Weak Monsoon Conditions Research Articles

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.

Composite analysis of precipitation intensity and distribution characteristics of western track landfall typhoons over China under strong and weak monsoon conditions

In this paper, 34 western track typhoons making landfall over China during the period of 2006–2016 from Jun. to Oct. are taken as research objects. They are divided into strong and weak monsoon influenced categories according to the East Asian summer monsoon index. Using precipitation product that was derived from hourly precipitation observed at automatic weather stations (AWS) in China and retrievals from CMORPH [CPC (Climate Prediction Center) morphing technique] satellite data, Japanese 55-year Reanalysis products (JRA-55) and Blackbody brightness temperature (TBB) data from FengYun 2E (FY-2E) satellite, precipitation intensity and distribution characteristics of strong monsoon influenced typhoons. In addition, the weak monsoon influenced typhoons was compared through composite analysis. Furthermore, to explore the water vapor conditions, dynamic factors, thermal structure, development conditions of mesoscale convective systems, and long-term typhoon maintenance mechanisms of typhoons under both conditions. This research is attempted to obtain the regularity of influences exerted on typhoon precipitation intensity and distribution by summer monsoon, and the possible influencing mechanisms of summer monsoon on typhoon precipitation.Results show that the stronger (weaker) the monsoon flow is, the stronger (weaker) the precipitation intensity of post-landfall western typhoon. Monsoon provides a sufficient amount of water vapor through direct moisture channel. The stronger monsoon flow is, the more obvious the coupling of low-level convergence and upper-level divergence, and thus provides a stronger lifting for typhoon precipitation. A strong monsoon condition is more conducive to the genesis and development of mesoscale convection system. A strong monsoon condition is also favorable for the maintenance of post-landfall typhoon's warm core structure, thereby prolonging the precipitation through delaying the decay of typhoons. All these factors lead up to the influences on precipitation of typhoons by summer monsoon directly or indirectly.

Intraseasonal Variability of Summer Monsoon Rainfall and Droughts over Central India

Rainfall over Madhya Pradesh (MP) in central India has large intra-seasonal variability causing droughts and floods in many years. In this study, rainfall variability in daily and monthly scale over central India has been examined using observed data. Consistency among various datasets such as rainfall, surface temperature, soil moisture and evapotranspiration has been examined. These parameters are from various different sources and critical for drought monitoring and prediction. It is found that during weak phases of monsoon, central India receives deficit rainfall with weaker monsoon circulation. This phase is characterized by an anticyclonic circulation at 850 hPa centered on MP. The EOF analysis of daily rainfall suggests that the two leading modes explain about 23–24% of rainfall variability in intraseasonal timescale. These two modes represent drought/flood conditions over MP. Relationship of weak phases of rainfall over central India with real-time multivariate (RMM) indices of Madden Julian Oscillation (MJO) has been examined. It is found that RMM-6, RMM-7, RMM-1 and RMM-2 describe the weak monsoon conditions over central India. However, frequency of drought occurrence over MP is more during RMM-7 phase. Surface temperature increases by about 0.5°–1° during weak phases of rainfall over this region. Soil moisture and evapotranspiration gradually reduce when rainfall reduces over the study region. Soil moisture and evapotranspiration anomalies have positive pattern during good rainfall events over central India and gradually reduce and become negative anomalies during weak phases.

Soil moisture variations in remotely sensed and reanalysis datasets during weak monsoon conditions over central India and central Myanmar

Variation of soil moisture during active and weak phases of summer monsoon JJAS (June, July, August, and September) is very important for sustenance of the crop and subsequent crop yield. As in situ observations of soil moisture are few or not available, researchers use data derived from remote sensing satellites or global reanalysis. This study documents the intercomparison of soil moisture from remotely sensed and reanalyses during dry spells within monsoon seasons in central India and central Myanmar. Soil moisture data from the European Space Agency (ESA)—Climate Change Initiative (CCI) has been treated as observed data and was compared against soil moisture data from the ECMWF reanalysis-Interim (ERA-I) and the climate forecast system reanalysis (CFSR) for the period of 2002–2011. The ESA soil moisture correlates rather well with observed gridded rainfall. The ESA data indicates that soil moisture increases over India from west to east and from north to south during monsoon season. The ERA-I overestimates the soil moisture over India, while the CFSR soil moisture agrees well with the remotely sensed observation (ESA). Over Myanmar, both the reanalysis overestimate soil moisture values and the ERA-I soil moisture does not show much variability from year to year. Day-to-day variations of soil moisture in central India and central Myanmar during weak monsoon conditions indicate that, because of the rainfall deficiency, the observed (ESA) and the CFSR soil moisture values are reduced up to 0.1 m3/m3 compared to climatological values of more than 0.35 m3/m3. This reduction is not seen in the ERA-I data. Therefore, soil moisture from the CFSR is closer to the ESA observed soil moisture than that from the ERA-I during weak phases of monsoon in the study region.

Real-Time Performance of a Multi-Model Ensemble-Based Extended Range Forecast System in Predicting the 2014 Monsoon Season Based on NCEP-CFSv2

The real-time validation of any strategy to forecast the Indian summer monsoon rainfall requires comprehensive assessment of performance of the model on sub-seasonal scale. The multi-model ensemble (MME) approach based on the NCEP-CFS version 2 models, as developed and reported earlier, has been employed to forecast the 2014 monsoon season on the extended range scale with 3-4 pentad lead time (where a pentad corresponds to five-day average). The present study reports the broad performance of the MME employed on experimental basis to forecast the salient features of the real-time evolution of the 2014 monsoon season during June to September. The MME is successful in predicting both these features well in advance (3-4 pentad or 15-20 days lead time). The assessment of the model performance at pentad scale lead time shows that the weak monsoon conditions that are evident in precipitation and lower level wind anomalies are well captured as a whole up to four pentad advance lead time. The subseasonal propagation during onset and withdrawal is also evident in the forecast. Finally, the region-wise performance shows that the spatial extent of the skillful forecast encompasses central India as well as the monsoon zone for the 2014 monsoon season. Considering the natural variation in the forecast skill of extended range forecast itself as reported in earlier studies, the 2014 monsoon forecast seems to be skillful for operational purposes. For other regions (e.g. North East India), the forecast could be skillful at times, but it still requires further research on how to improve the same.

Impact of the Intraseasonal Variability of the Western North Pacific Large-Scale Circulation on Tropical Cyclone Tracks

Abstract The life cycle of the Southeast Asian–western North Pacific monsoon circulation is established by the northward migrations of the monsoon trough and the western Pacific subtropical anticyclone, and is reflected by the intraseasonal variations of monsoon westerlies and trade easterlies in the form of an east–west seesaw oscillation. In this paper, an effort is made to disclose the influence of this monsoon circulation on tropical cyclone tracks during its different phases using composite charts of large-scale circulation for certain types of tracks. A majority of straight-moving (recurving) tropical cyclones appear during weak (strong) monsoon westerlies and strong (weak) trade easterlies. The monsoon conditions associated with straight-moving tropical cyclones are linked to the intensified subtropical anticyclone, while that associated with recurving tropical cyclones is coupled with the deepened monsoon trough. The relationship between genesis locations and track characteristics is evolved from the intraseasonal variation of the monsoon circulation reflected by the east–west oscillation of monsoon westerlies and trade easterlies. Composite circulation differences between the flows associated with the two types of tropical cyclone tracks show a vertically uniform short wave train along the North Pacific rim, as portrayed by the Pacific–Japan oscillation. During the extreme phases of the monsoon life cycle, the anomalous circulation pattern east of Taiwan resembles this anomalous short wave train. A vorticity budget analysis of the strong monsoon condition reveals a vorticity tendency dipole with a positive zone to the north and a negative zone to the south of the deepened monsoon trough. This meridional juxtaposition of vorticity tendency propagates the monsoon trough northward. The interaction of a tropical cyclone with the monsoon trough intensifies the north–south juxtaposition of the vorticity tendency and deflects the tropical cyclone northward. In contrast, during weak monsoon conditions, the interaction between a tropical cyclone and the subtropical high results in a northwestward motion steered by the intensified trade easterlies. The accurate prediction of the monsoon trough and the subtropical anticyclone variations coupled with the monsoon life cycle may help to improve the forecasting of tropical cyclone tracks.

Ocean-atmosphere interaction and synoptic weather conditions in association with the two contrasting phases of monsoon during BOBMEX-1999

Surface meteorological parameters acquired during the field phase experiment, BOBMEX-99, for the stationary periods (SP I and II) of the ship ORV Sagar Kanya over Bay of Bengal have been analysed. Active and weak monsoon conditions were observed during the first and the second phase of the experiment respectively over India as well as over the stationary ship location. The phase mean sea surface temperature (SST) is found to be the same in both the phases, however large differences have been observed in the phase mean values of wind speed, mean sea level pressure, latent heat and momentum fluxes at air-sea interface. Synoptic scale monsoon disturbances formed only during the period of strong north-south pressure gradient over the Bay region. Events of prominent fall in SST and in the upper 15 m ocean layer mean temperature and salinity values during typical rainfall events are cited. The impact of monsoon disturbances on ocean-atmosphere interface transfer processes has been investigated.

Meso-scale distribution of summer monsoon rainfall near the Western Ghats (India)

The spatial distribution of southwest monsoon rainfall is studied over Maharashtra State (India), which includes part of the well-known Western Ghats mountain range, near its western boundary, running almost from north to south, perpendicular to the summer monsoon current in the lower troposphere. Meso-scale analysis of daily rainfall is performed for Maharashtra State, including the Western Ghats, for the two mid-monsoon months of July and August, during the 10-year period of 1971-1980. Strong and weak monsoon days were identified for the 5-year period of 1976-1980. The meso-scale pattern of average daily rainfall is obtained separately for strong and for weak monsoon conditions. All these average patterns show the following features: (i) the rainfall increases rapidly from the Arabian Sea coast close to the line of maximum height of the Western Ghats; (ii) there are two rainfall maxima corresponding to the two mountain peaks parallel to the coast line; (iii) between the two mountain peaks, there is a valley which is narrow at the western end (upwind end), broadening towards the east (on the downwind side). Ground contour height of the valley rises eastwards and ends as a part of the Deccan Plateau east of the Ghats. Here the valley opens out like a funnel with higher mountains flanking its two sides. In the valley, the rainfall increases from the coast up to the line of maximum height of the Ghats, and then decreases eastwards towards the plateau. The rainfall isopleths also take a funnel-shaped configuration. An interesting feature is that near the wider section of the valley funnel, there is a rainfall minimum and then the rainfall increases further eastwards on the downwind side. This feature of rainfall minimum is somewhat similar to the rainfall minimum reported by Asnani and Kinuthia (personal communication); Asnani (Asnani GC. 1993. Tropical Meteorology, Vol. I. Prof. G.C. Asnani: Pune, India; 603) attributed the rainfall minimum to the Bernoulli effect. A somewhat similar phenomenon is assumed in the present study area.

Inter-annual and intra-seasonal variation of some characteristics of monsoon disturbances formed over the Bay

The characteristics of monsoon disturbances during drought and flood years for the period 1971-96 are studied to find out their inter-annual variations. Variations of some of the characteristics of monsoon disturbances formed over Bay during 1979-88, with respect to different monsoon conditions such as strong, weak and break monsoons, are also studied. The results show that monsoon disturbance days are higher during flood years than during drought years. Drought years are associated with higher chances of low pressure areas to intensity into depressions, less westward movement, more horizontal extent, intense pressure departure from normal in comparison with flood years. However, more monsoon disturbances tilt significantly during flood years. The rainfall associated with these disturbances is highly variable and does not depend on the density, horizontal and vertical extent of the individual system. More number of lows intensify into depressions during strong monsoon conditions compared to those of weak monsoon conditions. Lows and depressions during strong monsoons have more westward movement and longer life period. Generally, very few lows form during break monsoon and none of them intensify into depression. Hence, the presence of mid-tropospheric heating during strong and weak monsoons is essential for the formation of depression. Synoptic systems which abate break monsoon condition and re-establish normal monsoon are also discussed.

気象庁全球モデルでシミュレートされた北半球夏の熱帯の30-60日季節内振動

The JMA global model at T106 horizontal resolution has been integrated for 10 years with observed SST. Intraseasonal variations in the tropics during N.H. summer have been examined using 30-60 day filtered anomalies from the model simulated results. Eastward propagation of velocity potential anomalies on a 30-60 day time scale has been noticed throughout the equatorial region with a period of about 38 days. Composite analyses have been made using the principal components of an EOF analysis of the upper tropospheric velocity potential anomalies. The observed characteristics of the oscillation have been simulated by the model reasonably well. The equatorial oscillation modulates the strength of the Indian monsoon and plays an important role in deftning its intraseasonal variability on a 30-60 day time scale. The model simulates well the northward propagation of the rainfall anomalies from the equatorial Indian Ocean to the Indian region. An EOF analysis has been made using the filtered rainfall anomalies in the Indian longitudes. Composite analyses of rainfall, wind and surface fields have been made using the principal components of the EOF analysis. Circulation features in the different phases of the northward propagation resemble that of active and weak monsoon conditions and agree well with the observed analyses. It it noticed that latent heat flux variations over the sea on a 30-60 day time scale associated with the modulation of large scale monsoon circulation by the eastward propagating equatorial intraseasonal mode play a more important role than the land surface processes in the northward propagation.

On the variation of temperature structure parameter in cloud- and clear-air during the summer monsoon season

High resolution aicraft observations of temperature were made in cloud-air and clear-air at different heights in the lower atmosphere over the Deccan Plateu, India, during the summer monsoon season (June–September) of 1976. Temperature fluctuations in the horizontal at each flight level were utilised for computing the temperature structure parameter (C T 2 ) at that level for studying the hieght dependence ofC T 2 . The results were found to be in agreement with those obtained by other investigators. Also, the height variations ofC T 2 in cloud-air and clear-air during active and weak monsoon conditions are studied. Marked differences were noticed in the behaviour ofC T 2 during active and weak monsoon conditions.

Satellite observed cloud distribution over the Indian Ocean during the southwest monsoon season

The cloud distribution over the Indian Ocean has been studied for the two consecutive southwest monsoon seasons of the years 1973 and 1974 with the help of cloud imagery received from NOAA-2 spacecraft in the visible and thermal infrared channels. The cloud distribution shows an interesting mean pattern and demarcates the position of Southern Hemispheric Equatorial Trough (SHET). Weak tropical disturbances which appear as amorphous convective cloud clusters are predominant in the near equatorial regions of the Indian Ocean in this season. The study reveals a negative association between the southwest monsoon activity over India and the intensity of SHET. During weak monsoon conditions over India there is intense convective activity over the Indian Ocean between equator and Lat. to Sin association with active SHET.

A critical examination of the streamlines charts and Conclusions of Dixit and Jones on (a) active And (b) weak monsoon conditions

A critical examination has been made of the report of Dixit and Jones and it is shown that neither their drawings nor their conclusions regarding (i) the structure of the two cyclonic cells (the Gujarat low and the monsoon low) and the precipitation associated with them and (ii) the difference in the characteristics of the subtropical and subequatorial ridges can be accepted.

Mean upper air flow patterns associated with spells of strong and weak northeast monsoon conditions over the Madras State in the month of November

Mean upper air flow patterns have been worked out for spells of strong and weak northeast monsoon conditions over the Madras State in the month of November and compared with the normal patterns for the month. It is noticed that the winds at 1.5 and 3. 0 km a.s.l. over the Pemnsula are mainly easterlies when the monsoon is strong, but become northerlies, when the monsoon is weak. At 6.0 and 9.0 km a.s.l. anti cyclonic cell over the Bay of Bengal is prominent during strong monsoon conditions but becomes diffuse during weak monsoon conditions, when the high pressure cell over the Arabian Sea. becomes dominant.