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Entrainment and droplet spectral characteristics in convective clouds during transition to monsoon
Get PDFIn situ observations in growing deep cumulus in polluted and clean environments during the transition to monsoon over Indian peninsula are used to investigate entrainment effects on droplet size distribution and the width of possible cloud cores. Pre-monsoon clouds indicate reduction in spectral width in the diluted cloud volumes due to lateral entrainment whereas monsoon cloud has higher spectral width. Relative dispersion is observed to increase with the distance from cloud core of monsoon cloud while for pre-monsoon clouds it remained almost constant. Enhanced entrainment in polluted pre-monsoon clouds is responsible for narrow cloud core compared to monsoon cloud
Tethered balloon-born and ground-based measurements of black carbon and particulate profiles within the lower troposphere during the foggy period in Delhi, India
No full textThe ground and vertical profiles of particulate matter (PM) were mapped as part of a pilot study using a Tethered balloon within the lower troposphere (1000 m) during the foggy episodes in the winter season of 2015–16 in New Delhi, India. Measurements of black carbon (BC) aerosol and PM < 2.5 and 10 μm (PM2.5 & PM10 respectively) concentrations and their associated particulate optical properties along with meteorological parameters were made. The mean concentrations of PM2.5, PM10, BC370 nm, and BC880 nm were observed to be 146.8 ± 42.1, 245.4 ± 65.4, 30.3 ± 12.2, and 24.1 ± 10.3 μg m− 3, respectively. The mean value of PM2.5 was ~ 12 times higher than the annual US-EPA air quality standard. The fraction of BC in PM2.5 that contributed to absorption in the shorter visible wavelengths (BC370 nm) was ~ 21%. Compared to clear days, the ground level mass concentrations of PM2.5 and BC370 nm particles were substantially increased (59% and 24%, respectively) during the foggy episode. The aerosol light extinction coefficient (σext) value was much higher (mean: 610 Mm− 1) during the lower visibility (foggy) condition. Higher concentrations of PM2.5 (89 μg m− 3) and longer visible wavelength absorbing BC880 nm (25.7 μg m− 3) particles were observed up to 200 m. The BC880 nm and PM2.5 aerosol concentrations near boundary layer (1 km) were significantly higher (~ 1.9 and 12 μg m− 3), respectively. The BC (i.e BCtot) aerosol direct radiative forcing (DRF) values were estimated at the top of the atmosphere (TOA), surface (SFC), and atmosphere (ATM) and its resultant forcing were - 75.5 Wm− 2 at SFC indicating the cooling effect at the surface. A positive value (20.9 Wm− 2) of BC aerosol DRF at TOA indicated the warming effect at the top of the atmosphere over the study region. The net DRF value due to BC aerosol was positive (96.4 Wm− 2) indicating a net warming effect in the atmosphere. The contribution of fossil and biomass fuels to the observed BC aerosol DRF values was ~ 78% and ~ 22%, respectively. The higher mean atmospheric heating rate (2.71 K day− 1) by BC aerosol in the winter season would probably strengthen the temperature inversion leading to poor dispersion and affecting the formation of clouds. Serious detrimental impacts on regional climate due to the high concentrations of BC and PM (especially PM2.5) aerosol are likely based on this study and suggest the need for immediate, stringent measures to improve the regional air quality in the northern India
Clouds SST relationship and interannual variability modes of Indian summer monsoon in the context of clouds and SSTs:observational and modelling aspects
No full textThis study examines the relationship between clouds and sea surface temperatures (SSTs) during Indian Summer Monsoon (ISM). Observation reveals dominance of high-level clouds in the monsoon region. NCEP Climate Forecast System version 2 (CFSv2) is able to replicate the observed high-level cloud fractions although it underestimates the magnitude. Cloud–SST relationship for observation depicts dominance of positive correlation over equatorial Indian Ocean region which is well recapitulated in CFSv2 simulations. To investigate the most dominating patterns of interannual variability of rainfall, SST and clouds, empirical orthogonal function (EOF) analysis is performed on seasonal JJAS (June–September) dataset. EOF analysis signifies that high-level clouds are highly correlated with rainfall for observations during ISM season. This study has also investigated relationship of clouds and El Niño Southern Oscillations (ENSO) and Indian Ocean Dipole (IOD) during ISM period. Interannual variations of clouds connote significant correlation with ENSO index (Nino 3/Nino 3.4). First principal component (PC1) of high-level clouds and ENSO index indicate significant negative correlation. EOF analysis based on observation also connotes that first mode (second) of EOF analysis is associated with ENSO (IOD). It is also confirmed by maximum covariance analysis (MCA). CFSv2 is also able to depict the significant negative correlation between PC1 of high-level clouds and ENSO index. Observation based second principal component (PC2) of high-level clouds and IOD index exhibits significant positive correlation. It gives indication that PC2 of observed high-level clouds can be associated with IOD. In contrast, PC2 of CFSv2 simulated high-level cloud and IOD index are not correlated. EOF analysis based on CFSv2 shows that the first mode of EOF analysis is associated with ENSO; however, second mode of EOF is not related with IOD. MCA analysis also supports these findings. It means that CFSv2 has good ability to represent ENSO as compared with IOD
Interannual variability of upper ocean stratification in Bay of Bengal: observational and modeling aspects
No full textThe annual cycle and interannual variability of stratification in Bay of Bengal (BoB) are studied using both observations and Global Ocean Data Assimilation System (GODAS) analysis during 2003–2012. Annual cycle of stratification and sea surface temperature (SST) evolve coherently, highlighting its role on modulating air-sea interaction over this climatologically important region. Spatial distribution of stratification shows strong seasonality in ARGO observations, whereas it is highly underestimated in GODAS with highest discrepancies during fall and spring. The annual cycle of sea surface salinity (SSS) in GODAS is out of phase with observations implying potential feedbacks. During La Niña years, SSS drop in fall and winter and are lesser than those reported during El Niño years. All these features are misrepresented in GODAS. As stratification modulates air-sea interaction over BoB especially during El Niño and La Niña years, such misrepresentation of ocean stratification may lead to unrealistic thermocline-SST coupling in the models. The mean stratification and its interannual variability in GODAS are weaker than observed even though interannual variability in freshwater flux (P-E) is higher in GODAS. Detailed analysis of GODAS with in situ observations reveals that upper ocean current shear (vertical) is overestimated in GODAS, leading to unrealistically strong mixing which is primarily responsible for the deeper penetration of surface warm and freshwater resulting weaker stratification. As GODAS is used to initialize the ocean component of the coupled forecasting system for seasonal prediction of Asian monsoon, proper representation of stratification is essential. This study advocates the need of accurate representation of upper ocean salinity in GODAS for improved stratification. We speculate that improved stratification and mixing in the BoB improve summer monsoon forecast
Response of OH airglow emissions to mesospheric gravity waves and comparisons with full-wave model simulation at a low-latitude Indian station
Get PDFQuasi-monochromatic gravity-wave-induced oscillations, monitored using the mesospheric OH airglow emission over Kolhapur (16.8° N, 74.2° E), India, during January to April 2010 and January to December 2011, have been characterized using the Krassovsky method. The nocturnal variability reveals prominent wave signatures with periods ranging from 5.2 to 10.8 h as the dominant nocturnal wave with embedded short-period waves having wave periods of 1.5–4.4 h. The results show that the magnitude of the Krassovsky parameter, viz. |η|, ranged from 2.1 to 10.2 h for principal or long nocturnal waves (5.2–10.8 h observed periods), and from 1.5 to 5.4 h for the short waves (1.5–4.4 h observed periods) during the years of 2010 and 2011, respectively. The phase (i.e., Φ) values of the Krassovsky parameters exhibited larger variability and varied from −8.1 to −167°. The deduced mean vertical wavelengths are found to be approximately −60.2 ± 20 and −42.8 ± 35 km for long- and short-period waves for the year 2010. Similarly, for 2011 the mean vertical wavelengths are found to be approximately −77.6 ± 30 and −59.2 ± 30 km for long and short wave periods, respectively, indicating that the observations over Kolhapur were dominated by upward-propagating waves. We use a full-wave model to simulate the response of OH emission to the wave motion and compare the results with observed values
Premature mortality in India due to PM2.5 and ozone exposure
No full textThis bottom-up modeling study, supported by new population census 2011 data, simulates ozone (O3) and fine particulate matter (PM2.5) exposure on local to regional scales. It quantifies, present-day premature mortalities associated with the exposure to near-surface PM2.5 and O3 concentrations in India using a regional chemistry model. We estimate that PM2.5 exposure leads to about 570,000 (CI95: 320,000–730,000) premature mortalities in 2011. On a national scale, our estimate of mortality by chronic obstructive pulmonary disease (COPD) due to O3 exposure is about 12,000 people. The Indo-Gangetic region accounts for a large part (~42%) of the estimated mortalities. The associated lost life expectancy is calculated as 3.4 ± 1.1 years for all of India with highest values found for Delhi (6.3 ± 2.2 years). The economic cost of estimated premature mortalities associated with PM2.5 and O3 exposure is about 640 (350–800) billion USD in 2011, which is a factor of 10 higher than total expenditure on health by public and private expenditure
Considering the radiative effects of snow on tropical Pacific Ocean radiative heating profiles in contemporary GCMs using A‐Train observations
No full textThis study characterizes biases in water vapor, dynamics, shortwave (SW) and longwave (LW) radiative properties in contemporary global climate models (GCMs) against observations over tropical Pacific Ocean. The observations are based on Atmospheric Infrared Sounder for water vapor, CloudSat 2B-FLXHR-LIDAR for LW and SW radiative heating profiles, and radiative flux from Clouds and the Earth's Radiant Energy System products. The model radiative heating profiles are adopted from the coupled and uncoupled National Center for Atmospheric Research (NCAR) Community Earth System Model version 1 (CESM1) and joint Year of Tropical Convection (YOTC)/Madden Julian Oscillation (MJO) Task Force-Global Energy and Water Cycle Experiment Atmospheric System Studies (GASS) Multi-Model Physical Processes Experiment (YOTC-GASS). The results from the model evaluation for YOTC-GASS and NCAR CESM1 demonstrate a number of systematic radiative biases. These biases include excessive outgoing LW radiation and excessive SW surface radiative fluxes, in conjunction with a radiatively unstable atmosphere with excessive LW cooling in the upper troposphere over convectively active areas, such as the Intertropical Convergence Zone/South Pacific Convergence Zone (ITCZ/SPCZ) and warm pool. Using sensitivity experiments with the NCAR-uncoupled/NCAR-coupled CESM1, we infer that these biases partly result from the interactions between falling snow and radiation that are missing in most contemporary GCMs (e.g., YOTC-GASS, Coupled Model Intercomparison Project 3 (CMIP)3, and Atmospheric Model Intercomparison Project 5 (AMIP5)/CMIP5). A number of biases in the YOTC-GASS model simulations are consistent with model biases in CMIP3, AMIP5/CMIP5, and NCAR-uncoupled/NCAR-coupled model simulation without snow-radiation interactions. These include excessive upper level convection and low level downward motion with outflow from ITCZ/SPCZ. This generates weaker low-level trade winds and excessive precipitation in the Central Pacific Trade wind regions. The excessive LW radiative cooling in NCAR-coupled/NCAR-uncoupled GCM simulations is reduced by 10–20% with snow-radiative effects considered
Modulation of surface meteorological parameters by extratropical planetary-scale Rossby waves
Get PDFThis study examines the link between upper-tropospheric planetary-scale Rossby waves and surface meteorological parameters based on the observations made in association with the Ganges Valley Aerosol Experiment (GVAX) campaign at an extratropical site at Aryabhatta Research Institute of Observational Sciences, Nainital (29.45° N, 79.5° E) during November–December 2011. The spectral analysis of the tropospheric wind field from radiosonde measurements indicates a predominance power of around 8 days in the upper troposphere during the observational period. An analysis of the 200 hPa meridional wind (v200 hPa) anomalies from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis shows distinct Rossby-wave-like structures over a high-altitude site in the central Himalayan region. Furthermore, the spectral analysis of global v200 hPa anomalies indicates the Rossby waves are characterized by zonal wave number 6. The amplification of the Rossby wave packets over the site leads to persistent subtropical jet stream (STJ) patterns, which further affects the surface weather conditions. The propagating Rossby waves in the upper troposphere along with the undulations in the STJ create convergence and divergence regions in the mid-troposphere. Therefore, the surface meteorological parameters such as the relative humidity, wind speeds, and temperature are synchronized with the phase of the propagating Rossby waves. Moreover, the present study finds important implications for medium-range forecasting through the upper-level Rossby waves over the study region
UHF wind profiler observations of monsoon low-level jet (MLLJ) and its association with rainfall over a tropical Indian station
Get PDFHigh resolution data of horizontal winds profiles (zonal and meridional) in the lower troposphere, derived from a UHF wind profiler at a tropical Indian station, Pune (18º 32' N, 73º 51' E, 559 masl) during a 3-yr period (June 2003-May 2006) has been utilized to study seasonal and intra-seasonal variability of winds. Winds display a systematic seasonal evolution with behavior opposite in phase in the two altitude regimes below and above a height of 4-5 km. In the lower region, during the southwest monsoon months (June to September) winds are predominantly westerly with a peak in the 1.5-3.0 km range indicating the occurrence of the monsoon low-level jet (MLLJ). Soon after September, winds in this height region change from westerly to easterly and these easterlies continue in winter months (December to February). Above a height of 4 km, westerlies are observed during post-monsoon (October to November) and winter periods. The MLLJ is observed to be strong during normal/good monsoon years. On a day-to-day scale during southwest monsoon months, winds exhibit considerable intra-seasonal variability and periods of strong MLLJ seem to be associated with occurrence of spells of rainfall over the region
On the co-existence of high-energy low-frequency waves and locally-generated cyclone waves off the Indian east coast
No full textThe evolution of wave energy spectra obtained from a directional waverider buoy at the landfall location (Gopalpur) of the very severe cyclonic storm Phailin in the Bay of Bengal is discussed. The study reveals that swells generated approximately 8600 km away from the buoy location in the southern ocean (at 12 UTC on 3 October, 2013) travelled at a speed of 15.6 m/s and contributed to create a complex wave field at Gopalpur during cyclone Phailin. The rare co-existence of the low-frequency (0.055 Hz), high-energy (21.37 m2/Hz) southern ocean swells with local cyclone-generated swells during the period (8-12 October, 2013) is reported