Jack B. Snider

The albedo of fractal stratocumulus clouds

Abstract An increase in the planetary albedo of the earth-atmosphere system by only 10% can decrease the equilibrium surface temperature to that of the last ice age. Nevertheless, albedo biases of 10% or greater would be introduced into large regions of current climate models if clouds were given their observed liquid water amounts, because of the treatment of clouds as plane parallel. Past work has addressed the effect of cloud shape on albedo; here the focus is on the within-cloud variability of the vertically integrated liquid water. The main result is an estimate of the “plane-parallel albedo bias” using the “independent pixel approximation,” which ignores net horizontal photon transport, from a simple fractal model of marine stratocumulus clouds that ignores the cloud shape. The use of the independent pixel approximation in this context will be justified in a separate Monte Carlo study. The focus on marine stratocumulus clouds is due to their important role in cloud radiative forcing and also that, o...

Measurement of Stratus Cloud and Drizzle Parameters in ASTEX with a Kα-Band Doppler Radar and a Microwave Radiometer

Abstract Data are used from a Kα-band radar and microwave radiometer along with a droplet model to determine the droplet parameters of drizzle and clouds. Drizzle droplet parameters are determined from the zeroth, first, and second moments of the Doppler spectrum. Cloud droplet parameters are determined from the zeroth moment of the Doppler spectrum and the measured integrated liquid water. Measurements of stratus clouds were made during the Atlantic Stratocumulus Transition Experiment (ASTEX) on the island of Porto Santo in the Madeira Islands, Portugal. Potential applications of this technique would be in the long-term monitoring of stratus clouds and in determining the vertical profiles of cloud liquid water, number of cloud droplets, and modal radius.

Stratocumulus Cloud Properties Derived from Simultaneous Satellite and Island-based Instrumentation during FIRE

Abstract Cloud parameters derived from visible and infrared window data from the Geostationary Operational Environmental Satellite (GOES) are compared to corresponding properties determined from instrumentation on San Nicolas Island off the coast of California during the First ISCCP Regional Experiment (FIRE) marine stratocumulus intensive field observations period in July 1987. Mean cloud amounts derived by applying the hybrid bispectral threshold method to the GOES data were 5% less than the island ceilometer measurements. Examination of the satellite imagery revealed that the apparent bias can be explained by the persistence of the clouds over the northwest part of the island during periods of clearing around the island. Diurnal variations in the cloud cover were very significant; minimum cloudiness occurred during the late afternoon and maximum cloudiness early in the morning. The satellite retrievals track this variation quite well. Satellite-derived mean cloud-top height is 53 m lower than that obse...

Cloud physics studies with 8 mm wavelength radar

Abstract Results from recent cloud experiments with an 8 mm wavelength Doppler radar demonstrate that millimeter wavelength radar can provide important new information about nonprecipitating and lightly precipitating clouds. Millimeter wave radar can be used to document small-scale spatial structure of cirrus and marine stratus clouds. Its data can be used to estimate profiles of ice content, particle size and concentration in cirrus clouds and profiles of liquid water content and turbulence in marine stratus clouds. New results with mm wave radar suggest that plate-like crystals may be distinguishable from aggregates with polarization techniques. Quantitative information about cirrus cloud ice crystal fallspeeds, and therefore ice mass flux, can also be produced with newly developed techniques that exploit the sensitivity and velocity precision of such radars. The good sensitivity to cloud particles, the immunity from ground clutter contamination, and the good spatial resolution of millimeter wavelength radar make it an excellent instrument for documenting quantitative microphysical and dynamical properties of non-precipitating and lightly precipitating clouds.

Estimation of ice cloud parameters from ground‐based infrared radiometer and radar measurements

A technique is presented to estimate ice cloud particle characteristic sizes and concentrations as well as the integrated ice water path from simultaneous ground-based radar and infrared radiometer measurements. The approach is based on the theoretical consideration of infrared thermal radiative transfer within a cloud and can be applied to clouds that are semitransparent in the infrared “window” and horizontally extensive. The suggested technique is applied to radar and infrared radiometer data collected during the Cloud Lidar and Radar Exploratory Test (CLARET-I) experiment. Retrieved values of ice cloud microphysical parameters are in general agreement with results obtained by other methods.

Influences of Storm-Embedded Orographic Gravity Waves on Cloud Liquid Water and Precipitation

Abstract This study illustrates opportunities for much improved orographic quantitative precipitation forecasting, determination of orographic cloud seedability, and flash flood prediction through state-of-the-art remote sensing and numerical modeling of gravity wave clouds. Wintertime field observations with multiple remote sensors, corroborated in this and related papers with a mesoscale–cloud scale numerical simulation, confirm that storm-embedded gravity waves can have a strong and persistent influence on orographic cloud liquid water (CLW) and precipitation. Where parallel mountain ridges dominate the landscape, an upwind ridge can force the wave action, and a downwind ridge can receive the precipitation. The 1995 Arizona Program was conducted in such terrain. In the scenario examined, traveling waves cyclically caused prefrontal cross-barrier winds that produced gravity waves. Significant cloud bands associated with the waves carried substantial moisture to the area. With the passage and waning of t...

A Remote-Sensing View of a Freezing-Rain Storm

Abstract A destructive freezing-rain storm on 15 February 1990 was observed intensively with advanced ground-based remote sensors and conventional instruments by the Lake Ontario Winter Storms (LOWS) project in upstate New York. A deep layer of warm, moist, southwesterly flow overran a shallower layer of subfreezing, easterly flow ahead of a surface warm front. Precipitation at the surface changed from snowfall to ice pellets, to freezing rain, and, finally, to ordinary rain as an elevated layer of above-freezing air moved into the region and eventually extended to the ground. Measurements from a scanning Doppler radar, wind profilers, a microwave radiometer, and mobile rawinsondes provided detailed information on the storms kinematic and thermodynamic structure and evolution, and allowed its basic microphysical structure to be inferred. The remote sensors detected signatures of the melting aloft that may be useful for improving detection and forecasts of freezing-rain hazards.

The Potential of 8-mm Radars for Remotely Sensing Cloud Drop Size Distributions

Abstract This paper describes the use of a vertically pointing 8.6-mm-wavelength Doppler radar for measuring drop size spectra in clouds. The data used were collected in the Atlantic Stratocumulus Transition Experiment in 1992. This paper uses the full Doppler velocity spectrum from the time series of Doppler radial velocities to extract information farther into the small-drop regime than previously attempted. The amount of liquid residing in the cloud regime is compared with that found in the precipitation regime where drop fall velocities are resolvable. Total liquid is compared with that measured with a collocated three-channel microwave radiometer. Examples of number density spectra, liquid water spectra, and flux spectra are shown and compared with what is known of these quantities from various in situ measurements by aircraft in similar clouds. Error estimates and uncertainties are discussed. It is concluded that 8-mm Doppler radars have the potential for broader use in cloud and precipitation studi...

Liquid Water Path and Plane-Parallel Albedo Bias during ASTEX

Abstract Inhomogeneous distributions of liquid water like those observed in real clouds generally reflect less solar radiation than idealized uniform distributions assumed in plane-parallel theory. Here the authors determine cloud reflectivity and the associated plane-parallel albedo bias from distributions of liquid water path derived from 28 days of microwave radiometer measurements obtained on Porto Santo Island in the Madeiras during June 1992 as part of the Atlantic Stratocumulus Transition Experiment (ASTEX). The distributions are determined for each hour of the day, both for composites of the full act of 28 days and for a subset of 8 days having a high fraction of relatively thick cloud. Both sets are compared with results obtained from California stratocumulus during FM [First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment]. In FIRE the albedo bin was dominated by variability of the cloud optical depth, as measured by a fractal parameter, 0≤ f 0 ≤ 1, while the ASTEX ...

Investigations of a Winter Mountain Storm in Utah. Part II: Mesoscale Structure, Supercooled Liquid Water Development, and Precipitation Processes

Abstract A comprehensive analysis of a deep winter storm system during its passage over the Tushar Mountains of southwestern Utah is reported. The case study, drawn from the 1985 Utah/NOAA cooperative weather modification experiment, is divided into descriptions of the synoptic and kinematic properties in Part I, and storm structure and composition here in Part II. In future parts of this series, the turbulence structure and indicated cloud seeding potential will be evaluated. The analysis presented here in Part II focuses on multiple remote sensor and surface microphysical observations collected from a midbarrier (2.57 km MSL) field site. The collocated remote sensors were a dual-channel microwave radiometer, a polarization lidar, and a Ka-band Doppler radar. These data are supplemented by upwind, valley-based C-band Doppler radar observations, which provided a considerably larger-scale view of the storm. In general, storm properties above the barrier were either dominated by barrier-level orographic clo...