Kenneth Sassen

THE CLOUDSAT MISSION AND THE A-TRAIN A New Dimension of Space-Based Observations of Clouds and Precipitation

CloudSat is a satellite experiment designed to measure the vertical structure of clouds from space. The expected launch of CloudSat is planned for 2004, and once launched, CloudSat will orbit in formation as part of a constellation of satellites (the A-Train) that includes NASAs Aqua and Aura satellites, a NASA–CNES lidar satellite (CALIPSO), and a CNES satellite carrying a polarimeter (PARASOL). A unique feature that CloudSat brings to this constellation is the ability to fly a precise orbit enabling the fields of view of the CloudSat radar to be overlapped with the CALIPSO lidar footprint and the other measurements of the constellation. The precision and near simultaneity of this overlap creates a unique multisatellite observing system for studying the atmospheric processes essential to the hydrological cycle. The vertical profiles of cloud properties provided by CloudSat on the global scale fill a critical gap in the investigation of feedback mechanisms linking clouds to climate. Measuring these profi...

Asian dust events of April 1998

On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low-pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia. The April 19 dust cloud crossed the Pacific Ocean in 5 days, subsided to the surface along the mountain ranges between British Columbia and California, and impacted severely the optical and the concentration environments of the region. In east Asia the dust clouds increased the albedo over the cloudless ocean and land by up to 10–20%, but it reduced the near-UV cloud reflectance, causing a yellow coloration of all surfaces. The yellow colored backscattering by the dust eludes a plausible explanation using simple Mie theory with constant refractive index. Over the West Coast the dust layer has increased the spectrally uniform optical depth to about 0.4, reduced the direct solar radiation by 30–40%, doubled the diffuse radiation, and caused a whitish discoloration of the blue sky. On April 29 the average excess surface-level dust aerosol concentration over the valleys of the West Coast was about 20–50 μg/m3 with local peaks >100 μg/m3. The dust mass mean diameter was 2–3 μm, and the dust chemical fingerprints were evident throughout the West Coast and extended to Minnesota. The April 1998 dust event has impacted the surface aerosol concentration 2–4 times more than any other dust event since 1988. The dust events were observed and interpreted by an ad hoc international web-based virtual community. It would be useful to set up a community-supported web-based infrastructure to monitor the global aerosol pattern for such extreme aerosol events, to alert and to inform the interested communities, and to facilitate collaborative analysis for improved air quality and disaster management.

The polarization lidar technique for cloud research - A review and current assessment

Abstract The development of the polarization lidar field over the past two decades is reviewed, and the current cloud-research capabilities and limitations are evaluated. Relying on fundamental scattering principles governing the interaction of polarized laser light with distinctly shaped hydrometers, this remote-sensing technique has contributed to our knowledge of the composition and structure of a variety of cloud types. For example, polarization lidar is a key component of current climate-research programs to characterize the properties of cirrus clouds, and is an integral part of multiple remote-sensor studies of mixed-phase cloud systems, such as winter mountain storms. Although unambiguous cloud-phase discrimination and the identification of some ice particle types and orientations are demonstrated capabilities, recent theoretical approaches involving ice crystal ray-tracing and cloud microphysical model simulations are, promising to increase the utility of the technique. New results simulating the...

Subvisual-Thin Cirrus Lidar Dataset for Satellite Verification and Climatological Research

Abstract A polarization (0.694-μm wavelength) lidar dataset for subvisual and thin (bluish-colored) cirrus clouds is drawn from Project FIRE (First ISCCP Regional Experiment) extended time observations. The clouds are characterized by their day–night visual appearance; base, top, and optical midcloud heights and temperatures; measured physical (ΔZ) and estimated optical (τc) cloud thicknesses; integrated linear depolarization ratios; and derived k/2η ratios. A subset of the data supporting 30 NOAA polar-orbiting satellite overpasses is given in tabular form to provide investigators with the means to test cloud retrieval algorithms and establish the limits of cirrus detectability from satellite measurements under various conditions. Climatologically, subvisual–thin cirrus appear to be higher, colder, and more strongly depolarizing than previously reported midlatitude cirrus, although similar k/2η that decrease with height and temperature are found. A class of radiatively distinct subvisual cirrus with k/2η...

The Mixed-Phase Arctic Cloud Experiment

The Mixed-Phase Arctic Cloud Experiment (M-PACE) was conducted from 27 September through 22 October 2004 over the Department of Energys Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) on the North Slope of Alaska. The primary objectives were to collect a dataset suitable to study interactions between microphysics, dynamics, and radiative transfer in mixed-phase Arctic clouds, and to develop/evaluate cloud property retrievals from surface-and satellite-based remote sensing instruments. Observations taken during the 1977/98 Surface Heat and Energy Budget of the Arctic (SHEBA) experiment revealed that Arctic clouds frequently consist of one (or more) liquid layers precipitating ice. M-PACE sought to investigate the physical processes of these clouds by utilizing two aircraft (an in situ aircraft to characterize the microphysical properties of the clouds and a remote sensing aircraft to constraint the upwelling radiation) over the ACRF site on the North Slope of Alaska. The measureme...

Global distribution of cirrus clouds from CloudSat/Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) measurements

[1] The cirrus clouds of the upper troposphere are globally widespread and are important regulators of the radiative balance, and hence climate, of the Earth-atmosphere system. Despite their wide distribution, however, cirrus are difficult to study from satellite radiance measurements or from scattered ground observing sites because they can occur as part of multilayered cloud systems and are characteristically optically thin. The need to better characterize the global distribution of cirrus clouds was therefore a major justification for the formation flying of the CloudSat and CALIPSO satellites, which support a cloud radar and polarization lidar, respectively. Measurements by these active remote sensors, when analyzed by appropriate algorithms, have the ability to identify and accurately measure the locations and heights of this category of clouds. The combined CloudSat/ Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data cirrus cloud algorithm used in this study is aimed at identifying those clouds that would likely be classified as cirrus by a surface weather observer: it is based on previous experience with multiple remote sensor approaches and knowledge gleaned from extensive surface lidar and radar observations of visually identified cirrus clouds with a minimum of a priori assumptions. We report on the global and seasonal frequencies of cirrus clouds, and on their heights and thicknesses obtained over the initial 1 year of data collected. We find a global average frequency of cirrus cloud occurrence of 16.7%. These new results are compared with other cirrus cloud climatologies and are interpreted in terms of local cirrus cloud formation mechanisms and the responsible global weather phenomena.

Cloud Type and Macrophysical Property Retrieval Using Multiple Remote Sensors

A cloud detection algorithm based on ground-based remote sensors has been developed that can differentiate among various atmospheric targets such as ice and water clouds, virga, precipitation, and aerosol layers. Standard cloud type and macrophysical properties are identified by combining polarization lidar, millimeter-wave radar, infrared radiometer, and dual-channel microwave radiometer measurements. These algorithms are applied to measurements collected during 1998 from the Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed site in north-central Oklahoma. The statistical properties of clouds for this year are presented, illustrating how extended-time remote sensing datasets can be converted to cloud properties of concern to climate research.

Homogeneous Nucleation Rate for Highly Supercooled Cirrus Cloud Droplets

Abstract A mixed-phase hydrometer growth model has been applied to determining the nucleation mode and rate responsible for the glaciation of a highly supercooled liquid cloud studied jointly by ground-based polarization lidar and aircraft in situ probes. The cloud droplets were detected at the base of an orographically induced cirrus cloud at temperatures between −34.3° and −37.3°C. The vertical distribution above cloud base of two independent data quantities, the aircraft-measured water and ice particle concentrations and the lidar linear depolarization ratio, have been compared to model predictions for both the homogeneous and heterogeneous drop-freezing. modes. It is concluded that, although activated ice nuclei may have contributed to the glaciation of the cloud, homogeneous nucleation was the dominant mode. Accordingly, a homogeneous nucleation rate ∼106 times greater than that predicted by classical theory, but ∼103 times less than laboratory measurements would suggest is found to be appropriate at...

A Midlatitude Cirrus Cloud Climatology from the Facility for Atmospheric Remote Sensing. Part II: Microphysical Properties Derived from Lidar Depolarization

In Part II of this series of papers describing the results of the extended time observations of cirrus clouds from the University of Utah Facility for Atmospheric Remote Sensing (FARS), the information content of laser backscatter depolarization measurements in terms of cloud microphysical content is treated. The authors rely on scattering principles indicating that polarization lidar can be applied to identifying cloud phase, and describing ice particle shape and orientation. It is found that 0.694- mm lidar linear depolarization ratios d obtained in the zenith display a steady increase with height. With respect to temperature, a minimum of d 5 0.25 is found at 217.58C, where horizontally oriented planar ice crystals are to be expected, and the d increase up to 0.45 at 277.58C. This trend indicates a basic transition in cirrus ice crystal shape with temperature, likely reflecting not only the effects of crystal axis ratio (i.e., plate-to-column) but also internal and radial crystallographic features. Evidence for transient supercooled liquid clouds embedded in cirrus is found at temperatures generally .2208C. Off-zenith lidar data show that the effects of oriented plate crystals in lowering d are widespread in cirrus, especially at temperatures .2458C. Comparisons with other lidar studies are made, and it is concluded that the depolarization data reveal fundamental distinctions in cirrus cloud particle properties that vary with temperature and probably geographical location. It is important to understand such variations in order to improve the ability to model the effects of cirrus clouds on climate.

A Midlatitude Cirrus Cloud Climatology from the Facility for Atmospheric Remote Sensing. Part III: Radiative Properties

Abstract In Part III of a series of papers describing the extended time high-cloud observations from the University of Utah Facility for Atmospheric Remote Sensing (FARS) supporting the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment, the visible and infrared radiative properties of cirrus clouds over Salt Lake City, Utah, are examined. Using ∼860 h of combined ruby (0.694 μm) lidar and midinfrared (9.5–11.5 μm) radiometer data collected between 1992 and 1999 from visually identified cirrus clouds, the visible optical depths τ and infrared layer emittance ϵ of the varieties of midlatitude cirrus are characterized. The mean and median values for the cirrus sample are 0.75 ± 0.91 and 0.61 for τ, and 0.30 ± 0.22 and 0.25 for ϵ. Other scattering parameters studied are the visible extinction and infrared absorption coefficients, and their ratio, and the lidar backscatter-to-extinction ratio, which has a mean value of 0.041 sr−1. Differences among cirrus clouds generated by g...