Janet M. Intrieri

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...

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 ...

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.

An Arctic Springtime Mixed-Phase Cloudy Boundary Layer Observed during SHEBA

Abstract The microphysical characteristics, radiative impact, and life cycle of a long-lived, surface-based mixed-layer, mixed-phase cloud with an average temperature of approximately −20°C are presented and discussed. The cloud was observed during the Surface Heat Budget of the Arctic experiment (SHEBA) from 1 to 10 May 1998. Vertically resolved properties of the liquid and ice phases are retrieved using surface-based remote sensors, utilize the adiabatic assumption for the liquid component, and are aided by and validated with aircraft measurements from 4 and 7 May. The cloud radar ice microphysical retrievals, originally developed for all-ice clouds, compare well with aircraft measurements despite the presence of much greater liquid water contents than ice water contents. The retrieved time-mean liquid cloud optical depth of 10.1 ± 7.8 far surpasses the mean ice cloud optical depth of 0.2, so that the liquid phase is primarily responsible for the cloud’s radiative (flux) impact. The ice phase, in turn, ...

A comparison of cloud and boundary layer variables in the ECMWF forecast model with observations at Surface Heat Budget of the Arctic Ocean (SHEBA) ice camp

Cloud and boundary layer variables from the European Centre for Medium-Range Weather Forecasts (ECMWF) forecast model were compared with measurements made from surface instruments and from upward looking 8 mm wavelength radar and lidar at the Surface Heat Budget of the Arctic Ocean (SHEBA) ice camp during November and December of 1997. The precipitation accumulation, near-surface winds, and surface downward longwave irradiance predicted by the model were in good agreement with SHEBA observations during this period. However, surface downward longwave irradiance was underestimated by 10 W m−2 on average when low clouds were present in the model and observations. The model demonstrated considerable skill in predicting the occurrence and vertical extent of cloudiness over SHEBA, with some tendency to overestimate the frequency of clouds below 1 km. A synthetic radar reflectivity estimated from the ECMWF model variables was compared with 8 mm wavelength radar measurements. The two were broadly consistent only if the assumed snowflake size distribution over SHEBA had a smaller proportion of large flakes than was found in previous studies at lower latitudes. The ECMWF model assumes a temperature-dependent partitioning of cloud condensate between water and ice. Lidar depolarization measurements at SHEBA indicate that both liquid and ice phase clouds occurred over a wide range of temperatures throughout the winter season, with liquid occurring at temperatures as low as 239 K. A much larger fraction of liquid water clouds was observed than the ECMWF model predicted. The largest discrepancies between the ECMWF model and the observations were in surface temperature (up to 15 K) and turbulent sensible heat fluxes (up to 60 W m−2). These appear to be due at least partially to the ECMWF sea ice model, which did not allow surface temperatures to respond nearly as rapidly to changing atmospheric conditions as was observed.

Cloud Particle Phase Determination with the AVHRR

An accurate determination of cloud particle phase is required for the retrieval of other cloud properties from satellite and for radiative flux calculations in climate models. The physical principles underlying phase determination using the advanced very high resolution radiometer (AVHRR) satellite sensor are described for daytime and nighttime, cold cloud and warm cloud conditions. It is demonstrated that the spectral properties of cloud particles provide necessary, but not sufficient, information for phase determination, because the relationship between the cloud and surface temperatures is also important. Algorithms based on these principles are presented and tested. Validation with lidar and aircraft data from two Arctic field experiments shows the procedures to be accurate in identifying the phase of homogeneous water and ice clouds, though optically thin, mixed-phase, and multilayer clouds are problematic.

Cloud-aerosol interactions during autumn over Beaufort Sea

Cloud and aerosol properties were observed by aircraft in autumn over the Beaufort Sea during the 1994 Beaufort and Arctic Storms Experiment (BASE). The microphysical properties (particle size, concentration, mass, and phase) and vertical structure of autumn clouds are examined as a function of height and minimum in-cloud temperature, T min . Below 2 km, liquid clouds were observed at T min between -5° and -9°C, mixed-phase clouds were observed between -5° and -20°C, and clear-sky ice crystal precipitation was observed at T min as warm as -14°C. Between 2 and 5 km all clouds were mixed-phase and typically consisted of a thin layer of liquid with ice extending well below the liquid layer. These mixed-phase clouds were found at T min as low as -32°C. All clouds observed above 5.5 km were composed entirely of ice at T min as warm as -33°C. The concentration of ice crystals is observed to increase exponentially with decreasing T min . The Hallet-Mossop ice multiplication process did not appear to be an important in the production of ice crystals in the mixed-phase cloud observed in this study. The atmosphere was relatively clean with condensation nuclei (CN) concentrations rarely exceeding 300 cm -3 . The smallest CN concentrations (as low as 50 cm -3 ) were observed in the boundary layer and just above the surface where precipitation and nucleation scavenging have cleansed the air. Thin layers of very large CN concentrations were often observed within and just above low-level clouds possibly resulting from gas-to-particle conversion which requires clean and humid air typical of lower Arctic atmosphere.

Characteristics and Radiative Effects of Diamond Dust over the Western Arctic Ocean Region

Atmospheric observations from active remote sensors and surface observers, obtained in the western Arctic Ocean between November 1997 and May 1998, were analyzed to determine the physical characteristics and to assess the surface radiative contribution of diamond dust. The observations showed that diamond dust contributed only a negligible radiative effect to the sea ice surface. Surface radiative fluxes and radiative forcing values during diamond dust events were similar in magnitude when compared to observed clear-sky periods. Combined information from lidar, radar, and surface observers showed that diamond dust occurred ;13% of the time between November and mid-May over the Arctic Ocean and was not observed between mid-May and October. Diamond dust vertical depths, derived from lidar measurements, varied between 100 and 1000 m but were most often observed to be about 250 m. Lidar and radar measurements were analyzed to assess if precipitation from boundary layer clouds was present during times when surface observers reported diamond dust. This analysis revealed that surface observers had incorrectly coded diamond dust events ;45% of the time. The miscoded events occurred almost exclusively under conditions with limited or no illumination (December‐March). In 95% of the miscoded reports, lidar measurements revealed the presence of thin liquid water clouds precipitating ice crystals down to the surface.

Cloud Boundary Statistics during FIRE II

Abstract An 8-mm wavelength radar, 3-mm wavelength radar, and 10.6-µm wavelength lidar operated side by side in vertically pointing mode during the First ISCCP Regional Experiment (FIRE II). This data collection mode yielded detailed information on distribution of cloud and cloud boundaries as a function of altitude. Statistics on the location of cloud boundaries during the FIRE II experiment indicate that cloud bases tended to form at two discrete levels centered around 2.5 and 7.5 km, cirrus cloud tops formed most frequently at 9.5 km and cloud thickness were usually 2 km or less. The atmosphere had the highest incidence of cloudiness at 8.5 km AGL, with a secondary maximum at an altitude of 3.5 km AGL. The incidence of cloudiness fell off rapidly between 8 and 11 km; there was also a distinct minimum in cloudiness at 2 km AGL. The diurnal variation of upper-level cloud base and top heights was about 1.0 km AGL, with the highest bases and tops occurring at 0500 UTC and the lowest bases and tops occurrin...

The Emergence of Weather-Related Test Beds Linking Research and Forecasting Operations

Test beds have emerged as a critical mechanism linking weather research with forecasting operations. The U.S. Weather Research Program (USWRP) was formed in the 1990s to help identify key gaps in research related to major weather prediction problems and the role of observations and numerical models. This planning effort ultimately revealed the need for greater capacity and new approaches to improve the connectivity between the research and forecasting enterprise. Out of this developed the seeds for what is now termed “test beds.” While many individual projects, and even more broadly the NOAA/National Weather Service (NWS) Modernization, were successful in advancing weather prediction services, it was recognized that specific forecast problems warranted a more focused and elevated level of effort. The USWRP helped develop these concepts with science teams and provided seed funding for several of the test beds described. Based on the varying NOAA mission requirements for forecasting, differences in the orga...