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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 paper reports upper ocean thermohaline structure and variability observed during the life cycle of an intense Madden Julian Oscillation (MJO) event occurred in the southern tropical Indian Ocean (14°S–Eq, 70°E–81°E). Water column measurements for this study were collected using airborne expendable probes deployed from NOAA's WP-3D Orion aircraft operated as a part of Dynamics of MJO field experiment conducted during November–December 2011. Purpose of the study is twofold; (1) to provide a statistical analysis of the upper ocean properties observed during different phases of MJO and, (2) to investigate how the upper ocean thermohaline structure evolved in the study region in response to the MJO induced perturbation. During the active phase of MJO, mixed layer depth (MLD) had a characteristic bimodal distribution. Primary and secondary modes were at ∼34 m and ∼65 m, respectively. Spatial heterogeneity of the upper ocean response to the MJO forcing was the plausible reason for bimodal distribution. Thermocline and isothermal layer depth deepened, respectively, by 13 and 19 m from the suppressed through the restoring phase of MJO. Thicker (>30 m) barrier layers were found to occur more frequently in the active phase of MJO, associated with convective rainfalls. Additionally, the water mass analysis indicated that, in the active phase of this MJO event the subsurface was dominated by Indonesian throughflow, nonetheless intrusion of Arabian Sea high saline water was also noted near the equator.

The article of record as published may be found at http://dx.doi.org/10.1175/JAMC-D-18-0029.1 In this study, we use observational and numerical model data from the Coupled Air Sea Processes and Electromagnetic Ducting Research (CASPER) field campaign to describe the mean refractive conditions offshore Duck, North Carolina. The U.S. Navy operational numerical weather prediction model known as the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) performed well forecasting large-scale conditions during the experiment, with an observed warm bias in SST and cold and dry biases in temperature and humidity in the lowest 2000 m. In general, COAMPS underpredicted the number of ducts, and they were weaker and at lower height than those seen in observations. It was found that there is a noticeable diurnal evolution of the ducts, more over land than over the ocean. Ducts were found to be more frequent over land but overall were stronger and deeper over the ocean. Also, the evaporative duct height increases as one moves offshore. A case study was chosen to describe the electromagnetic properties under different synoptic conditions. In this case the continental atmospheric boundary layer dominates and interacts with the marine atmospheric boundary layer. As a result, the latter moves around 80 km offshore and then back inland after 2 h. Coupled Air–Sea Processes and Electromagnetic Ducting Research (CASPER); Office of Naval Research (ONR) under Multidisciplinary University Research Initiative (MURI) Office of Naval Research (ONR) N0001418WX01087 N0001417WX02083

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.

The vertical thermodynamic structure of the marine boundary layer exhibits significant spatial variability in the near-shore/littoral regions or in the vicinity of ocean sea surface temperature fronts due to a variety of coupled mesoscale atmospheric and oceanographic processes. As a result, the ubiquitous and tactically-relevant evaporative duct (ED) also exhibits significant spatial variability correspondingly [1]. This paper explores this variability in near-shore ED properties through aircraft and buoy data collected as part of the Coupled Air-Sea Processes and EM ducting Research (CASPER) project's East and West field campaigns. We further explore impact of such spatial variability on electromagnetic wave (EM) propagation. The results should provide some guidance to retrieval of evaporation duct properties using propagation loss measurements. The sensitivity of propagation loss to the spatial resolutions of ED characteristics such as evaporative duct height (EDH) and evaporative duct strength (EDS) will also be discussed.

2018 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting The article of record may be found at https://doi.org/10.1109/APUSNCURSINRSM.2018.8608750 The Coupled Air-Sea Processes and Electromagnetic ducting Research (CASPER) project involved two field campaigns. The most recent field study, CASPER-West, occurred during 27 September - 26 October 2017 offshore of Pt. Mugu, California. The most significant ducting conditions observed during this field campaign included evaporation ducts and surface-based ducts. However, most of the evaporation ducts are also topped by elevated duct layers at several hundred meters above the surface. This paper provides an overview of the CASPER-West measurements and the observed ducting conditions from multiple platforms in CASPER-West.

Physical control of the chlorophyll a (chl a) distribution in the western tropical Indian Ocean (WTIO, 8°N to 18°S along 65°E) was studied during the 2008 winter monsoon (WM) and the 2009 summer monsoon (SM). During both seasons, a prominent deep chlorophyll maximum (DCM, 0.3–0.5 mg m− 3) was observed at all stations between 8°N and 10°S in the depth range of 50–75 m, but south of 10°S, this phenomenon was observed as deeper (~ 120 m) and relatively weak (0.15–0.3 mg m− 3). During the SM, in addition to seasonal forcing, eddies and a freshened surface layer also played major roles in controlling the DCM and the surface chl a concentrations in the southern Arabian Sea and the equatorial Indian Ocean. During the WM, surface freshening controlled the chl a distribution in the Seychelles Chagos Thermocline Ridge (SCTR, 5°S–10°S) region by modulating the static stability and mixed layer depth. It appears that the surface freshening in this region is associated with the core of the South Equatorial Current. South of the SCTR, the chl a distribution was predominantly determined by the anti-cyclonic eddies in both seasons. The spatial patterns of the Sea Level Anomaly (SLA) followed most of the thermocline features observed during the study period.

Electromagnetic wave propagation that behaves differently from that in the standard atmosphere is known as anomalous propagation (AP). Ducting is one type of AP in which the radio and microwaves become trapped within a nearly horizontal layer known as the duct. This energy trapping leads to enhanced propagation range and weak propagation loss of electromagnetic energy within the duct. A corresponding reduction in the electromagnetic signal strength also occurs above or below the duct. Knowledge on ducting is imperative for radar and communication applications such as ship-to-ship and ship-to-shore radio communication and radar based ocean navigations. Ducting also affects the ground-to-satellite radio links and radar based precipitation measurements. Strong vertical gradients in temperature and/or moisture create favorable conditions for ducting.

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.

During the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) over India, high-resolution airborne measurements of the altitude profiles of the mass concentrations (MB) of aerosol black carbon (BC) were made off Bhubaneswar (BBR, 85.82°E, 20.25°N), over northwest Bay of Bengal, in the altitude region upto 3 km. Such high-resolution measurements of altitude profiles of aerosols are done for the first time over India. The profiles showed a near-steady vertical distribution of MB modulated with two small peaks, one at 800m and the other at ∼2000m. High resolution GPS (Global Positioning System) sonde (Vaisala) measurements around the same region onboard the research vessel Sagar Kanya (around the same time of the aircraft sortie) revealed two convectively well mixed layers, one from ground to ∼700m with an inversion at the top and the other extends from 1200m to ∼2000m with a second inversion at ∼2200m and a convectively stable region in the altitude range 700–1200m. The observed peaks in the MB profile are found to be associated with these temperature inversions. In addition, long-range transport from the Indo-Gangetic Plain (IGP) and deserts lying further to the west also influence the vertical profile of BC. Latitudinal variation of MB showed a remarkable land ocean contrast at the 500m altitude (within the well mixed region) with remarkably lower values over oceans, suggesting the impact of strong sources over the mainland. However, above the ABL (at 1500m), the latitudinal variations were quite weak, and this appears to be resulting from the impact of long-range transport. Comparison of the altitude profiles of MB over BoB off BBR with those obtained during the earlier occasion over the inland stations of Hyderabad and Kanpur showed similarities above ∼500m, with MB remaining around a steady value of ∼1 μg m−3. However, large differences are seen within the ABL. Even though the observed MB values are not unusually high, their near constancy in the vertical column will have important implications to radiative forcing.

[1] This paper discusses the turbulence characteristics of the free atmosphere over the Bay of Bengal and the Arabian Sea during the premonsoon season of 2006. The turbulence scaling concepts, developed over the past decades for application to oceanic mixing, are used for retrieving turbulence properties in the free atmosphere from high-resolution soundings, as described by Clayson and Kantha (2008). The variability in the vertical turbulence structure over a large spatial region is studied using this novel method for the first time. Investigations revealed the presence of a highly turbulent band at upper troposphere below the cold point tropopause (CPT) with significant enhancements in Thorpe length and eddy diffusion coefficient. This increased turbulence is studied in conjunction with the observations of the tropical tropopause layer (TTL). It is seen that the lower and upper boundaries of this turbulence band nearly coincide with the height where the potential temperature lapse rate begins to show a decrease (LRHmin) and cold point tropopause, respectively. The TTL is the transition regime from convectively dominated troposphere to radiatively dominated stratosphere, and is characterized by a decrease in the static stability. The enhanced zonal and meridional wind shear observed in this region shows that the stability in this region is further weakened because of the breaking of Kelvin–Helmholtz waves, which results in the enhancement of turbulence. The study reveals the potential of high-resolution soundings to be used in the investigations of turbulence in the free atmosphere.