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- Author or Editor: S. Sharma x
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Abstract
It is shown that genetic inversions can be used to recover lognormal aerosol size distributions from multiangle optical scattering cross-section data measured by a polar nephelometer at a wavelength of 0.532 μm. The inversions can also be used to recover the absolute calibration factor of the polar nephelometer. The method is demonstrated by applying it to polar nephelometer data measured during the Shoreline Environment Aerosol Study (SEAS) at Bellows Beach on the island of Oahu, Hawaii. Also, the inverted size distributions are compared with those inferred from direct measurements by particle sizers during SEAS. At 0.532 μm, the polar nephelometer data are dominated by the effect of coarse-mode hydrated sea salt. Although the inversion was unable to place constraints on the accumulation-mode size distribution, the modeled size distribution provides a good description of optical scattering at wavelengths of 0.532 μm and above.
Abstract
It is shown that genetic inversions can be used to recover lognormal aerosol size distributions from multiangle optical scattering cross-section data measured by a polar nephelometer at a wavelength of 0.532 μm. The inversions can also be used to recover the absolute calibration factor of the polar nephelometer. The method is demonstrated by applying it to polar nephelometer data measured during the Shoreline Environment Aerosol Study (SEAS) at Bellows Beach on the island of Oahu, Hawaii. Also, the inverted size distributions are compared with those inferred from direct measurements by particle sizers during SEAS. At 0.532 μm, the polar nephelometer data are dominated by the effect of coarse-mode hydrated sea salt. Although the inversion was unable to place constraints on the accumulation-mode size distribution, the modeled size distribution provides a good description of optical scattering at wavelengths of 0.532 μm and above.
Abstract
Laser scintillation observations were carried out over a flat surface in different atmospheric conditions on 33 separate days during March 1990–April 1991 and were analyzed and studied. The principal results of the analysis reveal (i) marked seasonal variations in optical turbulence (through the measurement of refractive-index structure function Cn 2) and scintillation intensity (measured in terms of percent modulation Pm ) with maximum Cn 2 or Pm during winter (December–February) and minimum during premonsoon (March–May) seasons; (ii) close correspondence among the variations in Cn 2, Pm , and atmospheric temperature; (iii) lower values of Cn 2 during cloudy sky as compared to clear sky conditions; and (iv) agreement between the observations and theory in respect of the pathlength dependence of Cn 2 and Pm . The results are discussed with reference to the possible meteorological origin of turbulence, and the importance of the study for making measurements of optical turbulence remotely over inaccessible regions is highlighted.
Abstract
Laser scintillation observations were carried out over a flat surface in different atmospheric conditions on 33 separate days during March 1990–April 1991 and were analyzed and studied. The principal results of the analysis reveal (i) marked seasonal variations in optical turbulence (through the measurement of refractive-index structure function Cn 2) and scintillation intensity (measured in terms of percent modulation Pm ) with maximum Cn 2 or Pm during winter (December–February) and minimum during premonsoon (March–May) seasons; (ii) close correspondence among the variations in Cn 2, Pm , and atmospheric temperature; (iii) lower values of Cn 2 during cloudy sky as compared to clear sky conditions; and (iv) agreement between the observations and theory in respect of the pathlength dependence of Cn 2 and Pm . The results are discussed with reference to the possible meteorological origin of turbulence, and the importance of the study for making measurements of optical turbulence remotely over inaccessible regions is highlighted.
Abstract
This chapter outlines the development of our understanding of several examples of mesoscale atmospheric circulations that are tied directly to surface forcings, starting from thermally driven variations over the ocean and progressing inland to man-made variations in temperature and roughness, and ending with forced boundary layer circulations. Examples include atmospheric responses to 1) overocean temperature variations, 2) coastlines (sea breezes), 3) mesoscale regions of inland water (lake-effect storms), and 4) variations in land-based surface usage (urban land cover). This chapter provides brief summaries of the historical evolution of, and tools for, understanding such mesoscale atmospheric circulations and their importance to the field, as well as physical processes responsible for initiating and determining their evolution. Some avenues of future research we see as critical are provided. The American Meteorological Society (AMS) has played a direct and important role in fostering the development of understanding mesoscale surface-forced circulations. The significance of AMS journal publications and conferences on this and interrelated atmospheric, oceanic, and hydrological fields, as well as those by sister scientific organizations, are demonstrated through extensive relevant citations.
Abstract
This chapter outlines the development of our understanding of several examples of mesoscale atmospheric circulations that are tied directly to surface forcings, starting from thermally driven variations over the ocean and progressing inland to man-made variations in temperature and roughness, and ending with forced boundary layer circulations. Examples include atmospheric responses to 1) overocean temperature variations, 2) coastlines (sea breezes), 3) mesoscale regions of inland water (lake-effect storms), and 4) variations in land-based surface usage (urban land cover). This chapter provides brief summaries of the historical evolution of, and tools for, understanding such mesoscale atmospheric circulations and their importance to the field, as well as physical processes responsible for initiating and determining their evolution. Some avenues of future research we see as critical are provided. The American Meteorological Society (AMS) has played a direct and important role in fostering the development of understanding mesoscale surface-forced circulations. The significance of AMS journal publications and conferences on this and interrelated atmospheric, oceanic, and hydrological fields, as well as those by sister scientific organizations, are demonstrated through extensive relevant citations.
Abstract
Scanning lidar measurements were carried out during the Shoreline Environment Aerosol Study (SEAS) experiment (19–30 April 2000) to characterize the aerosol scattering fields in the coastal marine boundary layer at Bellows Beach on the southeast side of Oahu, Hawaii. The sea salt was found to be well mixed throughout the mixed layer, although the depth of the trade wind mixed layer was found to vary significantly over short timescales. As expected, the frequency distribution of aerosol scatter had a lognormal distribution, with the exception of regions downwind of breaking waves, where the frequency distribution was bimodal. A spatial statistical study revealed that the island-blocking effects cause low-level clouds to develop as they approach the island, with enhanced drizzle near the coastline reaching all the way to the surface. The spray from waves breaking on an outer reef was found to be intermittent and contained to heights of 20 m (on average) for the average wind speed of 7 m s−1. Sea-salt concentrations and fluxes from the breaking waves were estimated from the lidar measurements and found to be within the range of values reported by other investigators.
Abstract
Scanning lidar measurements were carried out during the Shoreline Environment Aerosol Study (SEAS) experiment (19–30 April 2000) to characterize the aerosol scattering fields in the coastal marine boundary layer at Bellows Beach on the southeast side of Oahu, Hawaii. The sea salt was found to be well mixed throughout the mixed layer, although the depth of the trade wind mixed layer was found to vary significantly over short timescales. As expected, the frequency distribution of aerosol scatter had a lognormal distribution, with the exception of regions downwind of breaking waves, where the frequency distribution was bimodal. A spatial statistical study revealed that the island-blocking effects cause low-level clouds to develop as they approach the island, with enhanced drizzle near the coastline reaching all the way to the surface. The spray from waves breaking on an outer reef was found to be intermittent and contained to heights of 20 m (on average) for the average wind speed of 7 m s−1. Sea-salt concentrations and fluxes from the breaking waves were estimated from the lidar measurements and found to be within the range of values reported by other investigators.
Abstract
The characteristics of a small, lightweight portable lidar system for measuring aerosol (Mie) scatter at wavelengths of 1064 and 532 nm are described. It uses a 20-Hz Nd:YAG pulsed laser as a source and a 12.7-cm-diameter telescope as a receiver. By using a minimal number of commercially available components, the cost of construction has been reduced. The lidar has a useable range of 60–3000 m for clean marine conditions. Its performance has been demonstrated using measurements of tropospheric aerosols on the island of Hawaii.
Abstract
The characteristics of a small, lightweight portable lidar system for measuring aerosol (Mie) scatter at wavelengths of 1064 and 532 nm are described. It uses a 20-Hz Nd:YAG pulsed laser as a source and a 12.7-cm-diameter telescope as a receiver. By using a minimal number of commercially available components, the cost of construction has been reduced. The lidar has a useable range of 60–3000 m for clean marine conditions. Its performance has been demonstrated using measurements of tropospheric aerosols on the island of Hawaii.
Abstract
Coordinated experiments to study the nocturnal lower atmosphere were conducted on selected nights during April–August 1991 using an argon ion lidar and a Doppler sodar at the Indian Institute of Tropical Meteorology, Pune (18°32′N, 73°51′E, 559 m MSL), India. The lidar and the sodar have been operated simultaneously so as to detect the nocturnal atmospheric structure in the common air volume sampled by both the techniques. By analyzing the thermal and aerosol structures in the vertical profiles of the sodar and the lidar signal intensity, the nocturnal mixed-layer height or ground-based inversion height and the stably stratified or multiple elevated layers aloft have been determined. The top of the nocturnal ground-based inversion observed in the sodar records is taken as the height above the ground where the negative vertical gradient in aerosol concentration first reaches a maximum in the lidar records. The results of the study indicate an agreement between the lidar-derived mixing depth and the sodar-derived heights of the ground-based inversion and the low-level wind maximum.
Abstract
Coordinated experiments to study the nocturnal lower atmosphere were conducted on selected nights during April–August 1991 using an argon ion lidar and a Doppler sodar at the Indian Institute of Tropical Meteorology, Pune (18°32′N, 73°51′E, 559 m MSL), India. The lidar and the sodar have been operated simultaneously so as to detect the nocturnal atmospheric structure in the common air volume sampled by both the techniques. By analyzing the thermal and aerosol structures in the vertical profiles of the sodar and the lidar signal intensity, the nocturnal mixed-layer height or ground-based inversion height and the stably stratified or multiple elevated layers aloft have been determined. The top of the nocturnal ground-based inversion observed in the sodar records is taken as the height above the ground where the negative vertical gradient in aerosol concentration first reaches a maximum in the lidar records. The results of the study indicate an agreement between the lidar-derived mixing depth and the sodar-derived heights of the ground-based inversion and the low-level wind maximum.
Abstract
The interaction of global climate change and urban heat islands (UHI) is expected to have far-reaching impacts on the sustainability of the world’s rapidly growing urban population centers. Given that a wide range of spatiotemporal scales contributed by meteorological forcing and complex surface heterogeneity complicates UHI, a multimodel nested approach is used in this paper to study climate-change impacts on the Chicago, Illinois, UHI, covering a range of relevant scales. One-way dynamical downscaling is used with a model chain consisting of global climate (Community Atmosphere Model), regional climate (Weather Research and Forecasting Model), and microscale (“ENVI-met”) models. Nested mesoscale and microscale models are evaluated against the present-day observations (including a dedicated urban miniature field study), and the results favorably demonstrate the fidelity of the downscaling techniques that were used. A simple building-energy model is developed and used in conjunction with microscale-model output to calculate future energy demands for a building, and a substantial increase (as much as 26% during daytime) is noted for future (~2080) climate. Although winds and lake-breeze circulation for future climate are favorable for reducing energy usage by 7%, the benefits are outweighed by such factors as exacerbated UHI and air temperature. An adverse change in human-comfort indicators is also noted in the future climate, with 92% of the population experiencing thermal discomfort. The model chain that was used has general applicability for evaluating climate-change impacts on city centers and, hence, for urban-sustainability studies.
Abstract
The interaction of global climate change and urban heat islands (UHI) is expected to have far-reaching impacts on the sustainability of the world’s rapidly growing urban population centers. Given that a wide range of spatiotemporal scales contributed by meteorological forcing and complex surface heterogeneity complicates UHI, a multimodel nested approach is used in this paper to study climate-change impacts on the Chicago, Illinois, UHI, covering a range of relevant scales. One-way dynamical downscaling is used with a model chain consisting of global climate (Community Atmosphere Model), regional climate (Weather Research and Forecasting Model), and microscale (“ENVI-met”) models. Nested mesoscale and microscale models are evaluated against the present-day observations (including a dedicated urban miniature field study), and the results favorably demonstrate the fidelity of the downscaling techniques that were used. A simple building-energy model is developed and used in conjunction with microscale-model output to calculate future energy demands for a building, and a substantial increase (as much as 26% during daytime) is noted for future (~2080) climate. Although winds and lake-breeze circulation for future climate are favorable for reducing energy usage by 7%, the benefits are outweighed by such factors as exacerbated UHI and air temperature. An adverse change in human-comfort indicators is also noted in the future climate, with 92% of the population experiencing thermal discomfort. The model chain that was used has general applicability for evaluating climate-change impacts on city centers and, hence, for urban-sustainability studies.
Abstract
C-FOG is a comprehensive bi-national project dealing with the formation, persistence, and dissipation (life cycle) of fog in coastal areas (coastal fog) controlled by land, marine, and atmospheric processes. Given its inherent complexity, coastal-fog literature has mainly focused on case studies, and there is a continuing need for research that integrates across processes (e.g., air–sea–land interactions, environmental flow, aerosol transport, and chemistry), dynamics (two-phase flow and turbulence), microphysics (nucleation, droplet characterization), and thermodynamics (heat transfer and phase changes) through field observations and modeling. Central to C-FOG was a field campaign in eastern Canada from 1 September to 8 October 2018, covering four land sites in Newfoundland and Nova Scotia and an adjacent coastal strip transected by the Research Vessel Hugh R. Sharp. An array of in situ, path-integrating, and remote sensing instruments gathered data across a swath of space–time scales relevant to fog life cycle. Satellite and reanalysis products, routine meteorological observations, numerical weather prediction model (WRF and COAMPS) outputs, large-eddy simulations, and phenomenological modeling underpin the interpretation of field observations in a multiscale and multiplatform framework that helps identify and remedy numerical model deficiencies. An overview of the C-FOG field campaign and some preliminary analysis/findings are presented in this paper.
Abstract
C-FOG is a comprehensive bi-national project dealing with the formation, persistence, and dissipation (life cycle) of fog in coastal areas (coastal fog) controlled by land, marine, and atmospheric processes. Given its inherent complexity, coastal-fog literature has mainly focused on case studies, and there is a continuing need for research that integrates across processes (e.g., air–sea–land interactions, environmental flow, aerosol transport, and chemistry), dynamics (two-phase flow and turbulence), microphysics (nucleation, droplet characterization), and thermodynamics (heat transfer and phase changes) through field observations and modeling. Central to C-FOG was a field campaign in eastern Canada from 1 September to 8 October 2018, covering four land sites in Newfoundland and Nova Scotia and an adjacent coastal strip transected by the Research Vessel Hugh R. Sharp. An array of in situ, path-integrating, and remote sensing instruments gathered data across a swath of space–time scales relevant to fog life cycle. Satellite and reanalysis products, routine meteorological observations, numerical weather prediction model (WRF and COAMPS) outputs, large-eddy simulations, and phenomenological modeling underpin the interpretation of field observations in a multiscale and multiplatform framework that helps identify and remedy numerical model deficiencies. An overview of the C-FOG field campaign and some preliminary analysis/findings are presented in this paper.
Abstract
In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air–sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air–sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ∼20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10–12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ∼20 to 50 m), cooling SST (by ∼1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air–sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.
Abstract
In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air–sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air–sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ∼20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10–12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ∼20 to 50 m), cooling SST (by ∼1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air–sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.
