On the marine atmospheric boundary layer characteristics over Bay of Bengal and Arabian Sea during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB)

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

Correction of Depth Bias in Upper-Ocean Temperature and Salinity Profiling Measurements from Airborne Expendable Probes

The article of record as published may be found at http://dx.doi.org/10.1175/JTECH-D-14-00114.1During the Dynamics of Madden–Julian Oscillation (DYNAMO) Experiment in 2011, airborne expendable conductivity–temperature–depth (AXCTD) probes and airborne expendable bathythermographs (AXBTs) were deployed using NOAA’s WP-3D Orion aircraft over the southern tropical Indian Ocean. From initial analysis of the AXCTD data, about 95% of profiles exhibit double mixed layer structures. The presence of a mixed layer from some of these profiles were erroneous and were introduced because of the AXCTD processing software not being able to correctly identify the starting point of the probe descent. This work reveals the impact of these errors in data processing and presents an objective method to remove such erroneous data from the profiles using spectrograms from raw audio files. Reconstructed AXCTD/AXBT profiles are compared with collocated shipborne conductivity–temperature–depth (CTD) and expendable bathythermograph (XBT) profiles and are found to be in good agreement.This work was supported by ONR Award N0001413WX20025 and partly by NSF Award AGS1062300. Denny P. Alappattu is sponsored by the National Research Council research associateship program. Discussions and input from Grant Johnson and Peter Black were very helpful. The hard work of Lt. David Tramp, LCDR Heather Hornick Quilenderino, and LCDR Robin Corey Cherrett in data collection and processing are greatly appreciated. Dr. James Moum of Oregon State University provided the CTD cast data from R/V Revelle

Variability of upper ocean thermohaline structure during a MJO event from DYNAMO aircraft observations

AXCTD and AXBT data used for this study are available at http://data.eol.ucar.edu/ master_list/?project5DYNAMO.The article of record as published may be found at http://dx.doi.org/10.1002/2016JC012137This 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 (148S–Eq, 708E–818E). 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.This work was supported by ONR award N0001413WX20025 and partly by NSF award AGS1062300. Denny P. Alappattu was sponsored by the National Research Council research associate ship program

Anomalous propagation conditions over eastern Pacific Ocean derived from MAGIC data

The article of record as published may be found at http://dx.doi.org/10.1002/2016RS005994This 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 1 year (2012–2013) of ship-based measurements made during the Marine Atmospheric Radiation Measurement GPCI (GEWEX cloud system study 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 Coupled Ocean-Atmosphere Response Experiment 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 and 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 and 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.Office of Naval ResearchGrant number N0001416WX0046

Observations of the Marine Atmospheric Surface Layer Gradients during the CASPER-West Field Experiment

99th American Meteorological Society Annual MeetingThe 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

Warm Layer and Cool Skin Corrections for Bulk Water Temperature Measurements for Air-Sea Interaction Studies

Data used for this study are available at https://workspace.axiomdatascience.com/group/282520/projects.The article of record as published may be found at http://dx.doi.org/10.1002/2017JC012688The sea surface temperature (SST) relevant to air-sea interaction studies is the temperature immediately adjacent to the air, referred to as skin SST. Generally, SST measurements from ships and buoys are taken at depths varies from several centimeters to five meters below the surface. These measurements, known as bulk SST, can differ from skin SST up to O(1°C). Shipboard bulk and skin SST measurements were made during the Coupled Air- Sea Processes and Electromagnetic ducting Research east coast field campaign (CASPER-East). An Infrared SST Autonomous Radiometer (ISAR) recorded skin SST, while R/V Sharp’s Surface Mapping System (SMS) provided bulk SST from one-meter water depth. Since the ISAR is sensitive to sea spray and rain, missing skin SST data occurred in these conditions. However, SMS measurement is less affected by adverse weather and provided continuous bulk SST measurements. It is desirable to correct the bulk SST to obtain a good representation of the skin SST, which is the objective of this research.Bulk-skin SST difference has been examined with respect to meteorological factors associated with cool skin and diurnal warm layers. Strong influences of wind speed, diurnal effects and net longwave radiation flux on temperature difference are noticed. A three-step scheme is established to correct for wind effect, diurnal variability and then for dependency on net longwave radiation flux. Scheme is tested and compared to existing correction schemes. This method is able to effectively compensate for multiple factors acting to modify bulk SST measurements over the range of conditions experienced during CASPER-East

A Study on Bulk and Skin Temperature Difference using Observations from Atlantic and Pacific Coastal Regions of United States

The article of record as published may be found at  http://dx.doi.org/10.1117/12.2263533Analysis of bulk-skin sea surface temperature (SST) difference form the west and east coasts of United States is presented using the data collected from three field experiments. These experiments were conducted at offshore Duck, North Carolina and in the Monterey Bay of the California coastal region. Bulk SST measurements were made using conventional thermistors from a depth of one meter below the sea level. Infrared radiometers were used to measure the surface skin SST. Depending on measurement depth and prevailing conditions, the bulk SST can differ from skin SST by few tenths of a degree to O(1°C). Difference between bulk and skin SST arise from cools skin and warm layer effects. Bulk-skin SST difference (ΔSST) estimated from east coast observations varied from -0.46°C to 1.24°C. Here, the bulk SST was higher than skin SST most of the time during the observations. This indicates cool skin effect was the dominant factor determining the ΔSST in the east coast. For wind speeds less than 4 m s-1, we also noticed an increase in ΔSST. Additionally, for low winds (< 4 m s-1) ΔSST also varied diurnally with the occurrence of generally higher ΔSST in the nighttime in comparison with daytime. Moreover, increase in downwelling longwave radiation reduced the bulk-skin SST difference. ΔSST calculated from the observation in the Monterey bay varied between ~2.3° and ~-2.3°C. This was higher than the variability ΔSST observed at the east coast. Moreover, ΔSST variability observed at west coast was independent of wind speed.CASPER is funded by the Office of Naval Research (ONR) under its Multidisciplinary University Research Initiative (MURI) program, grant N0001416WX00469, under project managers Dr. Daniel Eleuterio and Steve Russell

Ducting Conditions During CASPER-West Field Campaign

2018 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science MeetingThe article of record may be found at https://doi.org/10.1109/APUSNCURSINRSM.2018.8608750The 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

An Evaluation of Monin–Obukhov Similarity Theory within the Marine Atmospheric Surface Layer: The Prevalence of the Constant Stress Layer

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

Mean Offshore Refractive Conditions during the CASPER East Field Campaign

The article of record as published may be found at http://dx.doi.org/10.1175/JAMC-D-18-0029.1In 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) N0001418WX01087N0001417WX0208