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Investigations of the development of thunderstorm with hail. Part 3. Numerical simulation of cloud evolution
No full textThe three-dimensional nonstationary model of a convective cloud is used for investigating a thunderstorm with hail which developed over Pyatigorsk on May 29, 2012 and produced a severe hailstorm. The values of cloud characteristics (liquid water content, ice content, vertical velocity, etc.) are obtained. The importance ofconsidering wind shear is noted. The simulation results are used to analyze the transformation of precipitation field and the electric charge structure of the analyzed cloud during its development
Impact of interdecadal Pacific oscillation on Indian summer monsoon rainfall: an assessment from CMIP5 climate models
No full textThe present study evaluates the fidelity of 32 models from the fifth Coupled Model Intercomparison Project (CMIP5) in simulating the observed teleconnection of Interdecadal Pacific Oscillation (IPO) with Indian summer monsoon rainfall (ISMR). Approximately two-thirds of the models show well-defined spatial pattern of IPO over the Pacific basin and most amongst these capture the IPO-ISMR teleconnection. In general, the models that fail to reproduce the IPO-ISMR teleconnection are the ones that are also showing a poor spatial pattern of IPO, irrespective of the extent to which they reproduce the precipitation climatology and seasonal cycle. The results reveal a strong relationship between the quality of reproducing the IPO pattern and the IPO-ISMR teleconnection in the models, in particular with respect to the tropical–extratropical as well as the equatorial Pacific-Indian Ocean sea surface temperature gradients during IPO phases. Furthermore, the CMIP5 models that are capable of reproducing the IPO-ISMR teleconnection also reasonably simulate the atmospheric circulation as well as the convergence/divergence patterns associated with the IPO. Thus, for the better understanding of decadal-to-multidecadal variability and to improve decadal prediction of rainfall over India it is therefore vital that models should simulate the IPO skillfully
Possible role of aerosols in the charge structure of isolated thunderstorms
No full textThe electric field and Maxwell current density measured below 32 small isolated thunderstorms over Pune (India) have been analyzed here. These data clearly show the presence of 10 out of 32 thunderstorms with inverted polarity charge structure. Values of Aerosol Optical Depth (AOD) on thunderstorm days taken from MODIS show that all the thunderstorms with inverted polarity occurred on days with significantly higher AOD compared to normal polarity thunderstorms. The peak flash rate did not show significant difference between normal polarity thunderstorms and inverted polarity thunderstorms. The dew point depression (DPD) during pre-monsoon thunderstorms shows good correlation with inverted polarity charge structure. Observations suggest that aerosol concentration plays an important role in the formation of inverted polarity charge structure in these thunderclouds. In presence of high aerosol concentration with adequate ice nuclei non-inductive charging mechanism can produce strong and wide spread positive charge region in the lower portion of cloud. However, observed good correlation of DPD with inverted polarity charge structure in the pre-monsoon period suggest that the effect of high cloud base height on inverted polarity charge structure as suggested by Williams et al. (2005) cannot be ruled out
Evidence for the existence of Persian Gulf Water and Red Sea Water in the Bay of Bengal
No full textThe high-salinity water masses that originate in the North Indian Ocean are Arabian Sea High-Salinity Water (ASHSW), Persian Gulf Water (PGW), and Red Sea Water (RSW). Among them, only ASHSW has been shown to exist in the Bay of Bengal. We use CTD data from recent cruises to show that PGW and RSW also exist in the bay. The presence of RSW is marked by a deviation of the salinity vertical profile from a fitted curve at depths ranging from 500 to 1000 m; this deviation, though small (of the order of ~0.005 psu and therefore comparable to the CTD accuracy of 0.003 psu), is an order of magnitude larger than the ~0.0003 psu fluctuations associated with the background turbulence or instrument noise in this depth regime, allowing us to infer the existence of RSW throughout the bay. PGW is marked by the presence of a salinity maximum at 200–450 m; in the southwestern bay, PGW can be distinguished from the salinity maximum due to ASHSW because of the intervening Arabian Sea Salinity Minimum. This salinity minimum and the maximum associated with ASHSW disappear east and north of the south-central bay (85°E, 8°N) owing to mixing between the fresher surface waters that are native to the bay (Bay of Bengal Water or BBW) with the high-salinity ASHSW. Hence, ASHSW is not seen as a distinct water mass in the northern and eastern bay and the maximum salinity over most of the bay is associated with PGW. The surface water over most of the bay is therefore a mixture of ASHSW and the low-salinity BBW. As a corollary, we can also infer that the weak oxygen peak seen within the oxygen-minimum zone in the bay at a depth of 250–400 m is associated with PGW. The hydrographic data also show that these three high-salinity water masses are advected into the bay by the Summer Monsoon Current, which is seen to be a deep current extending to 1000 m. These deep currents extend into the northern bay as well, providing a mechanism for spreading ASHSW, PGW, and RSW throughout the ba
North-East monsoon rainfall extremes over the southernpeninsular India and their association with El Ni˜no
No full textThe present study investigates the relationship between extreme north-east (NE) monsoon rainfall (NEMR) over the Indian peninsula region and El Niño forcing. This turns out to be a critical science issue especially after the 2015 Chennai flood. The puzzle being while most El Niños favour good NE monsoon, some don’t. In fact some El Niño years witnessed deficit NE monsoon. Therefore two different cases (or classes) of El Niños are considered for analysis based on standardized NEMR index and Niño 3.4 index with case-1 being both Niño-3.4 and NEMR indices greater than +1 and case-2 being Niño-3.4 index greater than +1 and NEMR index less than −1. Composite analysis suggests that SST anomalies in the central and eastern Pacific are strong in both cases but large differences are noted in the spatial distribution of SST over the Indo-western Pacific region. This questions our understanding of NEMR as mirror image of El Niño conditions in the Pacific. It is noted that the favourable excess NEMR in case-1 is due to anomalous moisture transport from Bay of Bengal and equatorial Indian Ocean to southern peninsular India. Strong SST gradient between warm western Indian Ocean (and Bay of Bengal) and cool western Pacific induced strong easterly wind anomalies during NE monsoon season favour moisture transport towards the core NE monsoon region. Further anomalous moisture convergence and convection over the core NE monsoon region supported positive rainfall anomalies in case-1. While in case-2, weak SST gradients over the Indo-western Pacific and absence of local low level convergence over NE monsoon region are mainly responsible for deficit rainfall. The ocean dynamics in the Indian Ocean displayed large differences during case-1 and case-2, suggesting the key role of Rossby wave dynamics in the Indian Ocean on NE monsoon extremes. Apart from the large scale circulation differences the number of cyclonic systems land fall for case-1 and case-2 have also contributed for variations in NE monsoon rainfall extremes during El Niño years. This study indicates that despite having strong warming in the central and eastern Pacific, NE monsoon rainfall variations over the southern peninsular India is mostly determined by SST gradient over the Indo-western Pacific region and number of systems formation in the Bay of Bengal and their land fall. The paper concludes that though the favourable large scale circulation induced by Pacific is important in modulating the NE monsoon rainfall the local air sea interaction plays a key role in modulating or driving rainfall extremes associated with El Niño
Moisture index during the last two centuries inferred from tree growth in the western Himalaya, India
Full text linkIn the present study, a regional tree-ring chronology prepared from two species (Picea smithiana and Cedrus deodara) over the western Himalaya has been used in relation to climate fluctuations. This analysis shows that moisture index and rainfall during February to May have significant positive relationship, whereas temperature and heat index show a negative relationship with tree growth. However, moisture and heat indices show greater impact on tree growth than rainfall and temperature. The strong association of tree-ring chronology with moisture demonstrates that tree rings are much more sensitive to the availability of moisture at the root zone, which enabled us to extend our analysis back to AD 1789; in the present reconstruction, moisture deficiency for two consecutive years was noticed during 1846–1847, 1908–1909, 1921–1922, 1931–1932, 1947–1948 and 1966–1967
Structure, characteristics, and simulation of monsoon low-pressure systems in CFSv2 coupled model
No full textIndian Summer Monsoon (ISM) synoptic scale systems (low‐pressure systems, LPS) are known to produce increased rainfall over central India (CI). Fidelity of the Climate Forecast System version 2 (CFSv2) at simulating the LPS and their characteristics is evaluated in this study using a feature tracking algorithm. The model is able to reproduce the clustering of LPS by monsoon intraseasonal oscillations and the associated precipitation over eastern‐central India. It is found that mean biases in circulation and moisture stem from cold sea surface temperature (SST) bias in the model which results in weak LPS linked rainfall events over central India. Two sensitivity experiments were carried out to study the effect of coupled dynamics of tropical basins on LPS. Suppression of active dynamics of the tropical Indian Ocean in CFSv2 causes a reduction in cold SST bias and enhanced cyclogenesis in the northern Bay of Bengal. The reduced low‐level anticyclonic bias and enhanced moisture availability result in a better simulation of LPS structure, and associated precipitation over CI. Suppression of active ocean dynamics in tropical Pacific Ocean causes a perennial El‐Niño type bias which restricts LPS propagation over the Indian landmass, possibly due to time‐mean subsidence induced by remote El‐Niño forcing. Sensitivity experiments indicate the need for improvements in the representation of tropical Indian Ocean coupled dynamics as well as convective parameterization schemes in the model for subsequent improvements in the simulation of ISM at various time scales
Importance of aerosol non-sphericity in estimating aerosol radiative forcing in Indo-Gangetic Basin
No full textAerosols are usually presumed spherical in shape while estimating the direct radiative forcing (DRF) using observations or in the models. In the Indo-Gangetic Basin (IGB), a regional aerosol hotspot where dust is a major aerosol species and has been observed to be non-spherical in shape, it is important to test the validity of this assumption. We address this issue using measured chemical composition at megacity Delhi, a representative site of the western IGB. Based on the observation, we choose three non-spherical shapes - spheroid, cylinder and chebyshev, and compute their optical properties. Non-spherical dust enhances aerosol extinction coefficient (βext) and single scattering albedo (SSA) at visible wavelengths by > 0.05 km− 1 and > 0.04 respectively, while it decreases asymmetry parameter (g) by ~ 0.1. Accounting non-sphericity leads top-of-the-atmosphere (TOA) dust DRF to more cooling due to enhanced backscattering and increases surface dimming due to enhanced βext. Outgoing shortwave flux at TOA increases by up to 3.3% for composite aerosols with non-spherical dust externally mixed with other spherical species. Our results show that while non-sphericity needs to be accounted for, choice of shape may not be important in estimating aerosol DRF in the IGB
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
Can the Southern Annular Mode Influence the Korean Summer Monsoon Rainfall?
No full textWe demonstrate that a large-scale longitudinally symmetric global phenomenon in the Southern Hemisphere sub-polar region can transmit its influence over a remote local region of the Northern Hemisphere traveling more than 100° of latitudes (from ~70°S to ~40°N). This is illustrated by examining the relationship between the Southern Annular Mode (SAM) and the Korean Monsoon Rainfall (KMR) based on the data period 1983-2013. Results reveal that the May-June SAM (MJSAM) has a significant in-phase relationship with the subsequent KMR. A positive MJSAM is favorable for the summer monsoon rainfall over the Korean peninsula. The impact is relayed through the central Pacific Ocean. When a negative phase of MJSAM occurs, it gives rise to an anomalous meridional circulation in a longitudinally locked air-sea coupled system over the central Pacific that propagates from sub-polar to equatorial latitudes and is associated with the central Pacific warming. The ascending motion over the central Pacific descends over the Korean peninsula during peak-boreal summer resulting in weakening of monsoon rainfall. The opposite features prevail during a positive phase of SAM. Thus, the extreme modes of MJSAM could possibly serve as a predictor for ensuing Korean summer monsoon rainfall