Labrador Sea Deep Convection Experiment Research Articles

Mean and Variability of the WHOI Daily Latent and Sensible Heat Fluxes at In Situ Flux Measurement Sites in the Atlantic Ocean*

Daily latent and sensible heat fluxes for the Atlantic Ocean from 1988 to 1999 with 1° × 1° resolution have been recently developed at Woods Hole Oceanographic Institution (WHOI) by using a variational object analysis approach. The present study evaluated the degree of improvement made by the WHOI analysis using in situ buoy/ship measurements as verification data. The measurements were taken from the following field experiments: the five-buoy Subduction Experiment in the eastern subtropical North Atlantic, three coastal field programs in the western Atlantic, two winter cruises by R/V Knorr from the Labrador Sea Deep Convection Experiment, and the Pilot Research Moored Array in the Tropical Atlantic (PIRATA). The differences between the observed and the WHOI-analyzed fluxes and surface meteorological variables were quantified. Comparisons with the outputs from two numerical weather prediction (NWP) models were also conducted. The mean and daily variability of the latent and sensible heat fluxes from the WHOI analysis are an improvement over the NWP fluxes at all of the measurement sites. The improved flux representation is due to the use of not only a better flux algorithm but also the improved estimates for flux-related variables. The mean differences from the observations in latent heat flux and sensible heat flux, respectively, range from 2.9 (3% of the corresponding mean measurement value) and 1.0 W m−2 (13%) at the Subduction Experiment site, to 11.9 (13%) and 0.7 W m−2 (11%) across the PIRATA array, to 15.9 (20%) and 10.5 W m−2 (34%) at the coastal buoy sites, to 8.7 (7%) and 9.7 W m−2 (6%) along the Knorr cruise tracks. The study also suggests that further improvement in the accuracy of latent and sensible heat fluxes will depend on the availability of high-quality SST observations and improved representation/observations of air humidity in the tropical Atlantic.

The Impact of Waves on Surface Currents

Abstract Ocean models usually estimate surface currents without explicit modeling of the ocean waves. To consider the impact of waves on surface currents, here a wave model is used in a modified Ekman layer model, which is imbedded in a diagnostic ocean model. Thus wave effects, for example, Stokes drift and wave-breaking dissipation, are explicitly considered in conjunction with the Ekman current, mean currents, and wind-driven pressure gradient currents. It is shown that the wave effect on currents is largest in rapidly developing intense storms, when wave-modified currents can exceed the usual Ekman currents by as much as 40%. A large part of this increase in velocity can be attributed to the Stokes drift. Reductions in momentum transfer to the ocean due to wind input to waves and enhancements due to wave dissipation are each of the order 20%–30%. Model results are compared with measurements from the Labrador Sea Deep Convection Experiment of 1997.

The Labrador Sea Deep Convection Experiment data collection

Between 1996 and 1998, a concerted effort was made to study the deep open ocean convection in the Labrador Sea. Both in situ observations and numerical models were employed with close collaboration between the researchers in the fields of physical oceanography, boundary layer meteorology, and climate. A multitude of different methods were used to observe the state of ocean and atmosphere and determine the exchange between them over the experiment's period. The Labrador Sea Deep Convection Experiment data collection aims to assemble the observational data sets in order to facilitate the exchange and collaboration between the various projects and new projects for an overall synthesis. A common file format and a browsable inventory have been used so as to simplify the access to the data.

Eddies in the Labrador Sea as Observed by Profiling RAFOS Floats and Remote Sensing

Abstract Data from profiling RAFOS floats, TOPEX/Poseidon altimetry, and the alongtrack scanning radiometer (ATSR) aboard ERS-1 have been used to describe the spatial and seasonal patterns of eddy variability in the Labrador Sea. Peaks in sea surface height (SSH) variability appear in two regions: off the west Greenland shelf near 61.5°N, 52°W where the 3000-m isobath separates from the shelf, and in the center of the basin at 58°N, 52°W. Both locations show seasonal ranges in SSH variability of up to 40 mm, with the Greenland site, having largest variability in January–March, leading the central site by 50 days. A sea surface temperature image from the ATSR at the Greenland site shows numerous eddies, both cyclonic and anticyclonic, being formed by injection of West Greenland Current water into the Labrador Sea interior. Data from profiling RAFOS floats launched in 1997 as part of the Labrador Sea Deep Convection Experiment are used to describe three of the West Greenland Current eddies in detail. One of...

Sampling Characteristics from Isobaric Floats in a Convective Eddy Field*

During the recent Labrador Sea Deep Convection Experiment, numerous isobaric floats were deployed. Interpretation of the quasi-Lagrangian measurements from these floats requires an understanding of any biases that may be introduced by the response of the floats to the flow in which they are embedded. To investigate the float measurement biases in convecting flow numerical simulations of isobaric floats in a domain containing several mesoscale eddies have been performed. When a surface heat loss is applied, spatially variable convective mixing and baroclinic instability result. The authors find that without surface cooling, probability density functions of Eulerian and isobaric float measurements of tracers and velocities are very similar, given an initial distribution of isobaric floats that is random with respect to the initial features of the tracer field. However, with cooling isobaric statistics are biased compared to the Eulerian statistics. In particular, in near-surface regions isobaric floats appear to oversample regions of dense downwelling fluid. Since in near-surface layers downwelling dense fluid is associated with convergence, a probable explanation of the isobaric float biases is a tendency for floats to concentrate in regions of horizontal convergence. Also, with cooling floats may be more easily exchanged between eddies and the ambient fluid. The escape of a float from an eddy can be identified from changes in the values of material tracers. The authors identify a positive skewness in the time derivative of the buoyancy measured by the individual near-surface floats as a indicator of convection in the presence of mesoscale eddies.

A Comparison of Surface Layer and Surface Turbulent Flux Observations over the Labrador Sea with ECMWF Analyses and NCEP Reanalyses

Comparisons are made between a time series of meteorological surface layer observational data taken on board the R/V Knorr, and model analysis data from the European Centre for Medium-Range Weather Forecasting (ECMWF) and the National Centers for Environmental Prediction (NCEP). The observational data were gathered during a winter cruise of the R/V Knorr, from 6 February to 13 March 1997, as part of the Labrador Sea Deep Convection Experiment. The surface layer observations generally compare well with both model representations of the wintertime atmosphere. The biases that exist are mainly related to discrepancies in the sea surface temperature or the relative humidity of the analyses. The surface layer observations are used to generate bulk estimates of the surface momentum flux, and the surface sensible and latent heat fluxes. These are then compared with the model-generated turbulent surface fluxes. The ECMWF surface sensible and latent heat flux time series compare reasonably well, with overestimates of only 13% and 10%, respectively. In contrast, the NCEP model overestimates the bulk fluxes by 51% and 27%, respectively. The differences between the bulk estimates and those of the two models are due to different surface heat flux algorithms. It is shown that the roughness length formula used in the NCEP reanalysis project is inappropriate for moderate to high wind speeds. Its failings are acute for situations of large air–sea temperature difference and high wind speed, that is, for areas of high sensible heat fluxes such as the Labrador Sea, the Norwegian Sea, the Gulf Stream, and the Kuroshio. The new operational NCEP bulk algorithm is found to be more appropriate for such areas. It is concluded that surface turbulent flux fields from the ECMWF are within the bounds of observational uncertainty and therefore suitable for driving ocean models. This is in contrast to the surface flux fields from the NCEP reanalysis project, where the application of a more suitable algorithm to the model surface-layer meteorological data is recommended

Mechanisms of Buoyancy Transport through Mixed Layers and Statistical Signatures from Isobaric Floats

Idealized nonhydrostatic numerical calculations that resolve both plumes and geostrophic eddies are used to mimic isobaric float observations taken in the Labrador Sea Deep Convection Experiment and study mechanisms of buoyancy transport through mixed layers. The plumes and eddies are generated in a periodic channel, initialized with a vertical profile of temperature, and cooled by surface heat loss varying across the channel. Probability density functions and time series of vertical velocity and temperature computed from the floats are interpreted in terms of the kinematics of plumes and geostrophic eddies, and their role in buoyancy transport. Estimates from Eulerian time series from the numerical model suggest that geostrophic eddies and plumes have a comparable contribution to vertical heat flux.

Marine wind analysis from remotely sensed measurements

A primary objective of this study is to demonstrate a methodology that can potentially be used to generate high spatial resolution gridded vector wind fields for the northwest Atlantic on synoptic time scales. This task is accomplished using optimal interpolation (OI) to blend satellite wind data from European remote sensing (ERS-2) satellite scatterometers, National Aeronautics and Space Administration (NASA) scatterometers (NSCAT), and TOPEX/Poseidon and ERS-2 altimeters with numerical weather prediction (NWP) model wind estimates. The NWP model wind fields are produced using the MC2 atmospheric model (Canadian mesoscale compressible community atmospheric model). The grid for these fields is 0.25°. OI correlation functions and variances are derived from remotely sensed data. A second objective of the study is to provide a methodology for validation of synthetic aperture radar (SAR) derived vector winds from RADARSAT-1 images collected during the 1997 Labrador Sea Deep Convection Experiment (LSDCE). These winds are derived from the observed radar cross section and a scatterometer wind retrieval model. The validation of the SAR-derived winds is based on the OI winds and in situ ship-based winds.

A regional climate model coupled to ocean waves: Synoptic to multimonthly simulations

The NCAR regional climate model RegCM is coupled to the WAM ocean model, using the sea‐state‐dependent roughness parameterization derived in the HEXOS experiment. Coupled model simulations are shown to give reduced wind speeds U10 compared to uncoupled simulations. This is in accord with other recent studies. However, the wave‐atmosphere coupling is effected through the friction velocity field u*, rather than through the wind field U10. Thus because the coupling gives enhanced resistive friction, U10 is weakened, whereas u* is enhanced. Wave heights, driven by u*, are also increased in our coupled model simulations compared to uncoupled model simulations. Coupled model outputs are shown to compare favorably with air‐sea observations collected during the recent Labrador Sea Deep Convection Experiment in 1997, both for synoptic storm timescales and for seasonal timescales.

High-resolution passive polarimetric microwave mapping of ocean surface wind vector fields

The retrieval of ocean surface wind fields in both one and two dimensions is demonstrated using passive polarimetric microwave imagery obtained from a conical-scanning airborne polarimeter. The retrieval method is based on an empirical geophysical model function (GMF) for ocean surface thermal emission and an adaptive maximum likelihood (ML) wind vector estimator. Data for the GMF were obtained using the polarimetric scanning radiometer/digital (PSR/D) on the NASA P-3 aircraft during the Labrador Sea Deep Convection Experiment in 1997. To develop the GMF, a number of buoy overflights and GPS dropsondes were used, out of which a GMF of 10.7, 18.7, and 37.0 GHz azimuthal harmonics for the first three Stokes parameters was constructed for the SSM/I incident angle of 53.1/spl deg/. The data show repeatable azimuthal harmonic coefficient amplitudes of /spl sim/2-3 K peak-to-peak, with a 100% increase in harmonic amplitudes as the frequency is increased from 10.7 to 37 GHz. The GMF is consistent with and extends the results of two independent studies of SSM/I data and also provides a model for the third Stokes parameter over wind speeds up to 20 m/s. The aircraft data show that the polarimetric channels are much less susceptible to geophysical noise associated with maritime convection than the first two Stokes parameters. The polarimetric measurement technique used in the PSR/D also demonstrates the viability of digital correlation radiometry for aircraft or satellite measurements of the full Stokes vector. The ML retrieval algorithm incorporates the additional information on wind direction available from multiple looks and polarimetric channels in a straightforward manner and accommodates the reduced SNRs of the first two Stokes parameters in the presence of convection by weighting these channels by their inverse SNR.

An Extreme Cold-Air Outbreak over the Labrador Sea: Roll Vortices and Air–Sea Interaction

Abstract Observational data from two research aircraft flights are presented. The flights were planned to investigate the air–sea interaction during an extreme cold-air outbreak, associated with the passage of a synoptic-scale low pressure system over the Labrador Sea during 8 February 1997. This is the first such aircraft-based investigation in this remote region. Both high-level dropsonde and low-level flight-level data were collected. The objectives were twofold: to map out the structure of the roll vortices that cause the ubiquitous cloud streets seen in satellite imagery, and to estimate the sensible and latent heat fluxes between the ocean and atmosphere during the event. The latter was achieved by a Lagrangian analysis of the flight-level data. The flights were part of the Labrador Sea Deep Convection Experiment, investigating deep oceanic convection, and were planned to overpass a research vessel in the area. The aircraft-observed roll vortices had a characteristic wavelength of 4–5 km, particular...

Mesoscale Forecasting during a Field Program: Meteorological Support of the Labrador Sea Deep Convection Experiment

Abstract This report discusses the design and implementation of a specialized forecasting system that was set up to support the observational component of the Labrador Sea Deep Convection Experiment. This ongoing experiment is a multidisciplinary program of observations, theory, and modeling aimed at improving our knowledge of the deep convection process in the ocean, and the air–sea interaction that forces it. The observational part of the program was centered around a cruise of the R/V Knorr during winter 1997, as well as several complementary meteorological research flights. To aid the planning of ship and aircraft operations a specially tailored mesoscale model was run over the Labrador Sea, with the model output postprocessed and transferred to a remote field base. The benefits of using a warm-start analysis cycle in the model are discussed. The utility of the forecasting system is illustrated through a description of the flight planning process for several cases. The forecasts proved to be invaluabl...

A Coupled Atmosphere-ocean Wave System For Understanding Air-sea Fluxes

Our model for atmosphere-ocean wave coupled dynamics consists of the NCAR RegCM2 climate model and the WAM ocean wave model. Our domain of implementation is the North Atlantic. Coupling of atmospheric and surface ocean dynamics is achieved by the HEXOS sea-state dependent roughness. No explicit sea-state dependent thermal mechanism such as sea spray is considered. Our time-scales for simulations range from synoptic to seasonal. Results are compared with data collected during LSDCE (Labrador Sea Deep Convection Experiment).