Taneil Uttal

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...

The Epic 2001 Stratocumulus Study

Abstract Overlaying the cool southeast Pacific Ocean is the most persistent subtropical stratocumulus cloud deck in the world. It produces a profound affect on tropical climate by shading the underlying ocean and radiatively cooling and stirring up turbulence in the atmosphere. In October 2001, the East Pacific Investigation of Climate undertook an exploratory cruise from the Galapagos Islands to Chile. The cruise gathered an unprecedented dataset, integrating radiosonde, surface, cloud remote sensing, aerosol, and ocean measurements. Scientific objectives included measuring the vertical structure of the ABL in this region, understanding what physical processes are determining the stra-tocumulus cloud albedo, and understanding the fluxes of heat and water that couple the atmosphere and ocean in this region. An unexpectedly well-mixed stratocumulus-capped boundary layer as a result of a strong inversion was encountered throughout. A strong diurnal cycle was observed, with thicker clouds and substantial dri...

Arctic Mixed-Phase Cloud Properties Derived from Surface-Based Sensors at SHEBA

Abstract Arctic mixed-phase cloud macro- and microphysical properties are derived from a year of radar, lidar, microwave radiometer, and radiosonde observations made as part of the Surface Heat Budget of the Arctic Ocean (SHEBA) Program in the Beaufort Sea in 1997–98. Mixed-phase clouds occurred 41% of the time and were most frequent in the spring and fall transition seasons. These clouds often consisted of a shallow, cloud-top liquid layer from which ice particles formed and fell, although deep, multilayered mixed-phase cloud scenes were also observed. On average, individual cloud layers persisted for 12 h, while some mixed-phase cloud systems lasted for many days. Ninety percent of the observed mixed-phase clouds were 0.5–3 km thick, had a cloud base of 0–2 km, and resided at a temperature of −25° to −5°C. Under the assumption that the relatively large ice crystals dominate the radar signal, ice properties were retrieved from these clouds using radar reflectivity measurements. The annual average ice par...

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, ...

On cloud radar and microwave radiometer measurements of stratus cloud liquid water profiles

We show a method for determining stratus cloud liquid water profiles using a microwave radiometer and cloud radar. This method is independent of the radar calibration and the cloud-droplet size distribution provided that the sixth moment of the size distribution can be related to the square of the third moment. We have calculated these moments with a wide variety of in situ measurements and show that this is a reasonable assumption. Examples of droplet distributions that meet this requirement are the lognormal and gamma distributions.

The Atmospheric Radiation Measurement Program Cloud Radars: Operational Modes

Abstract During the past decade, the U.S. Department of Energy (DOE), through the Atmospheric Radiation Measurement (ARM) Program, has supported the development of several millimeter-wavelength radars for the study of clouds. This effort has culminated in the development and construction of a 35-GHz radar system by the Environmental Technology Laboratory (ETL) of the National Oceanic and Atmospheric Administration (NOAA). Radar systems based on the NOAA ETL design are now operating at the DOE ARM Southern Great Plains central facility in central Oklahoma and the DOE ARM North Slope of Alaska site near Barrow, Alaska. Operational systems are expected to come online within the next year at the DOE ARM tropical western Pacific sites located at Manus, Papua New Guinea, and Nauru. In order for these radars to detect the full range of atmospheric hydrometeors, specific modes of operation must be implemented on them that are tuned to accurately detect the reflectivities of specific types of hydrometeors. The set...

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 ...

Daytime Global Cloud Typing from AVHRR and VIIRS: Algorithm Description, Validation, and Comparisons

Abstract Three multispectral algorithms for determining the cloud type of previously identified cloudy pixels during the daytime, using satellite imager data, are presented. Two algorithms were developed for use with 0.65-, 1.6-/3.75-, 10.8-, and 12.0-μm data from the Advanced Very High Resolution Radiometer (AVHRR) on board the National Oceanic and Atmospheric Administration (NOAA) operational polar-orbiting satellites. The AVHRR algorithms are identical except for the near-infrared data that are used. One algorithm uses AVHRR channel 3a (1.6 μm) reflectances, and the other uses AVHRR channel 3b (3.75 μm) reflectance estimates. Both of these algorithms are necessary because the AVHRRs on NOAA-15 through NOAA-17 have the capability to transmit either channel 3a or 3b data during the day, whereas all of the other AVHRRs on NOAA-7 through NOAA-14 can only transmit channel 3b data. The two AVHRR cloud-typing schemes are used operationally in NOAA’s extended Clouds from AVHRR (CLAVR)-x processing system. The ...

Cloud water contents and hydrometeor sizes during the FIRE Arctic Clouds Experiment

During the year-long Surface Heat Budget of the Arctic Experiment (1997- 1998) the NOAA Environmental Technology Laboratory operated a 35-GHz cloud radar and the DOE Atmospheric Radiation Measurement Program operated a suite of radiometers at an ice station frozen into the drifting ice pack of the Arctic Ocean. The NASA/FIRE Arctic Clouds Experiment took place during April-July 1998, with the primary goal of investigating cloud microphysical, geometrical, and radiative properties with aircraft and surface-based measurements. In this paper, retrieval techniques are utilized which combine the radar and radiometer measurements to compute height- dependent water contents and hydrometeor sizes for all-ice and all-liquid clouds. For the spring and early summer period, all-ice cloud retrievals showed a mean particle diameter of about 60 mm and ice water contents up to 0.1 g/m 3 , with the maximum sizes and water contents at approximately one fifth of the cloud depth from the cloud base. The all-liquid cloud retrievals had a mean effective particle radius of 7.4 mm, liquid water contents up to 0.7 g/m 3 , and a mean droplet concentration of 54 cm 23 . Maximum retrieved liquid drop sizes, water contents, and concentrations occurred at three fifths of the cloud depth from the cloud base. As a measure of how representative the FIRE-ACE aircraft flight days were of the April-July months in general, retrieval statistics for flight-day clouds are compared to the mean retrieval statistics. From the retrieval perspective the ice particle sizes and water contents on flight days were ;30% larger than the mean retrieved values for the April-July months. Retrieved liquid cloud parameters during flight days were all about 20% smaller. All-ice and/or all-liquid clouds acceptable for these retrieval techniques were observed about 34% of the time clouds were present; at all other times, mixed-phase clouds precluded the use of these single-phase retrieval techniques.

A Method for Determining Cirrus Cloud Particle Sizes Using Lidar and Radar Backscatter Technique

Abstract A method to determine cirrus cloud effective radii remotely using lidar and radar backscatter data is presented. The difference in backscattered returns from instruments widely separated in wavelength holds information on the characteristic sizes of the scatterers. The method compares theoretically expected backscatter coefficients to observed backscatter returns from NOAAs 3.2-cm and 8.6-mm radars and the 10.6-µm lidar. Measurements were taken during a two-phase cloud experiment held in northeastern Colorado from 6 September to 5 October 1989 and 15 February to 31 March 1991. It was found that the particle sizes estimated from the lidar-radar method agree closely with in situ aircraft measurements. Case studies are presented to demonstrate the method and the potential for multiwavelength remote sensing of cirrus cloud radiative properties.