Monsoon Situations Research Articles

Littoral drift analysis based on long-term observation of mesotidal beach profile in Kuta Beach, Bali for coastal retreat assessment

The beach profile survey in the intertidal zone is crucial for a temporal variability study of shoreline and beach profile change for coastal management. The combination of numerical modelling and field data has proven to be successful in identifying the primary hydrodynamic and sediment transport processes such as littoral and cross-shore drift. Those parameters are relevant to the sandbar migration process and shoreline changes. The purpose of the present study is to analyse the littoral drift that caused temporal variability shoreline change in mesotidal beach for coastal retreat mitigation. Beach profile data of Kuta Beach was analyzed by 7 years of long-term field observation data both east monsoon and west monsoon situation. The shoreline definition used mean sea level (MSL)1.3 m and high water level (HWL) 2.6 m as reference. By using the MeEPASoL program as a graphical user interface program, shoreline changes converging to an equilibrium state can be simulated by taking into account the existing breakwater. Temporal shoreline position resulting from littoral drift and beach width change from its initial position is estimated for coastal erosion analysis. The result showed that dominantly, the littoral drift pattern moved from south to north. Furthermore, this study can be used in the process of identifying the primary hydrodynamic analysis in erosion disaster management as assessment of the beach erosion.

Thermodynamic structure of monsoon boundary layer over the west coast of India

The study investigates thermodynamic structure of the atmospheric boundary layer (ABL) over the west coast of India during the southwest monsoon season. Radiosonde profiles at 00 UTC (0530 IST) are utilised for three stations, namely Thiruvananthapuram (TVM), Mangalore (MNG) and Mumbai (MUM) to represent south, central and north parts of the west coast, respectively. The high vertical resolution radiosonde data were collected from India Meteorological Department (IMD) for the period 1st May to 31st October 2018. Variations in the thermodynamic parameters associated with the monsoon and active and weak surges are examined. The active and weak monsoon conditions are identified using the values of OLR (outgoing longwave radiation) and rainfall in the respective stations. The study delineates interesting dynamic and thermodynamic features over the region. The thermodynamic parameters possess remarkable variations associated with the approach of the monsoon surges. TVM is characterised by absolutely stable or conditionally neutral (conditionally unstable) layer above a shallow conditionally unstable layer over the ground during the active (weak) monsoon situations; however, over MNG and MUM, ABL is conditionally unstable during both active and weak situations. Stable atmosphere is expected to cause stratiform clouds under favourable dynamic factors, whereas unstable atmosphere can aid vertical development of clouds and this can be attributed to the vertical shear of zonal wind. Zonal wind has a sharp increase from the ground to above ABL (~ 1200 m) in TVM; however, it is in the lower levels of ABL over MNG and MUM. In MUM, strong zonal wind is observed during the active (>20 m s−1) and weak (>12 m s−1) situations that causes intense mixing and maximum ABL height. The enhanced monsoon activity in MUM (MNG) is mainly associated with the active phase of the monsoon intraseasonal oscillation (active phase of the monsoon intraseasonal oscillation and strengthening of low level zonal wind), whereas in TVM it is mainly associated with the strengthening of low level zonal wind. The ABL heights are elevated during weak monsoon conditions due to convective turbulence added to the mechanical turbulence associated with the strong wind during the season.

Spatial Distribution of Extreme Rainfall Events During 2017 Southwest Monsoon over Indian Subcontinent

The extreme rainfall events over India during 2017 southwest monsoon and associated mechanisms are investigated using daily rainfall, winds and SST data sets. Extreme events occurred over mainly four locations according to the designed criteria, which are westcentral India (WC), central India (CI), northeast (NE) India and northwest coastal region (NWC). The events are identified mostly in the active phase of monsoon except in the northeast India, there the extremes are during break monsoon situation. As observed over the northwest coastal region and central India, most of the rainfall events are clustered during the peak monsoon (in the month of July). The seasonal frequencies of extreme events are significantly high in the west central and central India region. On the other hand, the relative contributions of these events are exceptionally high. In the northeast India, extreme events are followed the criterion of low-level wind regimes, where westerly regime induces a positive feed-back to enhance the extreme event. The temporal distribution and relative contribution of rainfall towards seasonal total in the northeast and northwest regions differ from other two selected regions. Global circulation pattern and local updraft contribute largely towards the seasonal rainfall in the west central and central India. A positive Indian Ocean Dipole mode together with the presence of a weak La Nina eventually favoured the rainfall in the northern parts during the month of July.

Characteristics of movement of low level clouds associated with onset / wet spells of northeast monsoon of Indian sub-continent as derived from high resolution INSAT OLR data

Characteristics of the northeast monsoon (NEM) have been studied utilising the outgoing long wave radiation (OLR) data derived over the north Indian Ocean and south peninsular India (SPI) from the series of Indian geostationary satellites. The contrasting feature of movement of the equatorial cloud zone from southeast to northwest direction during the onset phase of NEM has been reiterated using 1° × 1° gridded high resolution OLR data for the period 2000-2012. Presence of OLR values less than 180 Wm-2 over a large part of coastal Tamil Nadu on the date of onset and the simultaneous commencement of rainfall with clouding approaching SPI from southeast is clear from the study of superposed epoch analysis of the data. Triad means of OLR also substantiate this inference. During active spells of NEM which succeed prolonged dry spells, replication of the south to north movement of clouding by the OLR contours and the plausible reasons for such a movement have been brought out. The active monsoon situation existing over Sri Lankan region during the withdrawal phase of NEM over SPI is demonstrated with the depiction of the movement of OLR contours less than 230 Wm-2 over the region.

Wind‐driven changes of surface current, temperature, and chlorophyll observed by satellites north of New Guinea

AbstractSatellite observations of wind, sea level and derived currents, sea surface temperature (SST), and chlorophyll are used to expand our understanding of the physical and biological variability of the ocean surface north of New Guinea. Based on scarce cruise and mooring data, previous studies differentiated a trade wind situation (austral winter) when the New Guinea Coastal Current (NGCC) flows northwestward and a northwest monsoon situation (austral summer) when a coastal upwelling develops and the NGCC reverses. This circulation pattern is confirmed by satellite observations, except in Vitiaz Strait where the surface northwestward flow persists. We find that intraseasonal and seasonal time scale variations explain most of the variance north of New Guinea. SST and chlorophyll variabilities are mainly driven by two processes: penetration of Solomon Sea waters and coastal upwelling. In the trade wind situation, the NGCC transports cold Solomon Sea waters through Vitiaz Strait in a narrow vein hugging the coast. Coastal upwelling is generated in westerly wind situations (westerly wind event, northwest monsoon). Highly productive coastal waters are advected toward the equator and, during some westerly wind events, toward the eastern part of the warm pool. During El Niño, coastal upwelling events and northward penetration of Solomon Sea waters combine to influence SST and chlorophyll anomalies.

Planetary Boundary Layer height over the Indian subcontinent during extreme monsoon years

Planetary Boundary Layer (PBL) plays an important role in the regional monsoon circulation by exchanging heat and moisture between the earth surface and free atmosphere. However, PBL climatology over the Indian sub-continent in the context of extreme monsoon conditions is not available for its use in the regional climate models. In this study, a large-scale seasonal feature of PBL height over the Indian sub-continent is explored with 40 years (1961–2000) ERA-40 reanalysis data. The variations in the PBL height associated with excess and deficient monsoons over India are examined. The seasonal climatology showed higher PBL height during pre-monsoon (spring) and monsoon seasons as compared to post-monsoon and winter. For the extreme monsoon situations, PBL over the north-west (NW) India has greatly reciprocated. Over the NW India, during deficient monsoon years, composite anomalies of the PBL are observed to be negative indicating large reduction in PBL. Whereas during excess monsoon years, observed positive anomalies suggest increase in the PBL. During excess (deficient) monsoon years, the convection and the LLJ are found to be stronger (weaker) associated with the higher (lower) PBL height.

Extreme Rainstorm Events over the Northwest Himalayas during 1875–2010

AbstractA significant part of northern India is covered by the Himalayas, where a number of major Indian rivers originate. In the present study, a detailed analysis of rainstorms that affected the northwest region of the Himalayas has been made to assess the orographic effect of the Himalayas on precipitation in this region during the 135 years from 1875 to 2010. The study showed that the northwest Himalayas have experienced five most-severe rainstorms whose rain depths have not been surpassed so far. These severe rainstorms caused heavy to very heavy rainfall over the region that led to tremendous destruction to crops, communications, and livestock. Severe rainstorms have not occurred during the 2001–10 decade. It is also noticed that severe floods in this region have not occurred because of a “break monsoon situation” in the middle or eastern Himalayas.

On the parameterization of cloud droplet effective radius using CAIPEEX aircraft observations for warm clouds in India

Aircraft measurements of microphysical parameters in cumulus clouds taken during the Phase I of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX-1) project over the Indian region are used in this study. This data is found useful for verifying and tuning the schemes used in large scale models to parameterize the cloud effective radius (a representative parameter of cloud droplet spectra) using the cloud liquid water content and the cloud droplet number concentration. This analysis used 50-m altitude bin averaged vertical profiles of the microphysical parameters measured in clouds during pre-monsoon, transition-to-monsoon, and active monsoon situations sampled at various locations during June to August 2009. These monsoon cumulus clouds are found to be significantly diluted due to entrainment mixing. A linear relationship between the cloud droplet number concentration and adiabatic fraction (a measure of mixing between the cloud and its environment) is found for the each flight averaged data. The least squares best fit of this linear relationship is suggested as a simple formulation that will be useful for large scale models to predict cloud droplet number concentration, when only cloud liquid water content is diagnosed. The use of this simple formulation is shown to improve the skill of parameterized cloud effective radius for monsoon clouds.

Leaching losses of nitrate nitrogen and dissolved organic nitrogen from a yearly two crops system, wheat-maize, under monsoon situations

A large amount of nitrogen (N) fertilizers applied to the winter wheat–summer maize double cropping systems in the North China Plain (NCP) contributes largely to N leaching to the groundwater. A series of field experiments were carried out during October 2004 and September 2007 in a lysimeter field to reveal the temporal changes of N leaching losses below 2-m depth from this land system as well as the effects of N fertilizer application rates on N leaching. Four N rates (0, 180, 260, and 360 kg N ha−1 as urea) were applied in the study area. Seasonal leachate volumes were 87 and 72 mm in the first and second maize season, respectively, and 13 and 4 mm during the winter wheat and maize season in the third rotational year, respectively. The average seasonal flow-weighted NO3-N concentrations in leachate for the four N fertilizer application rates ranged from 8.1 to 103.7 mg N l−1, and seasonal flow-weighted dissolved organic nitrogen (DON) concentrations in leachate varied from 0.8 to 6.0 mg N l−1. Total amounts of NO3-N leaching lost throughout the 3 years were in the range of 14.6 to 177.8 kg ha−1 for the four N application rates, corresponding to N leaching losses in the range of 4.0–7.6% of the fertilizers applied. DON losses throughout the 3 years were 1.4, 2.1, 3.6, and 6.3 kg N ha−1 for the four corresponding fertilization rates. The application rate of 180 kg N ha−1 was recommended based on the balance between reducing N leaching and maintaining crop yields. The results indicated that there is a potential risk of N leaching during the winter wheat season, and over-fertilization of chemical N can result in substantial N leaching losses by high-intensity rainfalls in summer.

The evolution of mean conditions of surface meteorological fields during active/break phases of the Indian summer monsoon

The intra-seasonal variability of the Indian summer monsoon (ISM) is an important aspect that has been studied by various researchers, but still requires further examination to advance our understanding. A break monsoon situation, which is nothing more than a considerable reduction in rainfall over most parts of the Indian sub-continent, lasting from a few days to a few weeks, is one of the very significant periods of intra-seasonal variability of the ISM. In the present study, we examine the relationship between the break monsoon situation and changes in surface meteorological parameters using the National Centers for Environmental Prediction (NCEP) reanalysis datasets. Break monsoon situations persisting for a period of 3 days or more during 1968–1996 have been considered. An examination of average pentad data, commencing two pentads prior to the onset of the break situation to two pentads after the cessation of the break monsoon, is carried out in a sequential mode. The regions with statistically significant differences between prior and present break pentad are delineated by a Student’s t-test at the 95% confidence level.

High temporal and spatial resolution observations of meso-scale features of pre- and mature summer monsoon cloud systems over Bangladesh

Meso-scale characteristics of disturbances that bring about atmospheric disasters in pre- and mature monsoon seasons in Bangladesh are analyzed. Several types of meteorological instruments capable of observations with high temporal and spatial resolutions were introduced for the first time in this area to capture the meso-scale structure of rainfall systems. We installed an automatic weather station (AWS) and several automatic raingauges (ARGs) and utilized the weather radar of Bangladesh Meteorological Department (BMD). From the radar image in the summer of 2001 (16–18 July), a striking feature of the systematic diurnal variation in this area was elucidated. In these 3 days, the diurnal evolutions of convective activity were remarkably similar to each other, implying that this pattern can be understood as a typical response of local cloud systems to the diurnal variation of insolation under some summer monsoon situations. The ARG data show the difference in characteristics of rainfall between pre- and mature monsoon seasons. The short intense downpour tends to occur more frequently in the pre-monsoon season than in the mature monsoon season. The pre-monsoon rainfall also has clear diurnal variation with a peak that is more strongly concentrated in time. In the northern part the rainfall peak is found in between midnight and early morning, while it is observed in the daytime in central to western parts of the country. Two disaster cases caused by meso-scale disturbances are analyzed. Although they occurred in the same season, the structures of the cloud systems were largely different from each other. The disturbance brought about tornadoes on 14 April 2004, consisting of many spherical cloud systems of approximately 20 km size. On the other hand, another one that caused the tragic river water transport accident on 23 May 2004 had meso-scale rain band structure. The latter case was captured by the AWS located at Dhaka. Sudden changes in temperature, wind and pressure were observed clearly, showing the typical structure of convective rain bands.

The effects of climate and hydrology on the trophic status of Sélingué Reservoir, Mali, West Africa

AbstractThis work investigates the effects of climate and hydrology as factors controlling the trophic status of Sélingué, a monomictic reservoir in Mali, West Africa. Environmental (water transparency, hydrochemistry, nutrients) and biological (chlorophyll a concentrations) descriptors were studied on water samples collected biweekly from November 2000 to November 2001 in a station representative of the northern part of the water body. Statistical methods were used to estimate the existence and significance of breaks in the time series and environmental data were used to explain these breaks. In Sélingué Reservoir, the water column is stratified from March to May as a result of the cooling induced by the harmattan (North‐East trade winds). Stratification lasts a few weeks, until the beginning of the monsoon (South‐West trade winds), after which progressive warming allows vertical mixing of the lake. In harmattan or monsoon situations, the winds do not play a significant role except for storm events during the rainy season. Such events can change transiently, but markedly, the physical conditions prevailing in the whole water column. The water quality closely follows the hydrological cycle, characterized by a unique flood, which lasts a few weeks, from mid‐July. Input of nutrients during the flood events is limited and local mineralization of organic matter in the hypolimnion during the stratification period seems to play a major role in the availability of dissolved inorganic nitrogen and soluble reactive phosphorus. This availability is subject to vertical mixing which is only possible when the water height is sufficiently low and the wind‐induced energy is sufficiently high. Dry (dust during the harmattan period) and wet deposition (linked to rains) play a role in supplying nutrients to the lake and can be factors in the observed increased chlorophyll a concentrations. Thus, environmental factors in Sélingué change with the current hydrological conditions. Sélingué Reservoir can be regarded as oligotrophic during the high water period, but as meso‐eutrophic during the low water period. Each annual flood initializes the ecological conditions in this water body. This pattern limits the risks of trophic change (which can lead to algal blooms) to short periods of very low water levels immediately preceding flood events.

Breaks in monsoon and related precursors

A comprehensive analysis of eleven break monsoon situations that occurred during the period 1987 to 1997 have been attempted in the study. The various features like daily rainfall departures, wind anomalies and the satellite derived Outgoing Long wave Radiation (OLR) associated with the commencement/cessation of the break monsoon condition are studied with a view to identifying the precursors associate the break situation. The results reveal that there is progressive decrease of below normal rainfall departures 5 days prior to the actual break day in the latitude belts south of 20° N. During the period of the revival of the monsoon, the time section of the daily rainfall departures shows that the daily rainfall departure first starts becoming above normal in the southern most latitudinal belt 5° N to 10°N from the second day onwards after the cessation of the break. Similarly, the easterly anomalies in the zonal wind are first noticed in the southern latitude even 5 days prior to the starting of the break in the lower and middle troposphere. The maximum easterly anomalies in the lower and the middle troposphere move northwards upto 20° N. The composite latitudinal time section of OLR anomaly show a large area of negative OLR anomaly extending from 20°S to 10°N. The area is defined as the Southern. Hemispheric Convective Zone ( SHCZ). The negative OLR anomaly (10 Wm-2 is noticed around 5° S to 0° N. It increases to 20 Wm-2 on the second day of the break on the same latitudinal belt. The daily OLR anomaly pattern shows that the area of the negative OLR anomaly around the equatorial region increases with the approach of a break epoch. The forecasting aspects of the commencement / cessation of the break have been also discussed.

Performance of four intercropping systems under variable monsoon onset condi- tions in rainfed semi-arid region of Rajasthan

To provide a contingency cropping plan for the rainfed areas of hot semi-arid region of south-eastern Rajasthan a four seasons (2003 to 2007) kharif field experiment evaluated four intercropping systems viz. sorghum [ Sor- ghum bicolor (L.) Monech] + pigeon pea [ Cajanus cajan (L.) Millsp] (1:1), pigeon pea + black gram ( Vigna mungo L.) (1:2), castor ( Ricinus communis L.) + green gram [ Vigna radiata (L.) Wilczek] (1:2) and soybean [ Glycine max (L.) Merr.] + pigeon pea (4:1) under three dates of sowing, viz. first (normal) and third week of July, and first week of August. Four years mean pigeon pea grain equivalent data showed that sorghum + pigeon pea is the most re- munerative cropping system under normal monsoon conditions but the crop yields sharply decline with the de- layed onset of monsoon. The two week delayed planting of castor + green gram system did not show any yield re- duction while four weeks delayed planting reduced yield by 16% only as against corresponding yield reduction by 9 and 54%, 25 and 46% and 28 and 57% for sorghum + pigeon pea, pigeon pea + black gram and soybean + pi- geon pea intercropping systems respectively. Results indicated that if the monsoon onset is delayed by two weeks either castor + green gram or sorghum + pigeon pea should be the preferred cropping systems, whereas under four week delayed onset of monsoon situation only castor + green gram intercropping system has potential to maintain acceptable yield levels.

Rainfall based cropping system for rainfed vertisols of south Tamil Nadu

Field experiments were conducted at Regional Research Station, Aruppukottai to find out a suitable contingent cropping system for late monsoon situation. The treatment combination consisted of ten crops and three sowing times viz., 15th October, 30 October and 15th November. Three years pooled data revealed that pearl millet gave higher gross income of Rs. 5934 ha' when sown on 15th October with the normal onset of monsoon situation. If the rainfall delayed by 15-30 days, sunflower and coriander were. suitable for sowing during 30 October. Under very later monsoon situation upto 15th November, sunflower, horsegram and senna were suitable choice of crops for rainfed vertisols

Role of low pressure areas in the absence of tropical disturbances during monsoon months in India

In this study an effort has been made to find out the contribution of low pressure areas (or lows) towards the rainfall of northern and central India during the monsoon months of June to September in the absence of more intense cyclonic disturbances such as depressions, deep depressions and cyclonic storms. 15 years of data from 1983 to 1997 has been used to study this aspect. This study has shown that the occurrence of moderate to heavy rainfall mainly depends upon their frequency, life span, track followed and origin of these disturbances provided there are no inhibiting meteorological factors like ‘break’ monsoon situations. Copyright © 1999 Royal Meteorological Society

Role of low pressure areas in the absence of tropical disturbances during monsoon months in India

In this study an effort has been made to find out the contribution of low pressure areas (or lows) towards the rainfall of northern and central India during the monsoon months of June to September in the absence of more intense cyclonic disturbances such as depressions, deep depressions and cyclonic storms. 15 years of data from 1983 to 1997 has been used to study this aspect. This study has shown that the occurrence of moderate to heavy rainfall mainly depends upon their frequency, life span, track followed and origin of these disturbances provided there are no inhibiting meteorological factors like ‘break’ monsoon situations. Copyright © 1999 Royal Meteorological Society

GCMでシミュレートされたインドモンスーンとその変動に対する積雲対流ダウンドラフトの効果

The sensitivity of mean monsoon and its variability simulated by Hadley Centre Climate Model to changes in convection scheme, has been examined by integrating the model with and without parameterisation of convective downdrafts. Summer monsoon seasons during 7 years 1982, 1983, 1984, 1987, 1988, 1991 and 1994 have been simulated. These years have been selected for the study as the monsoon rainfall over India showed large interannual variability in these years. Results are compared with NCEP reanalysis circulation and Xie-Arkin precipitation data.Analysis of the model output shows that some of the systematic errors in the model in the monsoon simulation are reduced with incorporation of downdrafts. The systematic error by which the monsoon low level westerlies extend deep into the west Pacific ocean is substantially reduced by the inclusion of downdrafts. The lower tropospheric monsoon flow over the north Indian ocean is strengthened with downdrafts. TEJ simulated with downdrafts is closer to NCEP reanalysis. Inclusion of downdraft improves the precipitation simulation over land areas of India, and also simulates the correct position of the monsoon trough. When downdrafts are not included there is excessive precipitation over the west equatorial Indian ocean and less precipitation over India. Surface latent heat flux increases with the downdrafts. The systematic error of the model which simulates stronger low level circulation in El Niño years than in La Niña years is reduced, though not completely eliminated. The interannual variability of simulated monsoon precipitation is more realistic with downdrafts. Spells of active and ‘break, ’ or weak monsoon situations, are more prominently simulated when downdrafts are included. Larger number of strong synoptic disturbances are simulated over India with the inclusion of downdrafts.

Southwest monsoon rainfall in India: Part I-Spatial variability

Results of Principal Component Analysis (PCA) in spatial mode (S-Mode) applied on a 10 years' (1977-86) data set of weekly rainfall anomalies are presented in this study. The rainfall activity has been below average in the period under study with high standard deviations mostly over low rainfall areas. Inter - subdivisional correlation values suggest predominance of broad- scale weather systems over most parts of the country .The first principal component (PC) resemblance with the mean pattern. The second PC has been associated with active monsoon condition. The third and fourth PCs have been related to the northward progression of the Madden-Julian oscillation and the weak monsoon condition respectively.
 Resemblance of spatial patterns of alternative PCs with typical strong and weak monsoon activities suggest significant contributions from certain parts of the country (northeast, southeast) towards overall rainfall activity during weak monsoon situation.

日本海沿岸から脊梁山脈にかけた新雪中の主要化学組成の分布

Field work was carried out to investigate the regional characteristics of the concentration of major chemical constituents of snow cover from the coastal area of the Japan Sea to the mountain ranges of central Japan during the winter (January) of 1985. The snow cover in the interior highlands, such as Sugadaira and Takamine, was dry, and therefore the total snow cover retained the concentration of chemical constituents in the snow fall there. In contrast, in the Takada plain, a heavily snow-covered plain of the coastal area in Japan, the lower part held granular snow which contained melt water, and only the upper new snow layers retained the condition of accumulation of chemical constituents. The ratios of concentration of each of the chemical constituents in the new snow suggested that the Na+, Mg2+, and Cl- originated from sea water. The spatial distribution of the concentration of eash chemical constituent in the new snow which accumulated during one series of snowfalls under a winter monsoon situation was claritied. The concentration of chemical constituents originating in sea water decreased farther inland. This distribution was related not only to the distance from the coast but also to topography and the regional differences in cumulus development due to the topography. Excess Ca2+ and SO42- from the expected value originating in sea water were found. In particular, the distribution of excess Ca2+ showed strong regional characteristic, which suggested that there was another source of Ca2+ besides the sea water in the inland areas. Meanwhile, most of the samples contain excess SO42-, which decreases with decreasing amounts of accumulated new snow. The spatial distribution of NO3- concentration in the new snow cover in Sugadaira showed a large variation. The results suggested that the source of the nitrogen oxide was in the neighborhood of the sampling points.