The phenomenon of ducting, influenced by variations in air moisture and temperature above the sea surface, introduces non-standard propagation characteristics that can significantly affect marine communication and radar systems. Therefore, it is necessary to understand these duct formations in order to accurately predict system performance. Atmospheric ducts are characterized by variations in the vertical refractivity profile. The Refractivity-From-Clutter (RFC) technique, known for real-time low tropospheric refractivity profiling, is the focus of this study. This paper explores the concept of employing Multiple Launch Angles (MLA) to enhance RFC inversion precision. The optimal number and distribution of launch angles, multiple-angle cost functions, and corresponding MLA-RFC performance computations are demonstrated via simulations. An MLA-RFC-capable radar is built and deployed during the Coastal Land-Air-Sea Interaction (CLASI) Study in Moss Landing, CA in 2021 and 2022. The Lower ATmospheric PROPagation (LATPROP) radar system is a modified, low-cost marine radar. The capabilities of MLA-RFC are demonstrated using multiple launch angle LATPROP radar measurements collected during the CLASI campaign. The results show that the MLA-RFC method outperforms the single-angle RFC approach by generating reliable launch angle weights and applying them totheinversion algorithm, which leads to reduced bias and RMSerror inevaporation duct height estimates. These weights are derived from the averaged simulated energy trapped in a duct for a given evaporation duct height. Both simulation and experimental results indicate that MLA improves estimation performance by up to 77% and 67%, respectively, compared to the classical single-launch angle RFC.

Atmospheric aerosols, such as sea salt particles and those derived from air pollutants, provide a surface for water condensation leading to fog droplet formation in the lower atmosphere. Fog formation depends on synoptic to microphysical scale processes and this range involved in fog formation presents a challenge for forecasting. This study used aerosol (10-500 nm) and droplet (2-50 mu m) data collected on the northwest Atlantic Ocean during July 2022 as part of the FATIMA (Fog And Turbulence Interactions in the Marine Atmosphere) project to study marine fog. The objective was to evaluate the effect of atmospheric aerosols on fog properties such as droplet size distribution. The study was split into two regimes. While sampling during fog on the Grand Banks of Newfoundland, aerosol size distributions were bimodal, aerosol and fog droplet concentrations were low, and fogs were shorter (<12 hours). In contrast, near Sable Island, aerosol size distributions had a single broad mode that was likely influenced by continental emissions, corresponding with higher aerosol and droplet concentrations as well as longer (>40 hours) fog events. Droplet size distributions were bimodal for all fog events, with a predominant peak at small sizes of 3-5 mu m and a secondary peak around 20-30 mu m. The mean concentration of droplets smaller than 12 mu m (N-d,N-2-12) correlated better with the mean accumulation mode (100-500 nm) aerosol number concentration (N100-500) during the individual case studies with lower aerosol concentrations, consistent with an aerosol-limited regime. The correlation disappears at higher N100-500, suggesting that other processes were more important for droplet formation during those case studies. It should be noted that the small number of case studies limits our interpretation and additional numerical analysis would aid our understanding of fog droplets in this region.

Marine fog plays an important role in ship and aircraft operations as well as in marine ecosystems and climate change. Fog is affected by boundary layer processes occurring at varying time and space scales. Its monitoring and forecasting can be challenging in marine environments because of the limited number of observation sites, such as buoys and ships. Therefore, the Fog and Turbulence in Marine Atmosphere (FATIMA) field campaign was designed to study the marine fog life cycle with high fog occurrence. FATIMA-YS was established over the Yellow Sea region of the Republic of Korea from 20 June to 9 July 2023. Observations were collected using the South Korean research vessel (R/V) Onnuri, the Atmospheric Research Aircraft (NARA), three Korean Ocean Research Stations supported by R/V Gisang-1, meteorological stations including buoys, and the Korean Meteorological Administration weather stations. In-situ instruments on the R/V and aircraft provided extensive observations of various microphysical and dynamical parameters, aerosols and gases, and radiation parameters. Satellite platforms (Himawari and Geostationary Ocean Color Image) together with ship-based millimeter cloud radar, lidar, and microwave radiometer have also been used for fog monitoring and simulation validation. During the campaign, eight Intensive Observational Periods (IOPs) were performed, and visibility changed from tens of meters up to tens of kilometers, and liquid water content reached up to 0.5 g & centerdot;m-3 during heavy fog conditions. Marine fog usually occurs due to advection processes, but fog formation was found to be highly related to the mixing of air masses, turbulence, and ocean cold-water upwelling, whereas its maturity and dissipation were strongly correlated with mixing processes. Preliminary results, extensive new technologies, and the project campaign description with IOP summaries are provided. Finally, the future work and challenges are discussed.

It has been widely recognized that the refractive conditions in the stable boundary layers (SBL) are largely unknown, with critical issues related to refractivity retrieval or modeling of the refraction conditions in the atmosphere. These issues often boiled down to the validity of the Navy Atmospheric Vertical Surface Layer Model (NAVSLaM) or other Monin-Obukhov Similarity theory (MOST) based evaporation duct models under stable thermal stratifications.

A comprehensive study of aerosol chemistry, microphysics, physical processing, and meteorological parameters was conducted to investigate fog-aerosol interactions over the northwest Atlantic Ocean during the Fog And Turbulence Interactions in the Marine Atmosphere (Fatima) 2022 project. Chemical analysis of size-resolved aerosols and fog droplets revealed a reduction in coarse-mode sea salt ions (e.g., Cl-, Na+, ss-SO4 2-) compared to ambient conditions, attributed to their growth into fog droplets. In contrast, fine-mode aerosol processed-S (e.g., non-sea salt [nss]-SO4 2- and MSA-) and reduced-N (e.g., NH4 +) increased during fog events. Three distinct fog types were identified based on size-resolved ion mass distributions, combined with particle number concentrations and meteorological parameters, to provide insight into the drivers of aerosol chemical composition during each chemically distinct fog type. Type I fog was the most common, characterized by depleted coarse-mode sea salt aerosols, with fog droplets initially formed from freshly emitted coarse-mode aerosols and most likely sustained by growth of fine-mode aerosols. Type II fog was least common, sustained by ongoing production of coarse-mode sea salt aerosols due to elevated winds, and supplemented by fine-mode aerosol contributions. Type III fog had the highest mass loadings of sea salt ions at fog droplet sizes, with droplet formation primarily driven by coarse-mode growth with minor contributions from fine-mode aerosol. Reduction of coarse sea salt mass loadings during more than a dozen fog events confirms their expected role as effective fog condensation nuclei and the physical impact of fog processing in reducing their atmospheric mass loading. This dataset shows these aerosols are scavenged (similar to 70%) in a supersaturated atmosphere to form droplets. This is the most extensive marine fog dataset to date to study fog-aerosol interactions from a chemical perspective, with the potential to improve marine fog forecasting models to provide safer marine transportation.

In July 2022, the Fog and Turbulence Interactions in the Marine Atmosphere (FATIMA) field program included fog observations within surface-based air temperature inversions on Sable Island (SI). Twenty-six fog events under cyclonic and anticyclonic conditions were identified and one case each of these two conditions were analyzed in detail. A cyclonic case (CY) was characterized by a trough from a deep cyclone over northeast Canada moving eastward, generating southwesterly flow in the SI area, with a high cloud band. The approaching trough lowered the cloud base to 60 m at Sable Island that remained for the entire event. The saturation of the near-surface layer and subsequent fog occurred mainly due to low-level convergence causing vertical motion and mixing. In an anticyclonic case (AC), subsidence was maintaining a low cloud with a divergent low-level flow which was advected in a SW/S direction into the Sable Island area with fog. There were significant differences in thermal and dynamical instability between the CY and AC cases. According to the gradient Richardson number, the marine layer was more unstable during the CY case compared to the AC case with a larger buoyancy/shear term. This was confirmed by the higher turbulence kinetic energy in the CY case compared to the AC case. The liquid water content was much greater in the AC case than the CY case. The kinetic sensible and latent heat fluxes were largest during the mist periods and least during the fog periods in the CY case whereas they were much less in the AC case. Light rain began partway through the first fog and continued until the end of the event for the CY and the AC cases.

Research and operational numerical weather prediction models rely on bulk-layer parameterization techniques - primarily, the Monin-Obukhov Similarity theory - to compute vertical turbulent fluxes of momentum within the atmospheric surface layer (ASL). In this way, the aerodynamic roughness length and consequently the turbulent drag over land is assumed to be an intrinsic property of the surface, ignoring characteristics of the overlying flow. Although recognized to be invalid near heterogeneous surfaces, to date, no suitable alternatives have been developed for ASL parameterization near coastal environments. In these regions, drastic spatial gradients in surface thermal and roughness properties drive cross-coastal flows, leading to phenomena that directly contradict bulk-flux assumptions. Here, we define a flow-dependent, local, inland coastal aerodynamic roughness length Z_0c for onshore flow conditions. Analysis of observations collected from a cross-shore array of inland flux towers anchored at the Monterey Bay, CA coastline from June to October 2021 during the Coastal Land-Air-Sea Interaction (CLASI) campaign reveals significant departures in Z_0c from the expected homogeneous values for increasing wind speeds and inland fetches within 8 kilometers of the coast. These findings inform development of a physical framework describing a non-dimensional Z_0c as a function of the residence time of the inland flow, a reference height, and a representative homogeneous roughness length. We explore these relationships using large-eddy simulations of a coastal onshore flow scenario to achieve general understanding of the spatial variability in Z_0c . Finally, we present a baseline empirical relationship for Z_0c based on the CLASI dataset under near-neutral, onshore flow conditions.

Abstract Saharan air layer (SAL) events transport warm, dry, and dusty air from Africa across the tropical Atlantic, but how they influence the marine atmospheric boundary layer (MABL) is not well understood. Using unique observations from the August 2023 Moisture and Aerosol Gradients/Physics of Inversion Evolution (MAGPIE) field campaign on Barbados, we test the hypothesis that the SAL’s impact on the MABL is indirect, driven by large-scale dynamic adjustments. The SAL’s thermodynamic properties enhance large-scale subsidence, strengthening the trade-wind inversion. This enhanced inversion then acts as a “synoptic-scale governor,” controlling MABL depth and temperature from above. We verify this mechanism using a high-resolution mesoscale model and its adjoint, which provides quantitative evidence that MABL depth is most sensitive to the temperature of the SAL air mass far upstream, not local mixing. While global models capture the broad environmental changes, their coarse vertical resolution fails to resolve the fine-scale inversion structure. Our findings demonstrate that the evolution of the MABL under SAL conditions is influenced by a competition between the thermodynamic suppression from the warm, dry air and the dynamic deepening from shear-driven turbulence of the midlevel jet. This highlights a key process for improving tropical weather and climate models. Significance Statement The Saharan air layer (SAL) is a large-scale plume of dry, dusty air that extends from Africa into the Caribbean Sea that alters the temperature, humidity, cloud cover, and wind in this area, as well as negatively impacting public health. In this study, we examine the environment when the SAL is present and when it is absent in observations and models. We find that the SAL drastically changes the environment and modifies the near-surface conditions.

Saharan dust is frequently transported across the Atlantic, yet the chemical, physical, and morphological transformations dust undergoes within the marine atmospheric boundary layer (MABL) remain poorly understood. These transformations are critical for understanding dust's radiative and geochemical impacts, it's representation in atmospheric models, and detection via remote sensing. Here, we present coordinated observations from the Office of Naval Research's Moisture and Aerosol Gradients/Physics of Inversion Evolution (MAGPIE) August 2023 campaign at Ragged Point, Barbados. These include vertically resolved single-particle analyses, mass concentrations of dust and sea spray, and High Spectral Resolution Lidar (HSRL) retrievals. Single-particle data show that dust within the Saharan Air Layer (SAL) remains externally mixed, with a corresponding high HSRL-derived linear depolarization ratio (LDR) at 532 nm of similar to 0.3. However, at lower altitudes, dust becomes internally mixed with sea spray, and under the high humidity ( > 80 %) of the MABL undergoes hygroscopic growth, yielding more spherical particles, suppressing the LDR to < 0.1 ; even in the presence of high dust loadings (e.g., similar to 120 g m(-3)). This low depolarization in the MABL is likely due to a combination of the differences between the single scattering properties of dust and spherical particles, and the potential modification of the dust optical properties from an increased hygroscopicity of dust caused by the mixing with sea salt in the humid MABL. These results highlight the importance of the aerosol particle mixing state when interpreting LDR-derived dust retrievals and estimating surface dust concentrations in satellite products and atmospheric models.

We present a recent study to implement and test schemes for diagnostic calculations of skin sea surface temperature in the Navy’s Coastal Ocean Model (NCOM). This includes three schemes for estimating the cool anomaly in the viscous sublayer (i.e., the ocean skin), and a fourth scheme that adds an estimate of a warm anomaly in the solar radiation-driven, thermally stratified diurnal layer at near-surface depths. Applications of these schemes are made, and their performances are evaluated against field measurements from the Coupled Air-Sea Processes and Electromagnetic Ducting Research East campaign (CASPER-East), showing overall good agreements. The statistics of the model-observation comparisons are similar and do not indicate any systematic bias towards any scheme, but differences in the model performances are noticeable and vary depending on the surface wind and solar conditions. To understand the discrepancies among the schemes, inter-model comparisons are analyzed based on the conditions of surface wind stress and solar radiation flux. The issues associated with making the warm-layer correction are discussed, in particular, including the sensitivity of the diagnostic warm-layer anomaly to the layer thickness specified a priori, and the risk of double-counting the effect of solar radiation penetration when using the high-resolution NCOM temperature fields.

The differences in propagation losses between horizontally and vertically polarized radio frequency waves were measured using a large vertical antenna array during an overwater collection. These are compared with theoretical results and to wave buoy-collected sea state data from the same time period. Vertically polarized RF signals experienced greater propagation loss than horizontally polarized signals, especially during calm sea states. The deployment of a polarimetrically diverse transmitter and receiver will allow for retrieval of both atmospherics and sea-surface characteristics over a large, heterogeneous area.

Abstract Breaking waves are the dominant pathway by which energy is dissipated at the ocean surface, yet their definition, dynamics, and energetics remain subjects of ongoing debate. Here, we present results from a six-week field campaign during the Coastal Land–Air–Sea Interaction project, where wave breaking was detected across scales using an array of Air–Sea Interaction Spar buoys. Application of the wavelet-based Liberzon phase-discontinuity method identified more than 3.2 million wave crests and over 269,000 breaking events from high-frequency wave-wire measurements. From these elevation records, we developed an observation-constrained, crest-resolved parameterized framework for estimating breaking-induced energy dissipation. We introduce a breaking-strength parameterization consistent with the inertial scaling in terms of locally measured nonlinear steepness and consistent with inertial scaling after application of a constant empirical correction. Breaking fraction and associated dissipation depend strongly on wind forcing, wave age, and effective fetch, deviating from traditional whitecap-coverage power-law formulations. During active wind–wave coupling, breaking accounts for approximately 20–50% of the local atmospheric reference energy flux, underscoring its dynamical significance. Together, these results provide a physically grounded, observation-constrained, and crest-resolved picture of breaking-wave energetics from small-scale spilling events to larger, energetic whitecaps—and offer new constraints for improving dissipation schemes in spectral wave models under developing sea conditions.

Electromagnetic ducting in the atmospheric boundary layer (ABL) can markedly alter radio-wave propagation, reshaping link budgets and radar performance. We present an observational study of duct variability over the Salton Sea during the REDSAW campaign. The key measurement was made by an airborne Compact Raman Lidar (CRL) profiling water vapor mixing ratio and temperature at high vertical resolution. In the research, we complemented the CRL water vapor measurements by aircraft in situ thermodynamic measurements to derive the modified-refractivity (M) profiles and identify trapping layers. The CRL approach follows established airborne Raman-lidar methodology for daytime boundary-layer humidity/aerosol profiling and is adapted here to resolve the water vapor vertical gradient essential to forming evaporation and elevated ducts (B. Liu, Z. Wang, Y. Cai, P. Wechsler, W. Kuestner, M. Burkhart, and W. Welch, “Compact airborne Raman lidar for profiling aerosol, water vapor and clouds,” Optics Express, vol. 22, pp. 20613–20621, 2014, doi:10.1364/OE.22.020613). Our deployment and analysis align with REDSAW’s broader, multi-sensor design emphasizing stability classification and range-dependent propagation assessment.

This article explores the capabilities of phase-coherent electromagnetic (EM) data in estimating the lower atmospheric refractivity, specifically the evaporation duct. An evaporation duct is the most common refractive structure over the ocean, and it significantly affects the performance of marine radar and communication systems. Refractivity retrieval methods that rely on the EM signal amplitude have been extensively investigated in the literature. Here, we explore the performance improvement in duct estimation when both EM amplitude and phase (AP) measured across a vertical array are used instead of just the amplitude. This is done by computing and comparing the information content in AP using the Fisher information matrix. The evaporation ducts are retrieved using only the amplitude, only the phase, and both AP using both simulated and experimental data. To achieve this, an eight-element phase-coherent, software-defined radio-based vertical array is designed and built. This system is then deployed as part of multiple coastal land-air-sea interaction (CLASI) campaigns, collecting point-to-point phase-coherent propagation data across the vertical array, simultaneously with oceanographic and meteorological sensors measuring the evaporation duct. Phase-coherent inversion results are first compared in simulation to the classical amplitude-only inversion for varying frequency, signal-to-noise ratio (SNR), transmitter-receiver distance, number of array elements, overall array length, and ducting conditions. Then, these results were verified using the measured array data from two measurement campaigns. The results showed that there is overall less information in phase relative to the amplitude. However, the phase-coherent refractivity inversion algorithm still outperforms the amplitude-only one by up to 33%, depending on the system setup and the ducting conditions.

Existing theories explain that fluid interactions with isolated island topography generate leeward turbulent wakes through internal processes, such as hydraulic dissipation or baroclinic tilting. Realistic island wake flows and resulting marine atmospheric boundary layer (MABL) structures subjected to diurnal changes in surface heat fluxes are not well understood, especially over islands with low topographic peaks (<1 km). Here, we elucidate diurnal characteristics of the MABL downstream of Barbados (maximum elevation = 340 m) during summertime conditions coinciding with the Moisture and Aerosol Gradients/Physics of Inversion Evolution (MAGPIE) campaign. We synthesize composited coupled ocean-atmosphere, mesoscale numerical weather prediction model solutions over 18 days in August with remote estimates of the 10-m winds obtained from synthetic aperture radar, revealing contrasting spatial and temporal characteristics of the downstream wake MABL between night and day. Overnight, windward surface drag and leeward boundary layer separation result in a laminar, shallow wake. Enhanced low-level stability reduces lateral mixing between the wake and environment, permitting retention of long, laminar wakes downstream. During the daytime, standing mountain waves weaken in favor of thermally induced leeward surface convergence, fueling a deep, buoyant MABL immediately offshore. Finally, we contrast the evolution of the simulated flow between 2 days which differ by the strength of the upstream wind forcing, thermal stratification, and surface heating over the island. We find that nuanced differences in these upstream parameters impact the downstream extent and diurnal transition of the wake, providing a link between the local MABL and regional environment.

The atmospheric surface layer over the ocean has a strong impact on electromagnetic (EM) wave propagation in the lowest 100 m of the atmosphere. Due to surface evaporation, a strong vertical gradient in water vapor forms in this layer, resulting in an evaporation duct which may greatly increase detection ranges for sensors and targets within the duct. When a warm air mass moves over cooler water, the surface layer forms a stable thermal stratification whose ducting characteristics are much less understood compared to its counterpart, the unstable surface layer. In this study, we perform a sensitivity study to examine the characteristics of the stable surface layer profiles and how their impacts on EM propagation differ from those in the unstable cases. Using buoy-based measurements from both coasts of the United States as input to a surface layer model, refractivity profiles were generated as input to a propagation model to characterize path loss. The results suggest that the stable surface layers present more complex propagation scenarios with a broad range of propagation loss including the maximum propagation loss in the subrefractive conditions and maximum trapping in cases of deep evaporation duct heights. The results also suggest strong dependance of propagation loss on evaporation duct height when the transmitter height is above the duct. In contrast, propagation loss is no longer sensitive to evaporation duct height once the transmitter is within the duct. This research also reveals the role of 2-m curvature of the M-profile in defining the propagation regimes.

Recent military and civilian applications using Electromagnetic wave (EM) and Electro-Optic (EO) signals have underscored the importance of understanding the refractive environment in the atmosphere, particularly in the atmospheric surface layer. EM/EO propagation is greatly affected by thermodynamics and turbulence in the atmosphere, which is directly related to the wind and air-sea temperature differences. Accurate quantification of sea surface temperature (SST) is hence critical for EM/EO propagation studies. This research uses data from the Radar and Electromagnetic Ducting in a Stable Atmosphere over Water (REDSAW) campaign in the Salton Sea in May 2024 to quantify the spatial and temporal variability of SST.

A low-cost boundary-layer profiling system was developed to transmit GPS, temperature, humidity, and pressure at 10 Hz during descent. The novel miniDropsondes were deployed from a Twin Otter aircraft during the REDSAW field campaign over the Salton Sea, where over 150 drops achieved $\sim 4 \mathrm{~m} \mathrm{~s}^{-1}$ descent rates, surface data return, and 30 km reception ranges, demonstrating reliable performance for atmospheric sampling.

Marine fog is a multiscale phenomenon, where the largest to smallest length-scale ratio is of the order of 1013. Fog formation and evolution depend not only on large-scale (synoptic and mesoscale) weather systems but also on the intricate interactions and dynamics of smaller-scale processes, such as micrometeorological and microphysical factors, along with aerosol behavior. Large-eddy simulation (LES) effectively captures small-scale processes such as turbulence, microphysics, and radiation, but large-scale dynamics (LSD) are not considered or are poorly defined. To address this limitation, we modify the LES governing equations by adding specific terms to account for the effects of LSD. Additionally, we employ the Lagrangian cloud model (LCM) approach to analyze microphysical processeswithin fog. This combined method (LES, LCM, LSD), called L3 coupling, is used to simulatemarine fog observed during the Fog and Turbulence Interactions in the Marine Atmosphere (FATIMA) Multidisciplinary University Research Initiative (MURI) campaign. The simulations focused on two specific fog episodes observed on July 12 and 13, 2022. The simulation results for the liquid water content, mean volume diameter, droplet number concentration, and relative humidity were compared rigorously with measurements. The comparison highlighted the model's ability to capture the temporal and spatial characteristics of fog microphysics and dynamics, although some discrepancies in the onset of fog events were noted. The results demonstrate the utility of the L3 coupling method for improving the spatiotemporal representation of fog dynamics. This study reaffirms the critical role of mesoscale and microscale processes in the life cycle of fog, highlighting the importance of L3 coupling during the life cycle of fog. By integrating these scales effectively, the model is capable of simulating fog with realistic microphysical and dynamical properties and hence provides an effective approach for better understanding of fog lifecycle development.

The fair‐weather (wind speeds 10 m/s), surface ( 4 m), nearshore wind field modification is examined with multiple cross‐shore arrays spanning from the coastline to 40 km offshore, deployed within four‐month‐long field experiments, measuring winds and air‐water temperatures. The over‐water to coastline winds ratio, , was previously explored for offshore winds with limited observations and minimally for onshore and alongshore winds. Array observations provide a complete picture of the nearshore wind field for all wind orientations. The over‐water wind and temperature are linearly related to coastline values near the coastline, with decreasing with distance from shore, , with a decorrelation scale of 10 km. Mean as a function of differs per wind orientation, consistent with prior work. An model is developed from exponential Gaussian Process Regression (GPR), which accurately predicts the wind field with 20% data set holdback to elucidate the cross‐shore patterns and variable co‐dependence. The modeled Partial Dependence Plots provide dependency as a function of on coastline winds, and temperature differences without preconceptions. A consistent nearshore wind slowing occurs that varies in amplitude and distance, and changes with variable co‐dependence for wind orientation. The onshore wind slowing is counterintuitive, though consistent with sophisticated numerical models. Wind gustiness exhibits dependence, with linear normalization akin to the open ocean but larger. The observations and GPR highlight nearshore winds' cross‐shore extent and complexity, which are important for atmospheric and oceanic studies.