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An X-band vertical array system is developed for measuring and characterizing electromagnetic (EM) propagation in the marine atmospheric boundary layer. In particular, the evaporation duct that commonly forms over water is investigated as part of the Coupled Air-Sea Processes and Electromagnetic ducting Research (CASPER) at-sea experimental campaign conducted off the coast of Duck, NC, USA, during October–November of 2015. Monte Carlo simulations are first used to develop an optimal array of four vertically spaced receiving antennas. In the experiment, the antennas are mounted on the stern A-frame of a research vessel and measure EM signals transmitted from beacons mounted on another research vessel and on the pier at the Army Field Research Facility. While the propagation loss versus range provides a data set similar to previous work, the vertical array can provide sampling of modes in the leaky waveguide formed in the duct. Combining both the range and height sampling results in a robust inversion method for evaporation duct estimation. In this paper, the efficacy of the four-element array is demonstrated by estimation of the evaporation duct height through comparison with a library of precomputed propagation curves generated using the parabolic wave equation. Low model error gives confidence in the estimates, which are consistent with the concurrent environmental measurements performed by the CASPER team. It is found that the evaporation duct can vary significantly with range and time over the duration of a data collection run.

This study characterizes the evaporation and elevated ducts, the most common types of ducts observed over the ocean, along a track of around 4000 km between the California coast and Hawaii. We analyzed one year (2012-2013) of ship based measurements made during the Marine ARM (Atmospheric Radiation Measurement) GPCI (GCSS Pacific Cross-Section Intercomparison) Investigation of Clouds (MAGIC) campaign. During this period, the ship made multiple transects between southern California and Hawaii. While the ship based in situ measurements and the radiosonde data served as the primary data source, a marine atmospheric surface layer model adapted from the COARE 3.0 surface flux scheme is used to diagnose evaporative duct properties. Calculated mean evaporation duct heights based on shipboard measurements were found to increase steadily from 7 m offshore California to about 15 m near Hawaii. Overall 78% of duct heights are below 20 m. On average the evaporation duct strength is between ≈ 25-35 M units near Hawaii and about 15 M units near the California coast. A gradual transition from stratocumulus (Sc) dominated offshore California to trade wind regime with cumulus (Cu) clouds takes place along MAGIC track. The measured marine atmospheric boundary layer (MABL) height and the capping inversion characteristics are significantly different in the two regimes with MABL decoupling occurring in the longitude range between 125°W-140°W along the track. The characteristics of the elevated ducts, obtained from the rawinsonde sounding profiles, are also different in the two regions west and east of the MABL decoupling region. Approximately 70-80% of the elevated ducts occur below 1.5 km east of the decoupling region whereas, almost equal percentage of ducts forms at heights above 1.5 km on the west side.

This paper presents the observations of the thermodynamic structure of the Marine Atmospheric Boundary Layer (MABL) over Bay of Bengal (BoB), Northern Indian Ocean (NIO) and Arabian Sea (AS) regions, using radiosonde observations carried out as a part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) during March–May 2006. Most of the profiles show the general structure of tropical MABL which consists of surface layer, sub-cloud layer (mixed layer), transition layer, cloud layer and trade wind inversion (TWI). However, in few soundings over BoB and AS, it was observed that the cloud layer is remarkably well-mixed, forming a double mixed layer (DML) structure. This feature is seen to be closely associated with the low level anticyclonic system over these sectors. The average profiles of vertical wind (corresponding to the region of observation) obtained from NCEP showed that there is an increase in the vertical velocity on the days when such a double mixed layer was observed and the altitude region of this high ω coincides with that of the DML. The study also reveals some important differences in the MABL structure during convectively active and weak periods. The spatial variation of the parameters like mixed layer (ML) depth, height of lifting condensation level (LCL), level of free convection (LFC), and trade wind inversion (TWI) are presented and discussed in detail. The TWI observations are first of their kind from the BoB region. The TWI top is found to be at a higher height compared to the observations over other oceanic regions, and the relationship between the TWI altitude and lapse rate of specific humidity is used to explain why the TWI was formed at a higher height. The observations of the MABL parameters are also compared with those reported from the previous campaigns over these regions.

High-resolution Regional Model is used to study the structure and dynamics of Sea/Land Breeze Circulation along west coast of Indian Sub-continent during pre-monsoon season. Both the observations from ocean and land are in agreement with the model simulations. From the model simulations, a time delay of 3 hours is observed in the onset of sea breeze over ocean and land (∼ 28 km from the sea/land interface). The two main components of the sea breeze circulation cell, the sea breeze flow in the lower levels and the return current in the upper levels are clearly modeled. The vertical extent of sea breeze circulation cell is ∼ 1 km and, the horizontal seaward and landward extensions are ∼ 100 and ∼ 75 km respectively from the sea/land interface. Within the sea breeze circulation cell, during daytime, a shallow mixed layer (600–800 m) over ocean and a convective mixed layer (1000–1200 m) over the land are observed. Observations and simulations verified the pre-assumption of the near neutral lapse rate existing over ocean and transferring to the onshore regions.

In-situ measurements of number density, size distribution, and mass loading of near-surface aerosols were carried out at Kharagpur, a site on the eastern part of Indo-Gangetic Plains during the winter month of December 2004. The data have been used to investigate wintertime characteristics of aerosols and their effects on the occurrence of haze. The aerosol number density is found to be of the order of 109 m−3 and mass loading is ~265±70 μg m−3 (5–8 times that reported from south Indian sites). The diurnal patterns and day-to-day variations in aerosol number density and mass loading are closely associated with atmospheric boundary layer height. During haze events, the number density of submicron particles is found to be 2–5 times higher than that during non-hazy period. This could be attributed to the enhanced concentration of anthropogenic aerosols, low atmospheric boundary layer height/ventilation coefficient and airflow convergence.

The local weather and air quality over a region are greatly influenced by the atmospheric boundary layer (ABL) structure and dynamics. ABL characteristics were measured using a tethered balloon-sonde system over Kharagpur (22.32°N, 87.32°E, 40m above MSL), India, for the period 7 December 2004 to 30 December 2004, as a part of the Indian Space Research Organization-Geosphere Biosphere Program (ISRO-GBP) Aerosol Land Campaign II. High-resolution data of pressure, temperature, humidity, wind speed and wind direction were archived along with surface layer measurements using an automatic weather station. This paper presents the features of ABL, like ABL depth and nocturnal boundary layer (NBL) depth. The sea surface winds from Quikscat over the oceanic regions near the experiment site were analyzed along with the NCEP/NCAR reanalysis winds over Kharagpur to estimate the convergence of wind, moisture and vorticity to understand the observed variations in wind speed and relative humidity, and also the increased aerosol concentrations. The variation of ventilation coefficient (V C), a factor determining the air pollution potential over a region, is also discussed in detail.

We report the spatio-temporal variability of surface-layer turbulent fluxes of heat, moisture and momentum over the Bay of Bengal (BoB) and the Arabian Sea (AS) during the Integrated Campaign for Aerosols, gases Radiation Budget (ICARB) field experiment. The meteorological component of ICARB conducted during March – May 2006 onboard the oceanic research vessel Sagar Kanya forms the database for the present study. The bulk transfer coefficients and the surface-layer fluxes are estimated using a modified bulk aerodynamic method, and then the spatio-temporal variability of these air-sea interface fluxes is discussed in detail. It is observed that the sensible and latent heat fluxes over the AS are marginally higher than those over the BoB, which we attribute to differences in the prevailing meteorological conditions over the two oceanic regions. The values of the wind stress, sensible and latent heat fluxes are compared with those obtained for the Indian Ocean Experiment (INDOEX) period. The variation of drag coefficient (CD), exchange coefficients of sensible heat and moisture (CH = CE) and neutral drag coefficient (CDN) with wind speed is also discussed.

Quantitative estimates of the vertical structure and the spatial gradients of aerosol extinction coefficients have been made from airborne lidar measurements across the coastline into offshore oceanic regions along the east and west coasts of India. The vertical structure revealed the presence of strong, elevated aerosol layers in the altitude region of similar to 2-4 km, well above the atmospheric boundary layer (ABL). Horizontal gradients also showed a vertical structure, being sharp with the e(-1) scaling distance (D-0H) as small as similar to 150 km in the well-mixed regions mostly under the influence of local source effects. Above the ABL, where local effects are subdued, the gradients were much shallower (similar to 600-800 km); nevertheless, they were steep compared to the value of similar to 1500-2500 km reported for columnar AOD during winter. The gradients of these elevated layers were steeper over the east coast of India than over the west coast. Near-simultaneous radio sonde (Vaisala, Inc., Finland) ascents made over the northern Bay of Bengal showed the presence of convectively unstable regions, first from surface to similar to 750-1000 m and the other extending from 1750 to 3000 m separated by a stable region in between. These can act as a conduit for the advection of aerosols and favor the transport of continental aerosols in the higher levels (> 2 km) into the oceans without entering the marine boundary layer below. Large spatial gradient in aerosol optical and hence radiative impacts between the coastal landmass and the adjacent oceans within a short distance of < 300 km (even at an altitude of 3 km) during summer and the premonsoon is of significance to the regional climate.

Detailed measurements were carried out in the Marine Atmospheric Boundary Layer (MABL) during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) which covered both Arabian Sea and Bay of Bengal during March to May 2006. In this paper, we present the meteorological observations made during this campaign. The latitudinal variation of the surface layer turbulent fluxes is also described in detail.

[1] The Indo-Gangetic Plain (IGP) encompasses a vast area, (accounting for ∼21% of the land area of India), which is densely populated (accommodating ∼40% of the Indian population). Highly growing economy and population over this region results in a wide range of anthropogenic activities. A large number of thermal power plants (most of them coal fed) are clustered along this region. Despite its importance, detailed investigation of aerosols over this region is sparse. During an intense field campaign of winter 2004, extensive aerosol and atmospheric boundary layer measurements were made from three locations: Kharagpur (KGP), Allahabad (ALB), and Kanpur (KNP), within the IGP. These data are used (1) to understand the regional features of aerosols and BC over the IGP and their interdependencies, (2) to compare it with features at locations lying at far away from the IGP where the conditions are totally different, (3) to delineate the effects of mesoscale processes associated with changes in the local atmospheric boundary layer (ABL), (4) to investigate the effects of long-range transport or moving weather phenomena in modulating the aerosol properties as well as the ABL characteristics, and (5) to examine the changes as the season changes over to spring and summer. Our investigations have revealed very high concentrations of aerosols along the IGP, the average mass concentrations (MT) of total aerosols being in the range 260 to 300 μg m−3 and BC mass concentrations (MB) in the range 20 to 30 μg m−3 (both ∼5 to 8 times higher than the values observed at off-IGP stations) during December 2004. Despite, BC constituted about 10% to the total aerosol mass concentration, a value quite comparable to those observed elsewhere over India for this season. The dynamics of the local atmospheric boundary layer (ABL) as well as changes in local emissions strongly influence the diurnal variations of MT and MB, both being inversely correlated with the mixed layer height (Zi) and the ventilation coefficient (Vc). The share of BC to total aerosols is highest (∼12%) during early night and lowest (∼4%) in the early morning hours. While an increase in the Vc results in a reduction in the concentration almost simultaneously, an increase in Zimax has its most impact on the concentration after ∼1 day. Accumulation mode aerosols contributed ∼90% to the aerosol concentration at ALB, ∼77 % at KGP and 74% at KNP. The BC mass mixing ratio was ∼10% over all three locations and is comparable to the value reported for Trivandrum, a tropical coastal location in southern India. This indicates presence of submicron aerosols species other than BC (such as sulfate) over KGP and KNP. A cross-correlation analysis showed that the changes in MB at KGP is significantly correlated with those at KNP, located ∼850 km upwind, and ALB after a delay of ∼7 days, while no such delay was seen between ALB and KNP. Back trajectory analyses show an enhancement in MB associated with trajectories arriving from west, the farther from to the west they arrive, the more is the increase. This, along with the ABL characteristics, indicate two possibilities: (1) advection of aerosols from the west Asia and northwest India and (2) movement of a weather phenomena (such as cold air mass) conducive for build up of aerosols from the west to east. As the winter gives way to summer, the change in the wind direction and increased convective mixing lead to a rapid decrease in MB.

Enhanced aerosol loading over the Indo-Gangetic Plain (IGP) is a regular feature during winter months. In addition to the environmental degradation and reduced visibility, these aerosols can cause significant radiative impact also. In view of this, a campaign mode observation under ISRO-GBP was conducted in D ecember 2004 to characterize the aerosol properties over the IGP. As part of this, extensive measurements of aerosol BC were made from Kharagpur, an inland rural location lying at the eastern end of the Indo Gangetic Plain. It also lies close to several industrialized regions and area having lot of mining activities Results showed, extremely high BC co ncentration, often exceeding ~20 P g m -2 , prevailed during December. During this period, BC concentration also showed large diurnal vari ation. Simultaneous measuremen ts of the local atmospheric boundary layer height and wind fields revealed a very close association between the BC concentration and the ventilation coefficient (defined as the product of the boundary layer height and the transport wind). Back trajectory analyses using HYSPLIT revealed that in addition to the local boundary layer dynamics, the changes in the advection pathways also influence the concentration of BC. Keywords: Aerosols, BC, Atmospheric boundary layer, HYSPLIT

A case study of sea breeze circulation at a coastal region Thumba (8.5°N, 76.9°E) was carried out using Doppler Sodar, surface wind, temperature, humidity measurements and radiosonde ascents. The analysis of surface meteorological data showed that the onset of sea breeze on 12th April 2006 was at 0945 hrs. GPS sonde observation over sea at 1425 hrs and Radiosonde observation over land at 1730 showed a well developed sea breeze circulation over Thumba coast by afternoon hours. The vertical extent of sea breeze circulation was ~1000m over sea as well as on land. The Thermal Internal Boundary Layer (TIBL) depth associated with sea breeze circulation was about 400m at 8 km away from coast. The marine mixed layer height was ~500m about 12 km away from the coast. Numerical simulation of sea breeze was made using HRM (High Resolution Model) and compared the results with the observations.