Surface Heat Budget Of The Arctic Research Articles

Vertical Heat Fluxes beneath Idealized Sea Ice Leads in Large-Eddy Simulations: Comparison with Observations from the SHEBA Experiment

AbstractLarge-eddy simulations (Δx = Δz = 1 m) are used to examine vertical ocean heat fluxes driven by mechanical and buoyancy forcing across idealized sea ice leads. Forcing parameters approximate conditions from a shear event during the Surface Heat Budget of the Arctic (SHEBA) experiment in March 1998. In situ measurements near the lead showed isopycnal displacements of 14 m and turbulent vertical heat fluxes up to 400 W m−2, both of which were attributed to a strong cyclonic stress curl localized along the lead axis. By contrast, the large-eddy simulations show cyclonic shear across the lead to produce no turbulence, with vertical heat transport instead related to an overturning cell that connects a broad upwelling near the lead to downwelling farther away. Anticyclonic forcing produces an opposite-signed overturning cell, but with an intense, narrow downwelling jet and strong turbulent heat fluxes (~100 W m−2) near the lead. For both signs of curl, domain-integrated heat transport due to the overturning cells is somewhat larger than the turbulent heat flux, the latter being confined to the vicinity of the lead. Buoyancy forcing related to sea ice formation in the lead was found to increase both the turbulent and the cell-related heat fluxes (by up to 50% and 10%, respectively). Vertical isopycnal displacements for the upwelling case were found to increase linearly with the strength of the forcing. Possible reasons for the discrepancies with the observations include finer scale variation in the surface ocean stress and turbulence associated with the formation of a ridge during the shear event.

A comparison of the two Arctic atmospheric winter states observed during N‐ICE2015 and SHEBA

AbstractWinter time atmospheric observations from the 2015 Norwegian young sea‐ICE campaign (N‐ICE2015) are compared with data from the 1997–1998 Surface Heat Budget of the Arctic (SHEBA) campaign. Both data sets have a bimodal distribution of the net longwave radiative flux for January–February, with modal values of −40 W m−2 and 0 W m−2. These values correspond to the radiatively clear and opaquely cloudy states, respectively, and are likely to be representative of the wider Arctic. The new N‐ICE2015 observations demonstrate that the two winter states operate in the Atlantic sector of the Arctic and regions of thin sea ice. We compare the N‐ICE2015 and SHEBA data with ERA‐Interim and output from the coupled Arctic regional climate model HIRHAM‐NAOSIM. ERA‐Interim simulates two Arctic winter states well and captures the timing of transitions from one state to the other, despite underestimating the cloud liquid water path. HIRHAM‐NAOSIM has more cloud liquid water compared with ERA‐Interim but simulates the two states poorly. Our results demonstrate that models must simulate realistic synoptic forcing and temperature profiles to accurately capture the two Arctic winter states, and not only the presence of mixed‐phase clouds. Using ERA‐Interim, we find a positive trend in the number of opaquely cloudy days in the western Atlantic sector of the Arctic, and a strong correlation with the mean winter temperature over much of the Arctic Basin. Hence, the two Arctic winter states are important for understanding interannual variability in the Arctic. The N‐ICE2015 data set will help improve our understanding of these relationships.

Implications of Limited Liquid Water Path on Static Mixing within Arctic Low-Level Clouds

AbstractObservations of cloud properties and thermodynamics from two Arctic locations, Barrow, Alaska, and Surface Heat Budget of the Arctic (SHEBA), are examined. A comparison of in-cloud thermodynamic mixing characteristics for low-level, single-layer clouds from nearly a decade of data at Barrow and one full annual cycle over the sea ice at SHEBA is performed. These cloud types occur relatively frequently, evident in 27%–30% of all cloudy cases. To understand the role of liquid water path (LWP), or lack thereof, on static in-cloud mixing, cloud layers are separated into optically thin and optically thick LWP subclasses. Clouds with larger LWPs tend to have a deeper in-cloud mixed layer relative to optically thinner clouds. However, both cloud LWP subclasses are frequently characterized by an in-cloud stable layer above the mixed layer top. The depth of the stable layer generally correlates with an increased temperature gradient across the layer. This layer often contains a specific humidity inversion, but it is more frequently present when cloud LWP is optically thinner (LWP < 50 g m−2). It is suggested that horizontal thermodynamic advection plays a key role modifying the vertical extent of in-cloud mixing and likewise the depth of in-cloud stable layers. Furthermore, longwave atmospheric opacity above the cloud top is generally enhanced during cases with optically thinner clouds. Thermodynamic advection, cloud condensate distribution within the stable layer, and enhanced atmospheric radiation above the cloud are found to introduce a thermodynamic–radiative feedback that potentially modifies the extent of LWP and subsequent in-cloud mixing.

Synoptically Driven Arctic Winter States

Abstract The dense network of the Surface Heat Budget of the Arctic (SHEBA) observations is used to assess relationships between winter surface and atmospheric variables as the SHEBA site came under the influence of cyclonic and anticyclonic atmospheric circulation systems. Two distinct and preferred states of subsurface, surface, atmosphere, and clouds occur during the SHEBA winter, extending from the oceanic mixed layer through the troposphere and preceded by same-sign variations in the stratosphere. These states are apparent in distributions of surface temperature, sensible heat and longwave radiation fluxes, ocean heat conduction, cloud-base height and temperature, and in the atmospheric humidity and temperature structure. Surface and atmosphere are in radiative–turbulent–conductive near-equilibrium during a warm opaquely cloudy-sky state, which persists up to 10 days and usually occurs during the low surface pressure phase of a baroclinic wave, although occasionally occurs during the high surface pressure phase because of low, scattered clouds. Clouds occurring in this state have near-unity emissivity and the lowest bases in the vicinity of, or below, the temperature inversion peak. A cold radiatively clear-sky state persists up to two weeks, and occurs only in the high surface pressure phase of a baroclinic wave. The radiatively clear state has clouds that are too tenuous when surface based or, irrespective of opacity, located too far aloft to contribute significantly to the surface energy budget. There is a 13-K surface temperature difference between the two states, and atmospheric inversion peak temperatures are linearly related to the surface temperature in both states. The snow–sea ice interface temperature oscillates over the course of the winter season, as it cools during the radiatively clear state and is warmed from atmospheric emission above and ocean heat conduction from below during the opaquely cloudy state. Analysis of satellite data over the Arctic from 70°–90°N indicates that the radiatively clear and opaquely cloudy states observed at SHEBA may be representative of the entire Arctic basin. The results suggest that model formulation inadequacies should be easier to diagnose if modeled energy transfers are compared with observations using process-based metrics that acknowledge the bimodal nature of the Arctic ocean–ice–snow–atmosphere column, rather than monthly and regionally averaged quantities. Climate change projections of thinner Arctic sea ice and larger advective water vapor influxes into the Arctic could yield different frequencies of occupation of the radiatively clear and opaquely cloudy states and higher wintertime temperatures of SHEBA ocean, ice, snow, atmosphere, and clouds—in particular, a wintertime warming of the snow–sea ice interface temperature.

Development and testing of Polar Weather Research and Forecasting model: 2. Arctic Ocean

A version of the state‐of‐the‐art Weather Research and Forecasting model (WRF) has been developed for polar applications. The model known as “Polar WRF” is tested over the Arctic Ocean with a western Arctic grid using 25‐km resolution. The model is based upon WRF version 2.2, with improvements to the Noah land surface model and the snowpack treatment. The ocean surface treatment is modified to include fractional sea ice. Simulations consist of a series of 48‐h integrations initialized daily at 0000 UTC. The initial 24 h are taken as model spin‐up time for the atmospheric hydrology and boundary layer processes. Arctic conditions are simulated for the selected months: January 1998, June 1998, and August 1998 representing midwinter, early summer, and late summer conditions, respectively, from the Surface Heat Budget of the Arctic (SHEBA) study. The albedo of sea ice is specified as a function of time and latitude for June and as a function of time for August. Simulation results are compared with observations of the drifting ice station SHEBA in the Arctic ice pack. Polar WRF simulations show good agreement with observations for all three months. Some differences between the simulations and observation occur owing to apparent errors in the synoptic forecasts and the representation of clouds. Nevertheless, the biases in the simulated fields appear to be small, and Polar WRF appears to be a very good tool for studies of Arctic Ocean meteorology.

Observed Relationships between Arctic Longwave Cloud Forcing and Cloud Parameters Using a Neural Network

Abstract A neural network technique is used to quantify relationships involved in cloud–radiation feedbacks based on observations from the Surface Heat Budget of the Arctic (SHEBA) project. Sensitivities of longwave cloud forcing (CFL) to cloud parameters indicate that a bimodal distribution pattern dominates the histogram of each sensitivity. Although the mean states of the relationships agree well with those derived in a previous study, they do not often exist in reality. The sensitivity of CFL to cloud cover increases as the cloudiness increases with a range of 0.1–0.9 W m−2 %−1. There is a saturation effect of liquid water path (LWP) on CFL. The highest sensitivity of CFL to LWP corresponds to clouds with low LWP, and sensitivity decreases as LWP increases. The sensitivity of CFL to cloud-base height (CBH) depends on whether the clouds are below or above an inversion layer. The relationship is negative for clouds higher than 0.8 km at the SHEBA site. The strongest positive relationship corresponds to clouds with low CBH. The dominant mode of the sensitivity of CFL to cloud-base temperature (CBT) is near zero and corresponds to warm clouds with base temperatures higher than −9°C. The low and high sensitivity regimes correspond to the summer and winter seasons, respectively, especially for LWP and CBT. Overall, the neural network technique is able to separate two distinct regimes of clouds that correspond to different sensitivities; that is, it captures the nonlinear behavior in the relationships. This study demonstrates a new method for evaluating nonlinear relationships between climate variables. It could also be used as an effective tool for evaluating feedback processes in climate models.

From points to Poles: extrapolating point measurements of sea-ice mass balance

AbstractThe amount of ice growth and ablation are key measures of the thermodynamic state of the ice cover. While ice extent and even ice thickness can be determined using remote-sensing techniques, this is not the case for the mass balance. Mass-balance measurements require an ability to attribute the change, establishing whether a change in the thickness of the ice cover occurs at the top or bottom surface and whether it is a result of growth or ablation. We have developed and implemented a tool that can be used to measure thermodynamic changes in sea-ice mass balance at individual locations: the ice mass-balance buoy (IMB). The primary limitation of the IMB is that it provides a point measurement of the ice mass balance, defined by a particular combination of snow and ice conditions. Determining if, and how, such point measurements can be extrapolated is critical to understanding the large-scale mass balance of the sea-ice cover. We explore the potential for extrapolation using mass-balance observations from the Surface Heat Budget of the Arctic (SHEBA) field experiment. During SHEBA, mass-balance measurements were made at over 100 sites covering a 100 km2area. Results indicate that individual point measurements can provide reasonable estimates for undeformed and unponded multi-year ice, which represented more than two-thirds of the ice cover at SHEBA and is the dominant ice type in the perennial pack. A key is carefully selecting a representative location for the instrument package. The contribution of these point measurements can be amplified by integrating them with other tools designed to measure ice thickness and assimilating these combined data into sea-ice models.

Arctic Cloud Microphysics Retrievals from Surface-Based Remote Sensors at SHEBA

Abstract An operational suite of ground-based, remote sensing retrievals for producing cloud microphysical properties is described, assessed, and applied to 1 yr of observations in the Arctic. All measurements were made in support of the Surface Heat Budget of the Arctic (SHEBA) program and First International Satellite Cloud Climatology Project Regional Experiment (FIRE) Arctic Clouds Experiment (ACE) in 1997–98. Retrieval techniques and cloud-type classifications are based on measurements from a vertically pointing 35-GHz Doppler radar, microwave and infrared radiometers, and radiosondes. The retrieval methods are assessed using aircraft in situ measurements from a limited set of case studies and by intercomparison of multiple retrievals for the same parameters. In all-liquid clouds, retrieved droplet effective radii Re have an uncertainty of up to 32% and liquid water contents (LWC) have an uncertainty of 49%–72%. In all-ice clouds, ice particle mean sizes Dmean can be retrieved with an uncertainty of 26%–46% while retrieved ice water contents (IWC) have an uncertainty of 62%–100%. In general, radar-only, regionally tuned empirical power-law retrievals were best suited among the tested retrieval algorithms for operational cloud monitoring at SHEBA because of their wide applicability, ease of use, and reasonable statistical accuracy. More complex multisensor techniques provided a moderate improvement in accuracy for specific case studies and were useful for deriving location-specific coefficients for the empirical retrievals but were not as frequently applicable as the single sensor methods because of various limitations. During the yearlong SHEBA program, all-liquid clouds were identified 19% of the time and were characterized by an annual average droplet Re of 6.5 μm, LWC of 0.10 g m−3, and liquid water path of 45 g m−2. All-ice clouds were identified 38% of the time with an annual average particle Dmean of 73 μm, IWC of 0.014 g m−3, and ice water path of 30 g m−2.

Mesoscale Modeling of Springtime Arctic Mixed-Phase Stratiform Clouds Using a New Two-Moment Bulk Microphysics Scheme

Abstract A new two-moment bulk microphysics scheme is implemented into the polar version of the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) to simulate arctic mixed-phase boundary layer stratiform clouds observed during Surface Heat Budget of the Arctic (SHEBA) First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) Arctic Cloud Experiment (ACE). The microphysics scheme predicts the number concentrations and mixing ratios of four hydrometeor species (cloud droplets, small ice, rain, snow) and includes detailed treatments of droplet activation and ice nucleation from a prescribed distribution of aerosol obtained from observations. The model is able to reproduce many features of the observed mixed-phase cloud, including a near-adiabatic liquid water content profile located near the top of a well-mixed boundary layer, droplet number concentrations of about 200–250 cm−3 that were distributed fairly uniformly through the depth of the cloud, and continuous light snow falling from the cloud base to the surface. The impacts of droplet and ice nucleation, radiative transfer, turbulence, large-scale dynamics, and vertical resolution on the simulated mixed-phase stratiform cloud are examined. The cloud layer is largely self-maintained through strong cloud-top radiative cooling that exceeds 40 K day−1. It persists through extended periods of downward large-scale motion that tend to thin the layer and reduce water contents. Droplet activation rates are highest near cloud base, associated with subgrid vertical motion that is diagnosed from the predicted turbulence kinetic energy. A sensitivity test neglecting subgrid vertical velocity produces only weak activation and small droplet number concentrations (<90 cm−3). These results highlight the importance of parameterizing the impact of subgrid vertical velocity to generate local supersaturation for aerosol-droplet closure. The primary ice nucleation mode in the simulated mixed-phase cloud is contact freezing of droplets. Sensitivity tests indicate that the assumed number and size of contact nuclei can have a large impact on the evolution and characteristics of mixed-phase cloud, especially the partitioning of condensate between droplets and ice.

Impact of Aerosol Intrusions on Arctic Boundary Layer Clouds. Part II: Sea Ice Melting Rates

Abstract The potential impact of intrusions of polluted air into the Arctic basin on sea ice melting rates and the surface energy budget is examined. This paper extends a previous study to cloud-resolving simulations of the entire spring season during the 1998 Surface Heat Budget of the Arctic (SHEBA) field campaign. For that purpose, the Los Alamos National Laboratory sea ice model is implemented into the research and real-time versions of the Regional Atmospheric Modeling System at Colorado State University (RAMS@CSU). This new version of RAMS@CSU also includes a new microphysical module that considers the explicit nucleation of cloud droplets and a bimodal representation of their spectrum. Different aerosol profiles based on 4 May 1998 observations were used to characterize the polluted upper layer and the 2–3 daily SHEBA soundings were utilized to provide time-evolving boundary conditions to the model. Results indicate that entrainment of ice-forming nuclei (IFN) from above the inversion increases the sea ice melting rates when mixed-phase clouds are present. An opposite although less important effect is associated with cloud condensation nuclei (CCN) entrainment when liquid-phase clouds prevail.

Arctic Tropospheric Winds Derived from TOVS Satellite Retrievals

Abstract Accurate three-dimensional wind fields are essential for diagnosing a variety of important climate processes in the Arctic, such as the advection and deposition of heat and moisture, changes in circulation features, and transport of trace constituents. In light of recent studies revealing significant biases in upper-level winds over the Arctic Ocean from reanalyses, new daily wind fields are generated from 22.5 yr of satellite-retrieved thermal-wind profiles, corrected with a recently developed mass-conservation scheme. Compared to wind measurements from rawinsondes during the Surface Heat Budget of the Arctic (SHEBA) experiment, biases in satellite-retrieved winds are near zero in the meridional direction, versus biases of over 50% for reanalyses. Errors in the zonal component are smaller than those observed in reanalysis winds in the upper troposphere, while in the lower troposphere the effects of Greenland introduce uncertainty in the mass-conservation calculation. Further reduction in error may be achieved by incorporating winds retrieved from feature-tracking techniques using satellite imagers. Overall, satellite-retrieved winds are superior to reanalysis products over the data-sparse Arctic Ocean and provide increased accuracy for analyses requiring wind information. Trends and anomalies for the 22.5-yr record are calculated for both meridional and zonal winds at eight levels between the surface and 300 hPa. Annual mean trends are similar at varying levels, reflecting the relatively barotropic nature of the Arctic troposphere. Zonal winds are more westerly over Eurasia and the western Arctic Ocean, while westerlies have weakened over northern Canada. Combined with the corresponding pattern in meridional winds, these results suggest that the polar vortex has, on average, shifted toward northern Canada. Seasonal trends show that some changes persist throughout the year while others vary in magnitude and sign. Most striking are spring patterns, which differ markedly from the other seasons. Changes in meridional winds are consistent with observed trends in melt-onset date and sea ice concentration in the marginal seas. Anomalies in zonal wind profiles exhibit decadal-scale cyclicity in the eastern Arctic Ocean, while overall shifts in anomaly signs are evident and vary by region. The winter North Atlantic Oscillation (NAO) index correlates moderately with meridional wind anomalies in the Atlantic sector of the Arctic Ocean: positively (0.48) in the Barents Sea and negatively (−0.59) in the Lincoln Sea. These observed trends and anomalies are expected to translate to changes in advected heat and moisture into the Arctic basin, which are likely linked to trends in sea ice extent, melt onset, cloud properties, and surface temperature.

A New Double-Moment Microphysics Parameterization for Application in Cloud and Climate Models. Part II: Single-Column Modeling of Arctic Clouds

Abstract The new double-moment microphysics scheme described in Part I of this paper is implemented into a single-column model to simulate clouds and radiation observed during the period 1 April–15 May 1998 of the Surface Heat Budget of the Arctic (SHEBA) and First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment–Arctic Clouds Experiment (FIRE–ACE) field projects. Mean predicted cloud boundaries and total cloud fraction compare reasonably well with observations. Cloud phase partitioning, which is crucial in determining the surface radiative fluxes, is fairly similar to ground-based retrievals. However, the fraction of time that liquid is present in the column is somewhat underpredicted, leading to small biases in the downwelling shortwave and longwave radiative fluxes at the surface. Results using the new scheme are compared to parallel simulations using other microphysics parameterizations of varying complexity. The predicted liquid water path and cloud phase is significantly improved using the new scheme relative to a single-moment parameterization predicting only the mixing ratio of the water species. Results indicate that a realistic treatment of cloud ice number concentration (prognosing rather than diagnosing) is needed to simulate arctic clouds. Sensitivity tests are also performed by varying the aerosol size, solubility, and number concentration to explore potential cloud–aerosol–radiation interactions in arctic stratus.

Biogeographic Provinces of Total and Methyl Mercury in Zooplankton and Fish from the Beaufort and Chukchi Seas: Results from the SHEBA Drift

Samples of copepods (Calanus hyperboreus) and arctic cod (Boreogadus saida) were collected along the SHEBA (Surface HEat Budget of the Arctic) drift track, which commenced in the Canada Basin (October 1997) and finished in the Mendeleev Basin (October 1998). Here, we report total mercury (HgT) and CH3Hg concentrations in these biological samples and examine concentration variability along the drift track in the context of trophic variation, inputs from land, spring mercury depletion events (MDEs), and oceanographic provinces. We find background concentrations of HgT in C. hyperboreus as low as 0.02 microg/g (dw), with the Canada Basin samples exhibiting approximately 2-fold higher mercury concentrations than those from the Chukchi Plateau and Mendeleev Abyssal Plain. This east-to-west trend in mercury concentration is punctuated by two and possibly three intervals of elevated mercury (HgT, 0.10-0.12 microg/g (dw); CH3Hg, 0.023-0.028 microg/g (dw)) along the drift track. One interval of elevated HgT and CH3-Hg levels occurred during and shortly after melt. %CH3Hg reached a maximum of 60% during this time period, three times higher than any other time during the drift. This transient rise in C. hyperboreus CH3Hg concentration seems to strongly point to mercury accumulated in snow during MDEs. However, the alignment of elevated mercury samples with oceanographic fronts and the observed regional differences between basins suggest that variation of mercury concentration is primarily a consequence of ocean structure. Given that large animals such as whales selectively forage in regions of higher food concentration such as fronts, recent change in the ice climate of the western Arctic Ocean, perhaps mediated by changes in heat storage, may provide the means to change their exposure to mercury thus explaining observed increases in mercury concentrations in western beluga whales during the 1990s.

Near-Inertial Wave Propagation in the Western Arctic

Abstract From October 1997 through October 1998, the Surface Heat Budget of the Arctic (SHEBA) ice camp drifted across the western Arctic Ocean, from the central Canada Basin over the Northwind Ridge and across the Chukchi Cap. During much of this period, the velocity and shear fields in the upper ocean were monitored by Doppler sonar. Near-inertial internal waves are found to be the dominant contributors to the superinertial motion field. Typical rms velocities are 1–2 cm s−1. In this work, the velocity and shear variances associated with upward- and downward-propagating wave groups are quantified. Patterns are detected in these variances that correlate with underlying seafloor depth. These are explored with the objective of assessing the role that these extremely low-energy near-inertial waves play in the larger-scale evolution of the Canada Basin. The specific focus is the energy flux delivered to the slopes and shelves of the basin, available for driving mixing at the ocean boundaries. The energy and shear variances associated with downward-propagating waves are relatively uniform over the entire SHEBA drift, independent of the season and depth of the underlying topography. Variances associated with upward-propagating waves follow a (depth)−1/2 dependence. Over the deep slopes, vertical wavenumber spectra of upward-propagating waves are blue-shifted relative to their downward counterparts, perhaps a result of reflection from a sloping seafloor. To aid in interpretation of the observations, a simple, linear model is used to compare the effects of viscous (volume) versus underice (surface) dissipation for near-inertial waves. The latter is found to be the dominant mechanism. A parallel examination of the topography of the western Arctic shows that much of the continental slope is close to critical for near-inertial wave reflection. The picture that emerges is consistent with “one bounce” rather than trans-Arctic propagation. The dominant surface-generated waves are substantially absorbed in the underice boundary layer following a single roundtrip to the seafloor. However, surface-generated waves can interact strongly with nearby (<300 km) slopes, potentially contributing to dissipation rates of order 10−6–10−7 W m−3 in a zone several hundred meters above the bottom. The waves that survive the reflection process (and are not back-reflected) display a measurable blue shift over the slopes and contribute to the observed dependence of energy on seafloor depth that is seen in these upper-ocean observations.

A Spectral Technique for Estimating Turbulent Stress, Scalar Flux Magnitude, and Eddy Viscosity in the Ocean Boundary Layer under Pack Ice

Abstract The hypothesis is tested that, for the planetary boundary layer, turbulent vertical velocity (w) spectral density, normalized by u2∗/k (u2∗ is Reynolds stress magnitude and k is wavenumber: 2π times frequency divided by mean flow speed), is a “universal” function of nondimensional wavenumber k/kmax, where kmax is the wavenumber at the peak in the area-preserving log–log w spectrum. Data from clusters of turbulence-measuring instruments deployed through the ocean boundary layer beneath pack ice during the yearlong Surface Heat Budget of the Arctic (SHEBA) project were analyzed by averaging spectra in 3-h bins, then nondimensionalizing weighted w spectral density by directly measured Reynolds stress magnitude and wavenumber by kmax. In the outer boundary layer, normalized spectra were remarkably uniform, suggesting that (i) the fundamental turbulence scale is inversely proportional to kmax and (ii) the w wavenumber spectrum by itself may be used to estimate local stress magnitude and eddy viscosity...

Cloud Radiative Forcing of the Arctic Surface: The Influence of Cloud Properties, Surface Albedo, and Solar Zenith Angle

Abstract An annual cycle of cloud and radiation measurements made as part of the Surface Heat Budget of the Arctic (SHEBA) program are utilized to determine which properties of Arctic clouds control the surface radiation balance. Surface cloud radiative forcing (CF), defined as the difference between the all-sky and clear-sky net surface radiative fluxes, was calculated from ground-based measurements of broadband fluxes and results from a clear-sky model. Longwave cloud forcing (CFLW) is shown to be a function of cloud temperature, height, and emissivity (i.e., microphysics). Shortwave cloud forcing (CFSW) is a function of cloud transmittance, surface albedo, and the solar zenith angle. The annual cycle of Arctic CF reveals cloud-induced surface warming through most of the year and a short period of surface cooling in the middle of summer, when cloud shading effects overwhelm cloud greenhouse effects. The sensitivity of CFLW to cloud fraction is about 0.65 W m−2 per percent cloudiness. The sensitivity of ...

Atmospheric and Sea Ice Conditions During the Sheba Year: Historical and Spatial Assessment

The Surface Heat Budget of the Arctic (SHEBA) field project from 1997 to 1998 collected a suite of atmosphere, sea ice, and ocean data in order to improve our understanding of the surface energy and ice mass balances. Given the importance of the SHEBA field data, an assessment of the spatial and temporal characteristics of the SHEBA year is needed. Were atmospheric and sea ice conditions during SHEBA similar to climatology? Were data collected at or near the SHEBA field site representative of a larger spatial domain? The objectives of this paper are to examine the SHEBA year in regard to its historical and spatial context. Results show that atmospheric and sea ice conditions near the SHEBA field site were representative of a large spatial area, but not particularly representative of climatological values with the exception of sea level pressure.

Observation and Interpretation of Microwave Cloud Signatures over the Arctic Ocean during Winter

An analysis of satellite microwave brightness temperatures at 85 GHz (37 GHz) shows that these temperatures sometimes vary by more than 30 K (15 K) within 1 or 2 days at a single location over Arctic sea ice. This variation can be seen in horizontal brightness temperature distributions with spatial scales of hundreds of kilometers, as well as in brightness temperature time series observed at a single location. Analysis of satellite observations during winter shows that such brightness temperature warming frequently occurs in the Arctic Ocean, particularly in regions over which low pressure systems often pass. By comparing the observed microwave brightness temperature warming with ground-based measurements of geophysical variables collected during the Surface Heat Budget of the Arctic (SHEBA) experiment and with numerical prediction model analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF), it is found that brightness temperature anomalies are significantly correlated with clouds and precipitation. This finding raises the possibility of using satellite microwave data to estimate cloud liquid water path and precipitation in the Arctic. Factors contributing to the brightness temperature warming were examined, and it was found that the primary contributors to the observed warming were cloud liquid water and surface temperature change.

Potential for estimating cloud liquid water path over sea ice from airborne passive microwave measurements

[1] In this paper we investigate the feasibility of determining cloud liquid water path from passive microwave measurements over sea ice. Simulations using a 32-stream plane-parallel microwave radiative transfer model indicate a consistent increase in brightness temperature attributable to cloud liquid water for conditions observed in the Arctic during the Surface Heat Budget of the Arctic (SHEBA) experiment. Uncertainties in brightness temperature simulations due to variations in surface emissivity, surface temperature, cloud temperature, and atmospheric water vapor are investigated. Surface emissivity variations are found to cause the largest uncertainties in top-of-atmosphere brightness temperature over the range of liquid water paths examined. An algorithm previously developed for estimating liquid water path over oceans from SSM/I data is adapted for retrievals over sea ice. The algorithm is applied to brightness temperature measurements from airborne microwave radiometers. Mean retrieved liquid water path (LWP) is compared to in situ measurements from airborne cloud microphysical probes. The correlation coefficient for the two data sets is found to be 0.989 with an RMS error of 14 g m � 2 , although the retrieved LWP values show a high degree of variability. Accuracy is highest when LWP values are 100 g m � 2 and above. Poorest accuracies are obtained for small LWP (<50 g m � 2 ) where errors in the specification of surface emissivity have a large effect. These case studies provide evidence that liquid water clouds over sea ice produce detectable changes in microwave brightness temperature and suggest that interpretation of sea ice properties using certain microwave frequencies may be affected by the presence of liquid water clouds. INDEX TERMS: 0320 Atmospheric Composition and Structure: Cloud physics and chemistry, 3349 Meteorology and Atmospheric Dynamics: Polar meteorology, 3360 Meteorology and Atmospheric Dynamics: Remote sensing, 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques; KEYWORDS: cloud physics, microwave remote sensing, arctic clouds, liquid water path, airborne measurements

Airborne measurements of tropospheric ice‐nucleating aerosol particles in the Arctic spring

Instrumented aircraft flights were made during field experiments in the Arctic Ocean, the NASA FIRE Arctic Cloud Experiment and SHEBA (Surface Heat Budget of the Arctic). Airborne measurements of ice nucleating aerosol particles (IN) used a continuous flow diffusion (CFD) chamber, covering −10° to −30°C and humidities from ice saturation to water supersaturation. During selected time periods, ice crystals that grew on ice nuclei in the chamber were sampled onto electron microscope (EM) grids for later examination of the nucleating particles. Samples of total aerosol (IN and non‐IN) were also collected for comparative analyses. Concentrations of IN ranged from zero to rare very high values (hundreds per liter at −25°C), making the frequency distribution of IN highly skewed: when accumulated as 10 s average concentrations (volume ∼0.17 L), 50% were zero. Additional evidence of few IN was seen in thin low‐level stratus clouds at −10° to −20°C that persisted for several days with low concentrations of ice crystals (∼0.1 L−1) and a few tenths g m−3 liquid water. On occasion, small regions of high IN concentrations (hundreds per liter) were detected near the surface. The EM analyses indicated that ice nuclei a few tenths micrometer in size contained crustal materials (primarily Si) and had widely varying morphology. Many IN particles produced weak or no X‐ray signatures, suggesting a dominant low‐molecular‐weight component not detected by the energy dispersive X‐ray (EDX) system, and were probably carbonaceous. In contrast, for the total aerosol, S or S and Si were dominant components, and few particles had no X‐ray signature.