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The article of record as published may be found at https://doi.org/10.1029/2020JD032834 The constant flux layer assumption simplifies the problem of atmospheric surface layer (ASL) dynamics and is an underlying assumption of Monin‐Obukhov Similarity Theory, which is ubiquitously applied to model interfacial exchange and atmospheric turbulence. Within the marine environment, the measurements necessary to confirm the local ASL as a constant flux layer are rarely available, namely: direct observations of the near‐surface flux gradients. Recently, the Research Platform FLIP was deployed with a meteorological mast that resolved the momentum and heat flux gradients from 3 to 16 m above the ocean surface. Here, we present findings of a study assessing the prevalence of the constant flux layer within the ASL, using an approach that accounts for wave‐coherent turbulence, defines the wave boundary layer height, and empirically quantifies the observed flux divergence. Our analysis revealed that only 30‐40% of momentum flux gradients were approximately constant; for the heat fluxes, this increased to 50‐60%. The stationarity of local turbulence was critical to the constant flux layer's validity, but resulted in excising a large proportion of the observed profiles. Swell‐wind alignment was associated with momentum flux profile divergence under moderate wind speeds. In conjunction, our findings suggest that the constant flux layer, as it is conventionally defined, is not generally valid within the marine ASL. This holds significant implications for measuring air‐sea fluxes from single point sources and the application of Monin‐Obukhov similarity theory over the ocean. Office of Naval Research (ONR) This research was funded by the Office of Naval Research (ONR) Grant N0001418WX01087 under its Multidisciplinary University Research Initiative (MURI).

Near surface turbulent momentum flux estimates are performed over the Aegean Sea, using two different approaches regarding the drag coefficient formulation, a wave boundary layer model (referred here as KCM) and the most commonly used Coupled Ocean-Atmosphere Response Experiment (COARE) algorithm. The KCM model incorporates modifications in the energy-containing wave spectrum to account for the wave conditions of the Aegean Sea, and surface similarity to account for the stratification effects. Airborne turbulence data during an Etesian outbreak over Aegean Sea, Greece are processed to evaluate the simulations. KCM estimates found up to 10% higher than COARE ones, indicating that the wave-induced momentum flux may be insufficiently parameterized in COARE. Turbulent fluxes measured at about 150 m, and reduced to their surface values accounting for the vertical flux divergence, are consistently lower than the estimates. Under unstable atmospheric stratification and low to moderate wind conditions, the residuals between estimates and measurements are less than 40%. On the other hand, under stable stratification and strong winds, the majority of the residuals are more than 40%. This discrepancy is associated with the relatively high measurement level, shallow boundary layer, and the presence of a low level jet.

Approved for public release; distribution is unlimited Measuring atmospheric fluxes requires various steps of measurement quality control, in addition to experimental design and post-processing corrections, in order to provide robust and high-fidelity data for wider use. However, within the measurement community, methods for these control steps are still applied ad hoc. Regardless of the availability of several comprehensive references texts available in the literature and licensed software programs.The theoretical and technical design of an algorithm for eddy covariance flux sample quality control and assessment is presented. This algorithm, WISQ, is robust and efficient and can be readily incorporated into existing processing experimental software packages. The goal of this algorithm is to output a flagging system that can be used to judge the quality of individual flux samples, with the option for outputting more detailed information. WISQ is unique in that it directly and automatically assess the sample flux accumulation and convergence. WISQ is also a general method that can be utilized for flux measurement outside of the realm of meteorology and is open-sourced for ease in development and innovation.

Ocean Sciences Meeting 2020 Nonlinear internal ocean waves (NIWs) are regular features of the coastal ocean, where the hydrodynamic flow over changing bathymetry perturbs the isopycnal surfaces generating these high frequency waves. At the air-sea interface, these transient features may be characterized by quasilinear bands of smooth or rough ocean surface that propagate in the direction of the underlying NIWs. Theoretically, this roughness heterogeneity is driven by the phase-locked divergence and convergence of the NIW orbital motions. This NIW action modulates surface wavelengths within the capillary and gravity-capillary band, which also hold the majority of the tangential wind stress. Understanding the spatial-temporal distribution of these small-scale surface waves is critical to constraining air-sea coupling, which is significantly complicated in the case of a heterogeneous surface. The impact NIW-driven surface roughness has on the variability and structure of the atmospheric surface layer is unknown. During a Coupled Air Sea Processes and EM ducting Research (CASPER) field campaign, the Research Platform FLIP was deployed for five weeks in a coastal area with a suite of near-surface oceanographic and meteorological measurements, as well as near-field remote sensing of the surface using both radar, infrared, and optical visualization. This confluence of measurement capability from an ideal platform, enabled us to simultaneously identify and track NIWs while characterizing the variance and structure of the kinematic and thermodynamic state on either side of the interface. NIWs were regularly observed from FLIP, with their characteristic surface banding observed nearly every day of the campaign. Our analysis into one case revealed that NIWs exert a distinct and significant impact on the mean wind gradient, as well as the air-sea momentum flux (i.e. wind stress) on both the scale of individual wave fronts and an entire NIW packet. In particular, the MASL flow adjusts instantaneously to the smooth-rough transitions of individual bands, thereby enhancing the wind stress over the surface. Our presentation will focus on summarizing these findings, as well as highlighting additional NIW events observed during the CASPER campaign from FLIP to discern any underlying or general pattern in the nature of NIW-atmosphere interactions.

100th American Meteorological Society Annual Meeting. AMS, 2020. Monin-Obukhov Similarity Theory (MOST) forms the backbone for many studies of the atmospheric surface layer (ASL), whether over land or ocean, and is critical to predicting the electromagnetic (EM) propagation in the marine environment. Fundamentally, MOST extends the Richardson-Prandtl flux-gradient relationship to the general case of non-neutral conditions using dimensionless, empirical functions for momentum, temperature, moisture, and turbulent energy dissipation. These empirical functions provide a means to represent surface fluxes from mean quantities such as those from gridded numerical simulations. A critical component to the underlying basis for MOST, and the flux-gradient relationship, is the constant flux layer assumption. In the neutral conditions, the absence of stress divergence implies that only one relevant turbulent velocity scale is needed to close the surface flux problem (e.g., the friction velocity ). However, in the marine environment, flux profile measurements are rarely collected and the overwhelming majority of data sets cannot confirm the presence of this critical assumption over typical averaging windows. Using a complete (momentum and total heat), high-resolution flux profile (ranging 2-16 m above the surface) collected during the CASPER-West field campaign, we have conducted a study that systematically evaluates the prevalence of the constant flux layer model over the marine ASL (MASL). These measurements were taken from the Research Platform (R/P) FLIP, which is an ideal ocean-going platform for making near-surface measurements free from the contaminations endemic to typical ship-based measurements. We utilize a novel approach to empirically test each observed profile of momentum, sensible, and latent heat flux against a sufficiently constant profile. This enabled us to analyze the dependence of whether or not an individual profile was “constant” against the mean environmental state, e.g., wind speed, stability, and air-sea temperature difference. For the momentum flux, we found that only 33% of profiles could be considered non-divergent, which drops to 10% if only considering the profiles below 6 m. If only considering statistically stationary profiles (~20% of the total dataset), we found that 43% of profiles were sufficient constant stress. Similar findings were observed for sensible heat flux, with a dramatic increase in the prevalence of non-divergence for the latent heat flux. An analysis into the environmental dependence of these general results will be presented. These results question the generally held assumption that the MASL is typically a constant stress layer; this holds significant implications for how surface fluxes are parameterized and/or derived over the ocean, as well as the widespread reliance on MOST to accurately describe the vertical structure of the MASL.

2019 IEEE/OES Twelfth Current, Waves and Turbulence Measurement (CWTM) The article of record as published may be found at https://doi.org/10.1109/CWTM43797.2019.8955282 In the coastal environment, the oceanic flow over varying bathymetry can displace the isopycnal surfaces and, thus, generate nonlinear internal waves. These high frequency waves can propagate across large distances and over their lifetime significantly influence local currents and turbulence within a coastal region. These waves also create a common phenomenon that is recognized by even a casual observer: smooth, quasilinear bands of water that disrupt the typically rippled sea surface. While NIWs are an important oceanic process and their surface expression has been characterized and discussed for decades, investigators have not linked the presence of internal wave-driven surface roughness to an atmospheric response. Here we use a combination of oceanic and atmospheric measurements, as well as ocean surface visualization, to show that NIWs can alter the flow within the MASL and the subsequent momentum flux across the air-sea interface, at the dominant temporal-spatial scales of the NIWs. Our measurements were collected from the FLIP, which was deployed as part of the Coupled Air Sea Processes and Electromagnetic ducting Research (CASPER) West Coast field campaign. Using a thermistor chain, X band marine radar, upward- and downward-looking ADCP, as well as a visual field camera imaging the ocean surface near FLIP, we were able to identify several NIW events and track individual waves incident to the platform. This information was used to isolate the atmospheric response, as captured by a profile of meteorological flux sensors installed on a mast that was deployed from FLIP's boom. The observed NIW-interactions were found in multiple cases with different MASL conditions and internal wave properties. In the context of CASPER, the surface roughness associated with NIWs represents a persistent, quasi-Lagrangian heterogeneity that may impact the atmospheric gradients, which in turn modulates the index of refraction and the propagation of electromagnetic radiation. Office of Naval Research Funding provided by Office of Naval Research N0001418WX01087.

AGU Fall Meeting, 2019 Kolmogorov’s hypothesis for the presence of an inertial subrange exhibiting a power law scaling of -5/3 is one of the most widely recognized concepts in the fluid dynamics and remains an integral component of our general understanding of the turbulent cascade from energy-containing eddies to dissipation scales. While his theories have been debated since their proposal, its practical application to an observed turbulent field remains hampered by the lack of a systematic approach for identifying the bandwidth of this critical subrange. Within the atmospheric boundary layer literature, decades of studied has not yielded a standardized approach and the various methods reported appear ad hoc. This creates a significant hurdle to the applications of Kolmogorov’s theory to atmospheric boundary layer turbulence and the inter-comparison between disparate data sets. As part of a recent major field campaign conducted from the unique ocean- going platform the R/P FLIP, we developed the algorithm for robust identification of the inertial subrange (ARIIS) to study the inertial subrange characteristics within the atmospheric surface layer. ARIIS is a novel approach that explicitly uses the isotropic relationship between transverse velocity components to identify the onset and extent of the inertial subrange in the variance spectrum. After identifying the most plausible subrange bandwidth, ARIIS uses a robust, iterative fitting algorithm to derive an empirical estimate of the spectral slope, which can be compared to Kolmogorov’s expected -5/3 value. This presentation will discuss the implementation of ARIIS and describe the results of using this new method to conduct a systematic evaluation of Kolmogorov’s inertial subrange theory within the marine atmospheric surface layer using the FLIP data. Our findings provide compelling evidence for conditions where the inertial subrange slope diverges from -5/3, indicating that some other process(es), e.g. surface gravity waves, may play an integral role in the turbulence cascade near the air-sea interface.

Office of Naval Research v1.1: Additional text was added describing a review of some previous field campaigns conducted on the R/P FLIP, with focus placed on those investigators’ discussions of FLIP’s motion to various deployment schemes. This includes a new section (II.A.1) and an additional table (Table 1). These changes do not impact the results or analysis presented in v1. This document details the data processing, quality control and assessment analysis conducted on the instrumentation systems deployed by the Naval Postgraduate School Boundary Layer Processes research group as part of the CASPER-West field campaign. Approved for public release; distribution is unlimited.

99th American Meteorological Society Annual Meeting The bulk of our knowledge of air-sea exchange coefficients for momentum and heat derives from single-point measurements made at some height within the marine atmospheric surface layer (MASL). These point measurements rely on assumptions regarding the vertical structure of the MASL. Foremost among these assumptions, is the validity of Monin- Obukhov Similarity Theory (MOST), which postulates that the gradient-flux relationship is a universal function of surface layer stability. Under neutral conditions, this simplifies to the familiar logarithmic profile. While MOST has been validated over land, observations of the actual gradients within the MASL remain scarce, in part due to the challenges of making near-surface profile measurements over the ocean. The Research Platform FLIP was recently deployed on the west coast for the Coupled Air-Sea Processes and Electromagnetic ducting Research field campaign (CASPER-West), a large-scale air-sea interaction study that took place offshore of Pt. Mugu, CA. FLIP remains an ideal platform for making measurements in proximity to the air-sea interface, with minimal contamination from the platform. During CASPER, a meteorological mast was installed on FLIP that resolved both the bulk and flux profiles of momentum and heat, from 3 to 16 m above the surface. This mast included 7 flux levels, 10 mean wind measurements, and over 20 temperature and humidity probes. This presentation will focus on the vertical gradients measured from FLIP’s mast, with the specific aim of using these high-resolution measurements to test the variability predicted by MOST. As a preliminary step, linear regression was used to determine the natural prevalence of the logarithmic profile. For the mean wind profiles, only 10.2% of the profiles were strongly logarithmic (r2 > 0.9). For specific humidity, this increased to 40.9% of profiles, with no temperature profiles exhibiting a strong logarithmic relationship. Mean r2 was 0.624, 0.265, and 0.853 for wind, temperature, and specific humidity respectively, which increased to 0.761, 0.362, and 0.950 for wind speeds > 6 ms-1 (12.3% of the total data set). Wind speed exhibited positive, and temperature demonstrated negative, relationships with bulk air-sea temperature difference; for example, in stable conditions the mean r2 increased to 0.783 for wind speed, and decreased to 0.145 for temperature. Further analysis will focus on comparing strongly-logarithmic profiles to the empirical gradient-flux relationships available in the literature as well as, determining environmental factors driving the majority of profiles away from the expected logarithmic behavior. This unique dataset provides an opportunity to directly evaluate the prevalence and validity of the MASL vertical structure predicted by MOST, which is assumed to be generally valid over the ocean.

AGU Fall Meeting 2018 Nonlinear internal waves are readily observable in coastal waters, where flow over changing bathymetry may displace the isopycnal surfaces. These internal waves (IWs) may travel long distances, enhancing currents and turbulence within the upper ocean mixed layer. At the surface, IW fronts express as alternating smooth/rough water, creating transient heterogeneity that may disrupt the ambient gradients within the marine atmospheric surface layer (MASL) and hence the properties of the evaporation duct (ED). While the oceanic properties of IWs has been studied, their potential influence on the MASL remains largely unknown. Recently, the R/P FLIP was deployed for the Coupled Air-Sea Processes and Electromagnetic ducting Research (CASPER) program, an air-sea interaction study that took place offshore of Pt. Mugu, CA. As part of this effort, a meteorological mast with overlapping bulk and flux profiles for momentum, temperature, and water vapor was installed on FLIP, sampling the air column from 3 to 16 m above the surface. Using these data as well as high-resolution imagery of IW fronts propagating past FLIP, a study was conducted to determine the impact IWs have on MASL variability. The imagery of the IW bands was critical to enable tracking these features and directly relating measured variability to the presence of IWs. Preliminary findings suggest that the surface roughness variability associated with these IW fronts substantially affects the air-sea momentum flux. Strong flux divergence was found at the leading and trailing edges of each individual band, in some cases this divergence caused a sign reversal (i.e. upward momentum flux). This variability appeared to be trapped with individual bands, suggesting an impact on the MASL that may be distributed across the entire propagation range of a single IW front. Early analysis has not found a clear link between IW fronts and heat flux variability or divergence. However, these bands are visible at low winds and the observation of enhanced turbulence at their edges may impact the vertical scalar gradients (i.e. temperature and water vapor) and ultimately the predictability of the ED height. The results from this investigation, combining the measurements from the mast with observations from a co-located X-Band Marine Radar and a 40-m long thermistor chain, will be presented.