Peter S. Guest

Surface Heat Budget of the Arctic Ocean

A summary is presented of the Surface Heat Budget of the Arctic Ocean (SHEBA) project, with a focus on the field experiment that was conducted from October 1997 to October 1998. The primary objective of the field work was to collect ocean, ice, and atmospheric datasets over a full annual cycle that could be used to understand the processes controlling surface heat exchanges—in particular, the ice–albedo feedback and cloud–radiation feedback. This information is being used to improve formulations of arctic ice–ocean–atmosphere processes in climate models and thereby improve simulations of present and future arctic climate. The experiment was deployed from an ice breaker that was frozen into the ice pack and allowed to drift for the duration of the experiment. This research platform allowed the use of an extensive suite of instruments that directly measured ocean, atmosphere, and ice properties from both the ship and the ice pack in the immediate vicinity of the ship. This summary describes the project goal...

An annual cycle of Arctic surface cloud forcing at SHEBA

[1] We present an analysis of surface fluxes and cloud forcing from data obtained during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment, conducted in the Beaufort and Chuchki Seas and the Arctic Ocean from November 1997 to October 1998. The measurements used as part of this study include fluxes from optical radiometer sets, turbulent fluxes from an instrumented tower, cloud fraction from a depolarization lidar and ceilometer, and atmospheric temperature and humidity profiles from radiosondes. Clear-sky radiative fluxes were modeled in order to estimate the cloud radiative forcing since direct observation of fluxes in cloud-free conditions created large statistical sampling errors. This was particularly true during summer when cloud fractions were typically very high. A yearlong data set of measurements, obtained on a multiyear ice floe at the SHEBA camp, was processed in 20-day blocks to produce the annual evolution of the surface cloud forcing components: upward, downward, and net longwave and shortwave radiative fluxes and turbulent (sensible and latent heat) fluxes. We found that clouds act to warm the Arctic surface for most of the annual cycle with a brief period of cooling in the middle of summer. Our best estimates for the annual average surface cloud forcings are -10 W m -2 for shortwave, 38 W m -2 for longwave, and -6 W m -2 for turbulent fluxes. Total cloud forcing (the sum of all components) is about 30 W m -2 for the fall, winter, and spring, dipping to a minimum of -4 W m -2 in early July. We compare the results of this study with satellite, model, and drifting station data.

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

The Labrador Sea Deep Convection Experiment

In the autumn of 1996 the field component of an experiment designed to observe water mass transformation began in the Labrador Sea. Intense observations of ocean convection were taken in the following two winters. The purpose of the experiment was, by a combination of meteorological and oceanographic field observations, laboratory studies, theory, and modeling, to improve understanding of the convective process in the ocean and its representation in models. The dataset that has been gathered far exceeds previous efforts to observe the convective process anywhere in the ocean, both in its scope and range of techniques deployed. Combined with a comprehensive set of meteorological and air-sea flux measurements, it is giving unprecedented insights into the dynamics and thermodynamics of a closely coupled, semienclosed system known to have direct influence on the processes that control global climate.

The aerodynamic roughness of different types of sea ice

The aerodynamic roughness for all major types of sea ice is specified on the basis of surface layer measurements from ship and ice floe platforms. Median neutral surface drag coefficients, Cdn×1000, for grease and nilas ice are 0.7 and 1.6. Small, medium, large, and fused pancake ice have Cdn equal to 0.9, 1.6, 2.4, and 1.9. Young ice Cdn ranges from 2.3 to 3.1. Very smooth first-year or multiyear ice with no pressure ridges has Cdn equal to 1.5. Pack ice has median values of 2.0 and 2.2 for first-year and multiyear ice. Marginal ice zone ice averages 3.1 and 3.4 (first-year, multiyear) if unaffected by wave action. Wave-affected ice has Cdn values of 4.2 and 4.6 (first-year, multiyear). Extremely rough multiyear ice in intense shear or compaction regions has Cdn equal to 8.0. Open ocean areas within 100 km of an ice edge averaged 1.8, while ice-free water downwind of ice, with small or no waves is 1.4. These values can be adjusted if heat fluxes are present. The accuracy of the Cdn averages is estimated to be 20% or better.

The Arctic snow and air temperature budget over sea ice during winter

Arctic cooling through the fall-winter transition is calculated from a coupled atmosphere-sea ice thermal model and compared to temperature soundings and surface measurements made north of Svalbard during the Coordinated Eastern Arctic Experiment (CEAREX). A typical winter, clear-sky vertical temperature structure of the polar air mass is composed of a surface-based temperature inversion or an inversion above a very shallow (30–180 m) mechanically mixed boundary layer with temperatures −30° to −35°C, a broad temperature maximum layer of −20° to −25°C between 0.5 and 2 km, and a negative lapse rate aloft. Because the emissivity of the temperature maximum layer is less than that of the snow surface, radiative equilibrium maintains this low level temperature inversion structure. A 90-day simulation shows that heat flux through the ice is insufficient to maintain a local thermal equilibrium. Northward temperature advection by transient storms is required to balance outward longwave radiation to space. Leads and thin ice (<0.8 m) contribute 12% to the winter tropospheric heat balance in the central Arctic. CEAREX temperature soundings and longwave radiation data taken near 81°N show polar air mass characteristics by early November, but numerous storms interrupted this air mass during December. Snow temperature changes of 15°C occurred in response to changes in downward atmospheric longwave radiation of 90 W m−2 between cloud and clear sky. We propose that the strength of boundary layer stability, and thus the degree of air-ice momentum coupling, is driven by the magnitude of the radiation deficit (downward-outward longwave) at the surface and the potential temperature of the temperature maximum layer. This concept is of potential benefit in prescribing atmospheric forcing for sea ice models because a surface air temperature-snow temperature difference field is difficult to obtain and it may be possible to obtain a radiation deficit field via satellite sensors.

Evaluations of the von Kármán constant in the atmospheric surface layer

The von Karman constant

The Antarctic Zone Flux Experiment

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Parameterizing turbulent exchange over sea ice in winter

relates the flow speed profile in a wall-bounded shear flow to the stress at the surface. Recent laboratory studies in aerodynamically smooth flow report

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

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