Search Results
You are looking at 11 - 20 of 34 items for
- Author or Editor: K. Krishna x
- Refine by Access: All Content x
Abstract
An observational study was undertaken at selected sites in and around a mountain gap to understand the downwind increase of winds transiting the gap. Observations indicate that there is a fanning out of surface winds emerging out of the gap. Computations show that downwind of the gap, there exist divergence in the lower levels and convergence upward of the level of maximum winds.
A model of the atmospheric circulation in the lower layers east of the mountain gap in question is suggested.
Abstract
An observational study was undertaken at selected sites in and around a mountain gap to understand the downwind increase of winds transiting the gap. Observations indicate that there is a fanning out of surface winds emerging out of the gap. Computations show that downwind of the gap, there exist divergence in the lower levels and convergence upward of the level of maximum winds.
A model of the atmospheric circulation in the lower layers east of the mountain gap in question is suggested.
Abstract
A case study is presented on the impact of two isolated, strong thundershowers during a prevailing dry, sunny season on the spectral optical depths and inferred columnar size characteristics of atmospheric aerosols at a tropical station. Results show a remarkable decrease in the aerosol optical depth and change in the spectral slope after the rain. The scavenging was found to be dependent on the particle size distribution; the larger, supermicron particles were found to be removed faster during the first shower itself, even though it was of only moderate intensity, resulting in about a 64% decrease in the columnar mass loading. In the second shower, which was stronger and more widespread than the former, more of the submicron particles in the optically active submicron size range were removed, but the reduction in mass loading was very small. The effective radius decreased continuously and so too did the columnar mass loading (total aerosol volume). The data are used to estimate the apparent columnar scavenging efficiency. The inferred apparent scavenging efficiencies were ∼57% and 68% for the aerosol columnar number density in the optically active submicron size range for the two events, whereas for the coarse aerosols (r > 0.5 μm), it was ∼75% for the first event but insignificant for the subsequent event, in line with the pattern of mass loading. The prevailing synoptic conditions (continental air mass and dry weather) helped the atmosphere to “recharge” within about 1 week after the events (which removed more than 70% of the aerosol burden), unlike the case with extensive synoptic phenomena, like monsoons, for which the aerosol optical depths remain depleted over the entire season. This recharging is also dependent on the size distribution of aerosols; the fine and accumulation particles are replenished faster than the coarse particles.
Abstract
A case study is presented on the impact of two isolated, strong thundershowers during a prevailing dry, sunny season on the spectral optical depths and inferred columnar size characteristics of atmospheric aerosols at a tropical station. Results show a remarkable decrease in the aerosol optical depth and change in the spectral slope after the rain. The scavenging was found to be dependent on the particle size distribution; the larger, supermicron particles were found to be removed faster during the first shower itself, even though it was of only moderate intensity, resulting in about a 64% decrease in the columnar mass loading. In the second shower, which was stronger and more widespread than the former, more of the submicron particles in the optically active submicron size range were removed, but the reduction in mass loading was very small. The effective radius decreased continuously and so too did the columnar mass loading (total aerosol volume). The data are used to estimate the apparent columnar scavenging efficiency. The inferred apparent scavenging efficiencies were ∼57% and 68% for the aerosol columnar number density in the optically active submicron size range for the two events, whereas for the coarse aerosols (r > 0.5 μm), it was ∼75% for the first event but insignificant for the subsequent event, in line with the pattern of mass loading. The prevailing synoptic conditions (continental air mass and dry weather) helped the atmosphere to “recharge” within about 1 week after the events (which removed more than 70% of the aerosol burden), unlike the case with extensive synoptic phenomena, like monsoons, for which the aerosol optical depths remain depleted over the entire season. This recharging is also dependent on the size distribution of aerosols; the fine and accumulation particles are replenished faster than the coarse particles.
Abstract
Regular radiosonde measurements conducted by the India Meteorological Department at eleven stations spread over a latitude range of 8.5°–28.6°N for a period of nine years are used for the, study of the tropical tropopause characteristics, altitude (H), temperature (T) and pressure (P). The results are examined in the light of the hypothesis of Reid and Gage. It has been shown that large-scale cloud cover affects the tropopause parameters for annual and semiannual periods, while for longer periods (biennial) the cloud cover effect is not significant.
Abstract
Regular radiosonde measurements conducted by the India Meteorological Department at eleven stations spread over a latitude range of 8.5°–28.6°N for a period of nine years are used for the, study of the tropical tropopause characteristics, altitude (H), temperature (T) and pressure (P). The results are examined in the light of the hypothesis of Reid and Gage. It has been shown that large-scale cloud cover affects the tropopause parameters for annual and semiannual periods, while for longer periods (biennial) the cloud cover effect is not significant.
Abstract
During the second phase of the Arabian Sea Monsoon Experiment (ARMEX-II), extensive measurements of spectral aerosol optical depth, mass concentration, and mass size distribution of ambient aerosols as well as mass concentration of aerosol black carbon (BC) were made onboard a research vessel during the intermonsoon period (i.e., when the monsoon winds are in transition from northeasterlies to westerlies/southwesterlies) over the Arabian Sea (AS) adjoining the Indian Peninsula. Simultaneous measurements of spectral aerosol optical depths (AODs) were made at different regions over the adjoining Indian landmass. Mean AODs (at 500-nm wavelength) over the ocean (∼0.44) were comparable to those over the coastal land (∼0.47), but were lower than the values observed over the plateau regions of central Indian Peninsula (∼0.61). The aerosol properties were found to respond distinctly with respect to change in the trajectories, with higher optical depths and flatter AOD spectra associated with trajectories indicating advection from west Asia, and northwest and west-coastal India. On average, BC constituted only ∼2.2% to total aerosol mass compared to the climatological values of ∼6% over the coastal land during the same season.
These data are used to characterize the physical properties of aerosols and to assess the resulting short-wave direct aerosol forcing. The mean values were –27 W m−2 at the surface and −12 W m−2 at the top of the atmosphere (TOA), resulting in a net atmospheric forcing of +15 W m−2. The forcing also depended on the region from where the advection predominates. The surface and atmospheric forcing were in the range −40 to −57 W m−2 and +27 to +39 W m−2, respectively, corresponding to advection from the west Asian and western coastal India where they were as low as −19 and +10 W m−2, respectively, when the advection was mainly from the Bay of Bengal and from central/peninsular India. In all these cases, the net atmospheric forcing (heating) efficiency was lower than the values reported for northern Indian Ocean during northern winter, which is attributed to the reduced BC mass fraction.
Abstract
During the second phase of the Arabian Sea Monsoon Experiment (ARMEX-II), extensive measurements of spectral aerosol optical depth, mass concentration, and mass size distribution of ambient aerosols as well as mass concentration of aerosol black carbon (BC) were made onboard a research vessel during the intermonsoon period (i.e., when the monsoon winds are in transition from northeasterlies to westerlies/southwesterlies) over the Arabian Sea (AS) adjoining the Indian Peninsula. Simultaneous measurements of spectral aerosol optical depths (AODs) were made at different regions over the adjoining Indian landmass. Mean AODs (at 500-nm wavelength) over the ocean (∼0.44) were comparable to those over the coastal land (∼0.47), but were lower than the values observed over the plateau regions of central Indian Peninsula (∼0.61). The aerosol properties were found to respond distinctly with respect to change in the trajectories, with higher optical depths and flatter AOD spectra associated with trajectories indicating advection from west Asia, and northwest and west-coastal India. On average, BC constituted only ∼2.2% to total aerosol mass compared to the climatological values of ∼6% over the coastal land during the same season.
These data are used to characterize the physical properties of aerosols and to assess the resulting short-wave direct aerosol forcing. The mean values were –27 W m−2 at the surface and −12 W m−2 at the top of the atmosphere (TOA), resulting in a net atmospheric forcing of +15 W m−2. The forcing also depended on the region from where the advection predominates. The surface and atmospheric forcing were in the range −40 to −57 W m−2 and +27 to +39 W m−2, respectively, corresponding to advection from the west Asian and western coastal India where they were as low as −19 and +10 W m−2, respectively, when the advection was mainly from the Bay of Bengal and from central/peninsular India. In all these cases, the net atmospheric forcing (heating) efficiency was lower than the values reported for northern Indian Ocean during northern winter, which is attributed to the reduced BC mass fraction.
Abstract
The location of the 500-hPa ridge axis during April over India is one of the most important long-range predictors for the summer monsoon rainfall. This paper presents a comprehensive analysis on its space-time variability during the premonsoon season and its relation with the monsoon rainfall. Data on the daily latitudinal locations of the 500-hPa ridge axis along three longitudes during March, April, and May, as well as all-India rainfall and subdivisional monsoon rainfall for the period 1967–90, have been used.
The analysis involves correlations between the running means of the premonsoon ridge locations over windows of 15, 21, and 31 days, and the subsequent monsoon rainfall. The ridge location in March shows negative correlation with the all-India summer monsoon rainfall, while that in April shows positive correlation. The anticorrelation of the March ridge was more dominant with the monsoon rainfall of the peninsular India, while the positive correlation of the April ridge was more dominant with the monsoon rainfall of northern India. Regression equations for the prediction of the monsoon rainfall have also been developed.
Abstract
The location of the 500-hPa ridge axis during April over India is one of the most important long-range predictors for the summer monsoon rainfall. This paper presents a comprehensive analysis on its space-time variability during the premonsoon season and its relation with the monsoon rainfall. Data on the daily latitudinal locations of the 500-hPa ridge axis along three longitudes during March, April, and May, as well as all-India rainfall and subdivisional monsoon rainfall for the period 1967–90, have been used.
The analysis involves correlations between the running means of the premonsoon ridge locations over windows of 15, 21, and 31 days, and the subsequent monsoon rainfall. The ridge location in March shows negative correlation with the all-India summer monsoon rainfall, while that in April shows positive correlation. The anticorrelation of the March ridge was more dominant with the monsoon rainfall of the peninsular India, while the positive correlation of the April ridge was more dominant with the monsoon rainfall of northern India. Regression equations for the prediction of the monsoon rainfall have also been developed.
Abstract
Summer monsoon rains are a critical factor in Thailand’s water resources and agricultural planning and management. In fact, they have a significant impact on the country’s economic health. Consequently, understanding the variability of the summer monsoon rains over Thailand is important for instituting effective mitigating strategies against extreme rainfall fluctuations. To this end, the authors systematically investigated the relationships between summer monsoon precipitation from the central and northern regions of Thailand and large-scale climate features. It was found that Pacific sea surface temperatures (SSTs), in particular, El Niño–Southern Oscillation (ENSO), have a negative relationship with the summer monsoon rainfall over Thailand in recent decades. However, the relationship between summer rainfall and ENSO was weak prior to 1980. It is hypothesized that the ENSO teleconnection depends on the SST configuration in the tropical Pacific Ocean, that is, an eastern Pacific–based El Niño pattern, such as is the case in most of the post-1980 El Niño events, tends to place the descending limb of the Walker circulation over the Thailand–Indonesian region, thereby significantly reducing convection and consequently, rainfall over Thailand. It is believed that this recent shift in the Walker circulation is instrumental for the nonstationarity in ENSO–monsoon relationships in Thailand. El Niños of 1997 and 2002 corroborate this hypothesis. This has implications for monsoon rainfall forecasting and, consequently, for resources planning and management.
Abstract
Summer monsoon rains are a critical factor in Thailand’s water resources and agricultural planning and management. In fact, they have a significant impact on the country’s economic health. Consequently, understanding the variability of the summer monsoon rains over Thailand is important for instituting effective mitigating strategies against extreme rainfall fluctuations. To this end, the authors systematically investigated the relationships between summer monsoon precipitation from the central and northern regions of Thailand and large-scale climate features. It was found that Pacific sea surface temperatures (SSTs), in particular, El Niño–Southern Oscillation (ENSO), have a negative relationship with the summer monsoon rainfall over Thailand in recent decades. However, the relationship between summer rainfall and ENSO was weak prior to 1980. It is hypothesized that the ENSO teleconnection depends on the SST configuration in the tropical Pacific Ocean, that is, an eastern Pacific–based El Niño pattern, such as is the case in most of the post-1980 El Niño events, tends to place the descending limb of the Walker circulation over the Thailand–Indonesian region, thereby significantly reducing convection and consequently, rainfall over Thailand. It is believed that this recent shift in the Walker circulation is instrumental for the nonstationarity in ENSO–monsoon relationships in Thailand. El Niños of 1997 and 2002 corroborate this hypothesis. This has implications for monsoon rainfall forecasting and, consequently, for resources planning and management.
Abstract
The impact on tropical cyclone (TC) prediction from assimilating Doppler weather radar (DWR) observations obtained from the TC inner core and environment over the Bay of Bengal (BoB) is studied. A set of three operationally relevant numerical experiments were conducted for 24 forecast cases involving 5 unique severe/very severe BoB cyclones: Sidr (2007), Aila (2009), Laila (2010), Jal (2010), and Thane (2011). The first experiment (CNTL) used the NCEP FNL analyses for model initial and boundary conditions. In the second experiment [Global Telecommunication System (GTS)], the GTS observations were assimilated into the model initial condition while the third experiment (DWR) used DWR with GTS observations. Assimilation of the TC environment from DWR improved track prediction by 32%–53% for the 12–72-h forecast over the CNTL run and by 5%–25% over GTS and was consistently skillful. More gains were seen in intensity, track, and structure by assimilating inner-core DWR observations as they provided more realistic initial organization/asymmetry and strength of the TC vortex. Additional experiments were conducted to assess the role of warm-rain and ice-phase microphysics to assimilate DWR reflectivity observations. Results indicate that the ice-phase microphysics has a dominant impact on inner-core reflectivity assimilation and in modifying the intensity evolution, hydrometeors, and warm core structure, leading to improved rainfall prediction. This study helps provide a baseline for the credibility of an observational network and assist with the transfer of research to operations over the India monsoon region.
Abstract
The impact on tropical cyclone (TC) prediction from assimilating Doppler weather radar (DWR) observations obtained from the TC inner core and environment over the Bay of Bengal (BoB) is studied. A set of three operationally relevant numerical experiments were conducted for 24 forecast cases involving 5 unique severe/very severe BoB cyclones: Sidr (2007), Aila (2009), Laila (2010), Jal (2010), and Thane (2011). The first experiment (CNTL) used the NCEP FNL analyses for model initial and boundary conditions. In the second experiment [Global Telecommunication System (GTS)], the GTS observations were assimilated into the model initial condition while the third experiment (DWR) used DWR with GTS observations. Assimilation of the TC environment from DWR improved track prediction by 32%–53% for the 12–72-h forecast over the CNTL run and by 5%–25% over GTS and was consistently skillful. More gains were seen in intensity, track, and structure by assimilating inner-core DWR observations as they provided more realistic initial organization/asymmetry and strength of the TC vortex. Additional experiments were conducted to assess the role of warm-rain and ice-phase microphysics to assimilate DWR reflectivity observations. Results indicate that the ice-phase microphysics has a dominant impact on inner-core reflectivity assimilation and in modifying the intensity evolution, hydrometeors, and warm core structure, leading to improved rainfall prediction. This study helps provide a baseline for the credibility of an observational network and assist with the transfer of research to operations over the India monsoon region.
Abstract
Multiyear measurements of spectral aerosol optical depths (AODs) were made at Manora Peak in the central Himalaya Range (29°22′N, 79°27′E, ∼1950 m above mean sea level), using a 10-channel multiwavelength solar radiometer for 605 days during January 2002–December 2004. The AODs at 0.5 μm were very low (≤0.1) in winter and increased steeply to reach high values (∼0.5) in summer. It was observed that monthly mean AODs vary significantly (by more than a factor of 6) from January to June. Strong short-period fluctuations (within a daytime) were observed in the AODs. Further investigations of this aspect have revealed that boundary layer dynamics plays a key role in transporting aerosols from the polluted valley region to higher altitudes, causing large contrast in AODs between forenoon and afternoon. The seasonal variations in AODs, while examined in conjunction with synoptic-scale wind fields, have revealed that the transport of dust aerosols from arid regions to the valley regions adjacent to the observational site and their subsequent transport upward by boundary layer dynamics are responsible for the summer increases.
Abstract
Multiyear measurements of spectral aerosol optical depths (AODs) were made at Manora Peak in the central Himalaya Range (29°22′N, 79°27′E, ∼1950 m above mean sea level), using a 10-channel multiwavelength solar radiometer for 605 days during January 2002–December 2004. The AODs at 0.5 μm were very low (≤0.1) in winter and increased steeply to reach high values (∼0.5) in summer. It was observed that monthly mean AODs vary significantly (by more than a factor of 6) from January to June. Strong short-period fluctuations (within a daytime) were observed in the AODs. Further investigations of this aspect have revealed that boundary layer dynamics plays a key role in transporting aerosols from the polluted valley region to higher altitudes, causing large contrast in AODs between forenoon and afternoon. The seasonal variations in AODs, while examined in conjunction with synoptic-scale wind fields, have revealed that the transport of dust aerosols from arid regions to the valley regions adjacent to the observational site and their subsequent transport upward by boundary layer dynamics are responsible for the summer increases.
Abstract
Simultaneous and collocated measurements of total and hemispherical backscattering coefficients (σ and β, respectively) at three wavelengths, mass size distributions, and columnar spectral aerosol optical depth (AOD) were made onboard an extensive cruise experiment covering, for the first time, the entire Bay of Bengal (BoB) and northern Indian Ocean. The results are synthesized to understand the optical properties of aerosols in the marine atmospheric boundary layer and their dependence on the size distribution. The observations revealed distinct spatial and spectral variations of all the aerosol parameters over the BoB and the presence of strong latitudinal gradients. The size distributions varied spatially, with the majority of accumulation modes decreasing from north to south. The scattering coefficient decreased from very high values (resembling those reported for continental/urban locations) in the northern BoB to very low values seen over near-pristine environments in the southeastern BoB. The average mass scattering efficiency of BoB aerosols was found to be 2.66 ± 0.1 m2 g−1 at 550 nm. The spectral dependence of columnar AOD deviated significantly from that of the scattering coefficients in the northern BoB, implying vertical heterogeneity in the aerosol type in that region. However, a more homogeneous scenario was observed in the southern BoB. Simultaneous lidar and in situ measurements onboard an aircraft over the ocean revealed the presence of elevated aerosol layers of enhanced extinction at altitudes of 1 to 3 km with an offshore extent of a few hundred kilometers. Back-trajectory analyses showed these layers to be associated with advection from west Asia and western India. The large spatial variations and vertical heterogeneity in aerosol properties, revealed by the present study, need to be included in the regional radiative forcing over the Bay of Bengal.
Abstract
Simultaneous and collocated measurements of total and hemispherical backscattering coefficients (σ and β, respectively) at three wavelengths, mass size distributions, and columnar spectral aerosol optical depth (AOD) were made onboard an extensive cruise experiment covering, for the first time, the entire Bay of Bengal (BoB) and northern Indian Ocean. The results are synthesized to understand the optical properties of aerosols in the marine atmospheric boundary layer and their dependence on the size distribution. The observations revealed distinct spatial and spectral variations of all the aerosol parameters over the BoB and the presence of strong latitudinal gradients. The size distributions varied spatially, with the majority of accumulation modes decreasing from north to south. The scattering coefficient decreased from very high values (resembling those reported for continental/urban locations) in the northern BoB to very low values seen over near-pristine environments in the southeastern BoB. The average mass scattering efficiency of BoB aerosols was found to be 2.66 ± 0.1 m2 g−1 at 550 nm. The spectral dependence of columnar AOD deviated significantly from that of the scattering coefficients in the northern BoB, implying vertical heterogeneity in the aerosol type in that region. However, a more homogeneous scenario was observed in the southern BoB. Simultaneous lidar and in situ measurements onboard an aircraft over the ocean revealed the presence of elevated aerosol layers of enhanced extinction at altitudes of 1 to 3 km with an offshore extent of a few hundred kilometers. Back-trajectory analyses showed these layers to be associated with advection from west Asia and western India. The large spatial variations and vertical heterogeneity in aerosol properties, revealed by the present study, need to be included in the regional radiative forcing over the Bay of Bengal.
Abstract
Synergizing satellite remote sensing data with vertical profiles of atmospheric thermodynamics and regional climate model simulations, we investigate the relative importance, transport pathways, and seasonality of contribution of dust from regional (Thar Desert and adjoining arid regions) and remote (southwest Asia and northeast Africa) sources over the northeast Indian Ocean [i.e., the Bay of Bengal (BOB)]. We show that while over the northern BOB dust from the regional sources contribute more than 50% to the total dust load during the southwest monsoon period (June–September), interestingly; the remote dust sources dominate rest of the year. On the other hand, over the southern BOB, dust transported from the remote-source regions dominate throughout the year. During June, the dry elevated layer (at altitudes between 850 and 700 hPa) of dust, transported across the Indo-Gangetic Plain to the northern BOB, arises primarily from the Thar Desert. Dust from remote sources in the far west reaches the southern BOB after traversing over and around the southern Indian Peninsula. Since dust from these distinct source regions have different mineral composition (hence optical properties) and undergo distinct changes during atmospheric transport, it is important to understand source-specific dust contribution and transport pathways to address dust–climate feedback.
Abstract
Synergizing satellite remote sensing data with vertical profiles of atmospheric thermodynamics and regional climate model simulations, we investigate the relative importance, transport pathways, and seasonality of contribution of dust from regional (Thar Desert and adjoining arid regions) and remote (southwest Asia and northeast Africa) sources over the northeast Indian Ocean [i.e., the Bay of Bengal (BOB)]. We show that while over the northern BOB dust from the regional sources contribute more than 50% to the total dust load during the southwest monsoon period (June–September), interestingly; the remote dust sources dominate rest of the year. On the other hand, over the southern BOB, dust transported from the remote-source regions dominate throughout the year. During June, the dry elevated layer (at altitudes between 850 and 700 hPa) of dust, transported across the Indo-Gangetic Plain to the northern BOB, arises primarily from the Thar Desert. Dust from remote sources in the far west reaches the southern BOB after traversing over and around the southern Indian Peninsula. Since dust from these distinct source regions have different mineral composition (hence optical properties) and undergo distinct changes during atmospheric transport, it is important to understand source-specific dust contribution and transport pathways to address dust–climate feedback.
