Edgar L. Andreas

A theory for the scalar roughness and the scalar transfer coefficients over snow and sea ice

Although the bulk aerodynamic transfer coefficients for sensible (CH) and latent (CE) heat over snow and sea ice surfaces are necessary for accurately modeling the surface energy budget, they have been measured rarely. This paper, therefore, presents a theoretical model that predicts neutral-stability values of CH and CE as functions of the wind speed and a surface roughness parameter. The crux of the model is establishing the interfacial sublayer profiles of the scalars, temperature and water vapor, over aerodynamically smooth and rough surfaces on the basis of a surface-renewal model in which turbulent eddies continually scour the surface, transferring scalar contaminants across the interface by molecular diffusion. Matching these interfacial sublayer profiles with the semi-logarithmic inertial sublayer profiles yields the roughness lengths for temperature and water vapor. When coupled with a model for the drag coefficient over snow and sea ice based on actual measurements, these roughness lengths lead to the transfer coefficients. CE is always a few percent larger than CH. Both decrease monotonically with increasing wind speed for speeds above 1 m s−1, and both increase at all wind speeds as the surface gets rougher. Both, nevertheless, are almost always between 1.0 × 10−3 and 1.5 × 10−3.

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 New Sea Spray Generation Function for Wind Speeds up to 32 m s−1

Abstract The sea spray generation function quantifies the rate at which spray droplets of a given size are produced at the sea surface. As such, it is important in studies of the marine aerosol and its optical properties and in understanding the role that sea spray plays in transferring heat and moisture across the air–sea interface. The emphasis here is on this latter topic, where uncertainty over the spray generation function, especially in high winds, is a major obstacle. This paper surveys the spray generation functions available in the literature and, on theoretical grounds, focuses on one by M. H. Smith et al. that has some desirable properties but does not cover a wide enough droplet size range to be immediately useful for quantifying spray heat transfer. With reasonable modifications and extrapolations, however, the paper casts the Smith function into a new form that can be used to predict the production of sea spray droplets with radii from 2 to 500 μm for 10-m winds from 0 to 32.5 m s−1. The pap...

The spray contribution to net evaporation from the sea: A review of recent progress

The part that sea spray plays in the air-sea transfer of heat and moisture has been a controversial question for the last two decades. With general circulation models (GCMs) suggesting that perturbations in the Earths surface heat budget of only a few W m−2 can initiate major climatic variations, it is crucial that we identify and quantify all the terms in that heat budget. Thus, here we review recent work on how sea spray contributes to the sea surface heat and moisture budgets. In the presence of spray, the near-surface atmosphere is characterized by a droplet evaporation layer (DEL) with a height that scales with the significant-wave amplitude. The majority of spray transfer processes occur within this layer. As a result, the DEL is cooler and more moist than the atmospheric surface layer would be under identical conditions but without the spray. Also, because the spray in the DEL provides elevated sources and sinks for heat and moisture, the vertical heat fluxes are no longer constant with height. We use Eulerian and Lagrangian models and a simple analytical model to study the processes important in spray droplet dispersion and evaporation within this DEL. These models all point to the conclusion that, in high winds (above about 15 m/s), sea spray begins to contribute significantly to the air-sea fluxes of heat and moisture. For example, we estimate that, in a 20-m/s wind, with an air temperature of 20°C, a sea surface temperature of 22°C, and a relative humidity of 80%, the latent and sensible heat fluxes resulting from the spray alone will have magnitudes of order 150 and 15 W/m2, respectively, in the DEL. Finally, we speculate on what fraction of these fluxes rise out of the DEL and, thus, become available to the entire marine boundary layer.

SEA SPRAY AND THE TURBULENT AIR-SEA HEAT FLUXES

Heat and moisture carried by sea spray have long been suspected of contributing to the air-sea fluxes of sensible and latent heat. Using time scales that parameterize how long sea spray droplets reside in the air and how quickly they exchange heat and moisture with their environment, I estimate sea spray contributions to the air-sea heat fluxes. To make these estimates, I first develop a new sea spray generation function that predicts more realistic spume production than earlier models. Spray droplets with initial radii between 10 and 300 μm contribute most to the heat fluxes; the vast majority of these are spume droplets. The modeling not only demonstrates how spray droplets participate in the air-sea heat exchange but also confirms earlier predictions that the heat carried by sea spray (especially the latent heat) is an important component of the air-sea heat balance. In my examples, the maximum magnitude of the spray latent heat flux for a 20-m/s wind is 170 W/m2; the maximum spray sensible heat flux is 33 W/m2. For winds over 10 m/s, the spray latent heat flux is usually a substantial fraction of the interfacial (or turbulent) latent heat flux (estimated from the bulk-aerodynamic equations) and will thus confound measurements of the air-sea transfer coefficient for latent heat.

Effects of Sea Spray on Tropical Cyclone Intensity

The intensity of tropical cyclones is sensitive to the rates at which enthalpy and momentum are transferred between sea and air in the high-wind core of the storm. Present models of the wind dependence of these transfer rates suggest that the effective drag coefficient is more than twice the effective enthalpy transfer coefficient at wind speeds above 25 m s21. Using this ratio in numerical models, however, makes it impossible to sustain storms of greater than marginal hurricane intensity. Some other physical process must, therefore, enhance enthalpy transfer at very high wind speeds. This paper suggests that re-entrant sea spray explains this enhanced transfer. When a spray droplet is ejected from the ocean, it remains airborne long enough to cool to a temperature below the local air temperature but not long enough to evaporate an appreciable fraction of its mass. The spray droplet thus gives up sensible heat and returns to the sea before it has time to extract back from the atmosphere the heat necessary to continue its evaporation. Microphysical modeling, combined with data from the Humidity Exchange over the Sea Experiment (HEXOS), makes it possible to derive an expression for the net enthalpy transfer of re-entrant spray. This spray enthalpy flux is roughly cubic in wind speed. When this relation is used in a numerical simulation of a hurricane, the spray more than compensates for the observed increase in the ratio of drag and enthalpy transfer coefficients with wind speed. The momentum flux associated with sea spray is an important energy sink that moderates the effects of this spray enthalpy flux. Including a parameterization for this momentum sink along with wave drag and spray enthalpy transfer in the hurricane simulation produces results that are similar to ones based on equal transfer coefficients.

Bulk Transfer Coefficients for Heat and Momentum over Leads and Polynyas

Abstract Leads and polynyas are areas of open water surrounded by pack ice. In winter, when the polar oceans have extensive ice covers and the water-air temperature difference is typically 20°–40°C, they allow enormous amounts of sensible and latent heat to escaoe from the ocean to the atmosphere. Parameterizing these fluxes accurately is thus an important part of modeling the growth and decay of sea ice. To develop a unified method for parameterizing the turbulent transfer from open water surrounded by pack ice, we have reanalyzed data reported in the literature on momentum and heat transfer over Arctic leads and polynyas. The neutral stability value of the 10-m drag coefficient, CDN10 = 1.49×10−1, is independent of wind speed and open-water fetch for winds from 1 to 10 m s−1 and fetches from 7 to 500 m. That value is slightly higher than values typical of the open ocean at these wind speed probably because of the form drag over the upwind ice or at the ice edges and because the wave field is still activ...

Heat budget of snow‐covered sea ice at North Pole 4

The Russian drifting station North Pole 4 (NP-4) was within 5° latitude of the North Pole from April 1956 to April 1957. We use a wide-ranging set of snow and meteorological data collected at 3-hourly intervals on NP-4 during this period to investigate energy and mass transfer in the snow, sea ice, and atmospheric surface layer in the central Arctic. SNTHERM, a one-dimensional energy and mass balance model, synthesizes these diverse NP-4 data and thereby yields energetically consistent time series of the components of the surface heat budget. To parameterize the sensible heat flux during extremely stable stratification, we replace the usual log-linear stability function with the “Dutch” formulation and introduce a windless coefficient in the bulk parameterization. This coefficient provides sensible heat transfer at the surface, even when the mean wind speed is near zero, and thereby prevents the surface temperature from falling to unrealistically low values, a common modeling problem when the stratification is very stable. Several other modifications to SNTHERM introduce procedures for creating a realistic snowpack that has continuously variable density and is subject to erosion and wind packing. The NP-4 data provide for two distinct simulations: one on 2-year ice and one on multiyear ice. We validate our modeling by comparing simulated and observed temperatures at various depths in the snow and sea ice. Simulations for both sites show the same tendencies. During the summer, the shortwave radiation is the main term in the surface heat budget. Shortwave radiation also penetrates into the snow and causes a subsurface temperature maximum that both the data and the model capture. During the winter, the net longwave balance is the main term in the surface heat budget. The snow and sea ice cool in response to longwave losses, but the flux of sensible heat from the air to the surface mitigates these losses and is thus nearly a mirror image of the emitted longwave flux.

Low-Level Atmospheric Jets And Inversions Over The Western Weddell Sea

For four months in the fall and earlywinter of 1992, as Ice Station Weddell (ISW) driftednorthward through the ice-covered western Weddell Sea,ice station personnel profiled the atmosphericboundary layer (ABL) with radiosondes. These showedthat the ABL was virtually always stably stratifiedduring this season: 96% of the soundings found anear-surface inversion layer. Forty-four percent ofthese inversions were surface-based. Eighty percentof the soundings that yielded unambiguous windprofiles showed an atmospheric jet with speeds as highas 14 m s-1 in a core below an altitude of 425 m. This paper documents the features of these inversionsand low-level jets. Because the inversion statistics,in particular, are like those reported in and aroundthe Arctic Ocean, similar local processes seem tocontrol the ABL over sea ice regions in bothhemispheres. A simple two-layer model, in which anelevated layer becomes frictionally decoupled from thesurface, does well in explaining the ISW jetstatistics. This model also implies a new geostrophicdrag parameterization for sea-ice regions that dependson the magnitude of the geostrophic wind, the 10-mdrag coefficient CDN10, and the ABL height, butnot explicitly on any stratification parameter.

Spray Stress Revisited

Abstract In winds approaching hurricane strength, spray droplets proliferate. Once created, these droplets accelerate to the local wind speed in 1 s or less and thereby extract momentum from the wind. Because these droplets have substantial mass, they eventually plunge back into the ocean, delivering their horizontal momentum to the surface in the form of a spray stress. Inadequate information on the production rate and size distribution of spray droplets, however, hampered previous attempts to estimate the magnitude of this spray-mediated momentum exchange. This paper therefore uses recent estimates of the spray generation function to reconsider sprays ability to alter air–sea momentum exchange. Conservation of momentum requires that spray cannot enhance the air– sea stress beyond what the large-scale flow dictates. However, spray can redistribute stress in the near-surface atmosphere since the wind must slow if the spray droplets accelerate. For a wind of 30 m s−1, spray supports about 10% of the surfa...