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Study on the role of land-atmosphere coupling on the south Asian monsoon climate variability using a regional climate model
No full textLand-atmosphere coupling over the south Asian monsoon region is examined using a regional climate model. For this purpose, the Weather Research and Forecasting (WRF) model with a resolution of 45 km was used. In the control experiment (CTL), the model was integrated from the year 2000 to 2011 and allowed the soil moisture interaction with the atmosphere using a coupled land surface model. In the second experiment (CSM), the soil moisture evolution at each time step was replaced with the climatology of soil moisture taken from the control run. The results reveal that land-atmosphere coupling plays a critical role in influencing the south Asian monsoon climate variability. Soil moisture is found to have stronger impacts on daily maximum temperature compared to minimum temperature. Soil moisture also makes a significant contribution to monsoon rainfall variability over the monsoon region. The coupling strength for large-scale rainfall is found to be higher compared to that of cumulus rainfall. Soil moisture is found more strongly coupled to sensible heat flux over most of the monsoon region
Western Himalaya Trees Growth Study and its Association with Droughts in India: A Case Study
No full textTree ring
-
width index chronology based on a w
ell replicated tree core samples from the western Himalaya
showed significant positive relationship with standardized precipitation potential evapotranspiration (SPEI) and
standardized soil index (SSI) during summer season (April
-
June). However, SSI that d
escribes the drought index over
the region is found more compatible with tree growth variations than SPEI in controlling the annual ring
-
width patterns. It
shows high temporal stability with trees growth compared to SPEI. The results showed that the SSI wh
ich is an indicator
of drought index has strong localized effects on patterns of annual tree growth and forest dynamic, working as booster in
limiting of trees growth over western Himalaya
Microphysical characteristics of rainfall during different seasons over a coastal tropical station using disdrometer
No full textDrop size distribution (DSD) over the tropical region exhibit pronounced variations during different monsoon seasons. Measurements from an impact type Joss–Waldovgel disdrometer is used for characterization of drop size distribution and its integral parameters over a tropical coastal station (Thiruvananthapuram, 8.31°N, 76.54°E, 20 m asl). Rain events were identified during the winter, premonsoon, summer monsoon and postmonsoon seasons from 8 years, computed rain duration (min) and accumulated rain water (mm). Rain intensity (mm h−1), mean drop diameter (Dm, mm) and total number concentration of raindrops (NT, m−3) were calculated on each sampling interval and classified in to different bins.
The different range bins of rain intensity and their relative contributions towards total rainfall are different for different seasons. Maximum events were reported on the R2 (heavy drizzle/light rain) type, but the contribution of rainfall (mm) is mainly registered on R4 (heavy rain) type. Similarly, the NT and Dm are also showing different characteristics during different monsoon seasons. Frequency of occurrence of Dm is higher in Dm2 (1–2 mm) followed by Dm1 (Dm < 1 mm) and then Dm3 (2–3 mm) with difference in magnitudes for different seasons. On analysing relative rainfall contribution from different mean diameter bins, it can be observed that Dm2 and Dm3 (1–3 mm) are the major contributors to the total rainfall. In the case of NT, both frequency and accumulated water are almost same or comparable for the different bins during all the seasons. The Dm and NT are positively related with different intensity bins. The lower rainfall intensity bins show higher duration during the summer monsoon season and lower duration during the premonsoon season, the higher intensity range bins show lower duration for the premonsoon season and higher duration for the postmonsoon season
How distinct are the two flavors of El Niño in retrospective forecasts of Climate Forecast System version 2 (CFSv2)?
No full textTwo different flavors of El Niño-Southern Oscillation (ENSO, canonical east Pacific type and Modoki/central Pacific type) are reported in the recent decades and are found to influence the global climate in different ways. The success of a seasonal prediction system is dependent on its ability to capture these two ENSO flavors accurately, together with associated teleconnections. The present study analyses the ability of Climate Forecast System version 2 (CFSv2) in simulating the two El Niño flavors and their teleconnections. The study uses two versions of CFSv2 in which the atmospheric model horizontal resolutions are different (T126 at 100 km and T382 at 38 km) and are initialized from a calendar month, ranging from February to June. The canonical ENSO pattern is captured as prominent mode of tropical Pacific sea surface temperature (SST) by both resolutions of CFSv2. Even though the tri-polar structure of ENSO Modoki is simulated as second mode, it has some disagreement with observations. The canonical El Niño induced SST, rainfall and atmospheric circulation in the tropical Pacific in summer and fall seasons are comparable with observations in both models. Meanwhile, the teleconnections in the tropical Indian Ocean and Indian monsoon regions are close to observations in T382 only. Teleconnections associated with El Niño Modoki are proper in T382 hindcasts, in which SST bias in Indian Ocean is slightly warm and the cold bias in central Pacific is marginal (mainly for Feb IC hindcasts). The present study indicates that the distinction of ENSO flavors in summer is the major reason for the higher skill of Indian summer monsoon rainfall (ISMR) in CFSv2 T382 Feb IC hindcasts. However, teleconnections associated with two flavors of ENSO are not distinguishable in fall and winter seasons, even in higher resolution model due to the presence of strong cold SST bias in central Pacific and warm SST bias in extreme east Pacific. Thus present study confirms that, higher resolution CFSv2 is required to differentiate the flavors of ENSO and their teleconnections properly. It has better prediction skill for ISMR at a lead time of 4 months, which is significant for seasonal prediction of Indian summer monsoon
Shift in MONSOON–SST teleconnections in the tropical Indian Ocean and ENSEMBLES climate models fidelity in its simulation
No full textThe climate shift in 1976–1977 sets a clear distinction in the teleconnection between Indian summer monsoon rainfall (ISMR) and sea surface temperatures (SST) in the tropical oceans, particularly in the tropical Indian Ocean (TIO). Before the climate shift (Pre-77), the observed correlation between ISMR and SST was positive in the eastern equatorial Indian Ocean (EIO) and negative correlation prevailed in the western equatorial Indian Ocean (WIO), whereas after the climate shift (Post-77), the correlation pattern has reversed (i.e. positive correlation in WIO and negative correlation in EIO). The major reason for this shift is the consistent warming of SSTs throughout the tropics and frequent occurrence of IOD events. Further, it is noticed that during the earlier decades, the Indian Ocean was responding to the atmospheric forcing, particularly along the axis of strong cross equatorial flow, whereas in the recent decades, Indian Ocean dynamics are playing a crucial role in determining the SST in major parts of the Indian Ocean particularly in the EIO. Seasonal re-forecasts from six state-of-the-art global coupled atmosphere–ocean climate models which participated in ENSEMBLES faithfully capture the pre-77 dipole structure of correlations. But the post-77 correlation pattern is not simulated by any of the six models considered in this study. Two possible reasons are pointed out for the models' inability to capture the reversal of teleconnection: (1) in the coupled models, unchanged mean state of the winds inhibits ISMR to be governed by coupled dynamics in the TIO; (2) the strong, local SST–rainfall relationship in models even in post-77 period limits the influence of the teleconnections on the ISMR
Climate indicators for lightning over sea, sea-land mixed and land-only surfaces in India
No full textIn this article, we investigate indicators for lightning activity by analysing the data of surface heat flux, Bowen ratio (ratio of sensible to latent heats) and cloud base height for a period of 16 years (1998–2013) over Indo-Gangetic Plain (IGP) (25°–27°N, 80°–88°E), Indian land excluding IGP (8°–36°N, 68°–98°E) and 10 years (2000–2009) over Indian seas [Arabian Sea (5°–20°N, 65°–80°E) + Bay of Bengal (5°–20°N, 80°–98°E)]. Lightning activity varies with the surface heat flux, Bowen ratio and cloud base height over India. Over Indian land, annual lightning flash counts are found to be more by 32 and 24% than those over IGP and Indian seas, respectively. Total heat flux (sensible + latent) and lightning flash counts show a strong correlation coefficient of 0.75 for Indian land and 0.73 for IGP relative to that of Bowen ratio with lightning flash count (0.63 for Indian land and 0.19 for IGP). Hence, the total heat flux represents the best parameter for describing lightning activity over IGP and Indian land. Bowen ratio ≥1 in pre-monsoon increases lightning flash counts over IGP and Indian land. Cloud base height (a measure of moisture) and lightning flash counts show values in the order as Indian land > Indian seas > IGP. Geographic asymmetry of Indian land, IGP and Indian seas drive the continental and sea surface–atmosphere interactive processes that corroborate: (1) asymmetric synoptic scale delivery of moisture to Indian land and IGP from Indian seas revises the Bowen ratio, cloud base height and lightning activity, (2) increase in lightning activity with the total heat flux over Indian land and (3) enhance the lightning activity with the cloud base height/liquid condensation level
Potential of collocated radiometer and wind profiler observations for monsoon studies
No full textCollocated observations from microwave radiometer and wind profiler are used in a pilot study during the monsoon period to derive information on the thermodynamics and winds and association with rainfall characteristics. These instruments were operated throughout the monsoon season of 2015. Continuous vertical profiles of winds, temperature and humidity show significant promise for understanding the low-level jet, its periodicity and its association with moisture transport, clouds and precipitation embedded within the monsoon large-scale convection. Observations showed mutually beneficial in explaining variability that are part of the low frequency oscillations and the diurnal variability during monsoon. These observations highlight the importance of locally driven convective systems, in the presence of weak moisture transport over the area. The episodic moisture convergence showed a periodicity of 9 days which matches with the subsequent convection and precipitation and thermodynamic regimes. Inferences from the diurnal cycle of moisture transport and the convective activity, relationship with the low-level jet characteristics and thermodynamics are also illustrated
Unidentified heavy rainfall station ‘Tamhini’ in the northern region of Western Ghats of India
No full textTamhini, a station located in the leeward side of northern parts of Western Ghats (WG) of India, receives a mean-monsoon rainfall of 6498.4 mm. The station has no record of any meteorological agencies in India. Therefore, the existence of such heavy rainfall (HR) station remained unknown. In this study, many special features of Tamhini rainfall have been discovered hitherto unknown to monsoon meteorologists. Statistical analysis of daily rainfall data of monsoon season (June–September) of Tamhini for the period 1975–2013 has been carried out. The analysis showed that in spite of lying on the leeward side of main WG, Tamhini's monsoon rainfall is more than that of stations lying on the windward side of the WG. This is in contrast to widely accepted view that rainfall is lower on the leeward side of mountains. Tamhini rainfall ranks number one in the north Peninsula and fifth in India. It is more compared to Cherrapunji monsoon rainfall (second highest rainfall-receiving station in India) in 8 non-El Nino years. It has the highest 1-day (695 mm) and 3-day (1055 mm) rainfalls in the Peninsular WG region. There are many extended HR spells during the season with a mean duration of 10 days. Other stations in the same area viz. Mulshi-camp, Shirgaon, Davdi and Ambawane also receive high monsoon season rainfalls. Area-mean rainfall of the region has been studied to understand the frequency distribution of HR spells. Such spells are of concern as total accumulated water during these spells is conducive for occurrence of floods. No significant trends in Tamhini and area-mean seasonal and HRs are observed. The discovery of this station has shown the occurrence of high rainfall on the leeward side, which is a unique feature in the orographic rainfall distribution in the world
Lightning activity with rainfall during El Nino and La Nina events over India
No full textThis paper appraises the association of lightning flash count (FC) with rainfall using the satellite-borne Lightning Imaging Sensor’s (LIS) data along with gridded rainfall data (0.5o × 0.5o) for Indian summer monsoon seasons over 10 years (2001–2010). During strong El Nino years, 2002 and 2009, FCs were greater in magnitude by about 26.5 % and 37 %, than the long-term average, respectively, while during weak El Nino year (2004), it was more by 8 %. During the same years, the rainfall was deficient by about 10 % than the long-term average. Similarly, a rise in aerosol optical depth (AOD) over its average value (by about 15 % and 20 %) reduces the ratio of rainfall to FC (RLR) by 41 % and 44 % for strong El Nino years 2002 and 2009, respectively, and for weak El Nino year (2004), a 6.5 % rise in AOD lowers the RLR by 20 %. Bowen ratio more by 11 % and 17 % of its average value reduces the RLR by 41 % and 44 % for strong El Nino years 2002 and 2009, respectively, and, also, Bowen ratio higher by 8 % for 2004 declines RLR by 20 %. On the other hand, Bowen ratio less by 9 % and 6 % raises the RLR by 19 % and 56 % for moderate La Nina year (2007) and strong La Nina year (2010), respectively. Results for the daily rainfall, AOD and Bowen ratio over Indian regions, are discussed for strong El Nino and La Nina years. Correlations of FC with AOD and Bowen ratio of 0.66 and 0.71, respectively, while, that of FC with ONI of 0.56 indicates numerous (fewer) break days during El Nino (La Nina) years
Summer monsoon rainfall variability over North East regions of India and its association with Eurasian snow, Atlantic Sea Surface temperature and Arctic Oscillation
No full textThis observational study during the 29-year period from 1979 to 2007 evaluates the potential role of Eurasian snow in modulating the North East-Indian Summer Monsoon Rainfall with a lead time of almost 6 months. This link is manifested by the changes in high-latitude atmospheric winter snow variability over Eurasia associated with Arctic Oscillation (AO). Excessive wintertime Eurasian snow leads to an anomalous cooling of the overlying atmosphere and is associated with the negative mode of AO, inducing a meridional wave-train descending over the tropical north Atlantic and is associated with cooling of this region. Once the cold anomalies are established over the tropical Atlantic, it persists up to the following summer leading to an anomalous zonal wave-train further inducing a descending branch over NE-India resulting in weak summer monsoon rainfall