algorithm based on fuzzy logic, and its arrival time was 17:30 on 25 September. The boundary between the sea breeze and the return flow was separated by the edge enhancement algorithm. From this, the height of the sea breeze head (about 1100 m) and the thickness of the sea breeze layer (about 700 m) can be obtained. The fluctuated height of the boundary and the spiral airflow nearby revealed the Kelvin–Helmholtz instability. The influence of the Kelvin–Helmholtz instability could be delivered to the near-surface, which was verified by the spatiotemporal change of the horizontal wind speed and momentum flux. The intensity of the turbulence under the control of the sea breeze was significantly lower than that under the land breeze. The turbulent intensity was almost 0.1, and the dissipation rate was between 10−4 and 10−2 m2·s−3 under the land breeze. The turbulent intensity was below 0.05, and the dissipation rate was between 10−5 and 10−3 m2·s−3 under the sea breeze. The turbulent parameters showed peaks and large gradients at the boundary and the sea breeze front. The peak value of the turbulent intensity was around 0.3, and the dissipation rate was around 0.1 m2·s−3. The round-trip effect of sea–land breeze caused circulate pollutants. The recirculation factor was maintained at 0.5–0.6 at heights where the sea and land breeze alternately controlled (below 600 m), as well as increasing with a decreasing duration of the sea breeze. The factor exceeded 0.9 under the control of the high-altitude breeze (above 750 m). The convergence and rise of the airflow at the front led to collect pollutants, causing a sharp decrease in air quality when the sea breeze front passed.

“Late-stage” deforestation enhances storm trends in coastal West Africa

Deforestation affects local and regional hydroclimate through changes in heating and moistening of the atmosphere. In the tropics, deforestation leads to warming, but its impact on rainfall is more complex, as it depends on spatial scale and synoptic forcing. Most studies have focused on Amazonia, highlighting that forest edges locally enhance convective rainfall, whereas rainfall decreases over drier, more extensive, deforested regions. Here, we examine Southern West Africa (SWA), an example of “late-stage” deforestation, ongoing since 1900 within a 300-km coastal belt. From three decades of satellite data, we demonstrate that the upward trend in convective activity is strongly modulated by deforestation patterns. The frequency of afternoon storms is enhanced over and downstream of deforested patches on length scales from 16 to 196 km, with greater increases for larger patches. The results are consistent with the triggering of storms by mesoscale circulations due to landscape heterogeneity. Near the coast, where sea breeze convection dominates the diurnal cycle, storm frequency has doubled in deforested areas, attributable to enhanced land–sea thermal contrast. These areas include fast-growing cities such as Freetown and Monrovia, where enhanced storm frequency coincides with high vulnerability to flash flooding. The proximity of the ocean likely explains why ongoing deforestation across SWA continues to increase storminess, as it favors the impact of mesoscale dynamics over moisture availability. The coastal location of deforestation in SWA is typical of many tropical deforestation hotspots, and the processes highlighted here are likely to be of wider global relevance.

A numerical investigation of the influence of land-sea breeze on the atmospheric effluents released at a coastal site

ABSTRACT. Mesoscale features of a coastal atmospheric boundary layer such as the land-sea circulation and the thermal internal boundary layer (TIBL) structure have been simulated using a two-dimensional numerical boundary layer model. Using Boussinesq approximation for horizontal momentum equations and hydrostatic approximation for vertical momentum equation the model solves the 'shallow water' equations year over a grid domain 80 km length on either side of the coastline and 2 km height. The influence of the land-sea breezes on the dispersion of pollutants released from a continuous point source located at the roast has been studied. The fumigation of pollutants from an offshore source into TIBL over the land has also been illustrated. The limitations associated with the model are also discussed.

Analysis of Ocean-Atmospheric features associated with extreme temperature variation over east coast of India-A special emphasis to Orissa heat waves of 1998 and 2005

During the decade of 1998-2007, both Orissa and Andhra Pradesh at east Coast of India have been affected by heat waves more frequently and more severely causing very high damages to human lives. The most severe heat wave years for the region in the recent past are summer of 1998 over Orissa and 2003 over Andhra Pradesh when 2,042 and nearly 3054 people lost their lives respectively. In summer of 2005, though severe heat wave conditions were experienced for some days over Orissa and adjoining east coasts, the damages were not high as before. In view of such extreme temperature events have been regularly affected the region during the period where their normal frequency is low, analyses of their long period temperature data and study of their relationship with various regional and global ocean-atmospheric features are very much necessary, to find possible causes and then use them in forecasting. In the present study, an attempt has been made to analyze various temperature time series as available, varying from large domain to small domain, e.g., all India temperature, east coast of India temperature etc., to understand whether years which had recorded extreme temperatures in these larger domains have any relationship with that occurred over its very smaller domain, e.g., Orissa from station data, of which later is a part. To understand the relation between the magnitude of heat indices and loss to total human lives it caused during respective whole periods of heat waves, different heat indices, viz., general heat indices, Thom’s discomfort and Webb’s comfort indices have been computed during these extreme years over Orissa and Andhra Pradesh states and compared with total heat wave related human deaths over the respective states for the corresponding years. In addition to various heat indices, various Ocean-atmospheric characteristics, e.g., monthly SST over Bay of Bengal, day-to-day synoptic flow pattern, recurving Cyclonic Storms which strengthen low-level westerly and prohibit onset of Sea breeze over the coastal stations in the region causing persistent of heat waves, have also been critically analyzed both spatially and temporally to find role of these features in such occurrences. Their statistical lag correlations if any with ensuing temperature rise have been tested to explore the possibility of using them in forecasting these events much in advance.

Sensitivity studies on the air flow characteristics In Kharagpur using a meso-scale model

A three-dimensional hydrostatic model has been employed for the study of sea breeze circulations over south West Bengal with special reference to an inland station Kharagpur (22°.21' N, 87° 19'E). A series of sensitivity experiments have been performed to stress the Importance of differential heating on circulation over south West Bengal during pre-monsoon period. It is found that due to differential heating rate between land and sea surfaces, sea breezes can penetrate to the inland station Kharagpur and beyond even in case of moderate gradient wind. Surface observations at Kharagpur and pilot balloon observation at nearby station Kalaikunda are used to compare the model results. The onset of sea breezes, variation of the air temperature and humidity are In fairly good agreement whereas It over estimates the depth of the circulation and cannot predict the variation of the late morning hours surface wind.

See breeze like cloud-free zones during Monsoon months

Characteristic features of district cloud-free zones and their day to day evolution along Indian coasts as observed in INSAT cloud imageries during southwest monsoon months have been analysed and discussed along with sea surface and surface air temperatures and monsoon condition situation. It was a weak or break monsoon condition. Appearance of early morning clear zone just off shore along Indian Peninsula coasts is attributed to the gradual lowering of sea surface temperate due to upwelling caused by persistent favorable surface wind and slow-setting of air above colder water. With the advance of the day, wide extension of clear area over water where it ends abruptly and propagation of front-like zone inland manifest as a typical sea breeze. It is postulated that this is the effect of sea breeze circulation and shrinking of air above colder water. It is inferred that time-to-time appearance of such phenomenon may be an Indication of longer weak or break monsoon over the Peninsula.