Search Results

You are looking at 1 - 10 of 12 items for

  • Author or Editor: Jack B. Snider x
  • Refine by Access: All Content x
Clear All Modify Search
Allen B. White, C. W. Fairall, and Jack B. Snider

Abstract

Surface-based measurements are used to define some of the important macrophysical and optical properties of marine clouds. These measurements were taken during five different marine field programs. A progression is made from a midlatitude marine stratocumulus regime with an average cloud fraction of 0.7 and a median cloud base of 460 m to a marine tropical regime with an average cloud fraction of 0.2 and a median cloud base of 1050 m. Measurements of the solar transmission coefficient taken during the Atlantic Stratocumulus Transition Experiment (ASTEX) were used in a radiative transfer algorithm to produce values of albedo, absorption, and optical depth. A microwave radiometer provided measurements of the liquid water path (LWP). For a given LWP, the ASTEX optical depths averaged a factor of 2 smaller than the optical depths observed during the marine stratocumulus phase of the First International Cloud Climatology Program Regional Experiment (FIRE) at San Nicolas Island, off the coast of southern California. The variability of boundary-layer aerosol concentrations measured during ASTEX is sufficient to produce a factor of 2 change in optical depth. Further evidence suggests that the cloud droplet effective radius was nearly a factor of 2 larger during ASTFX than during FIRE.

Full access
Steven M. Gollmer, Harshvardhan, Robert F. Cahalan, and Jack B. Snider

Abstract

To improve radiative transfer calculations for inhomogeneous clouds, a consistent means of modeling inhomogeneity is needed. One current method of modeling cloud inhomogeneity is through the use of fractal parameters. This method is based on the supposition that cloud inhomogeneity over a large ranges of scales is related. An analysis technique named wavelet analysis provides a means of studying the multiscale nature of cloud inhomogeneity. In this paper, the authors discuss the analysis and modeling of cloud inhomogeneity through the use of wavelet analysis.

Wavelet analysis as well as other windowed analysis techniques are used to study liquid water path (LWP) measurements obtained during the marine stratocumulus phase of the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment. Statistics obtained using analysis windows, which are translated to span the LWP dataset, are used to study the local (small scale) properties of the cloud field as well as their time dependence. The LWP data are transformed onto an orthogonal wavelet basis that represents the data as a number of times series. Each of these time series lies within a frequency band and has a mean frequency that is half the frequency of the previous band. Wavelet analysis combined with translated analysis windows reveals that the local standard deviation of each frequency band is correlated with the local standard deviation of the other frequency bands. The ratio between the standard deviation of adjacent frequency bands is 0.9 and remains constant with respect to time. This ratio defined as the variance coupling parameter is applicable to all of the frequency bands studied and appears to be related to the slope of the data's power spectrum.

Similar analyses are performed on two cloud inhomogeneity models, which use fractal-based concepts to introduce inhomogeneity into a uniform cloud field. The bounded cascade model does this by iteratively redistributing LWP at each scale using the value of the local mean. This model is reformulated into a wavelet multiresolution framework, thereby presenting a number of variants of the bounded cascade model. One variant introduced in this paper is the “variance coupled model”, which redistributes LWP using the local standard deviation and the variance coupling parameter. While the bounded cascade model provides an elegant two parameter model for generating cloud inhomogeneity, the multiresolution framework provides more flexibility at the expense of model complexity. Comparisons are made with the results from the LWP data analysis to demonstrate both the strengths and weaknesses of these models.

Full access
Robert F. Cahalan, David Silberstein, and Jack B. Snider

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 results are more complex. Mean cloud fraction above a 10 g m−1 threshold is about 50% in the 28-day set, compared to 76% in the 8-day subset and 82% in FIRE. Cloud fraction is sensitive to the threshold for the 28 ASTEX days, probably due to a large fraction of thin cloud below the threshold, but this is not the case for the 8-day subset or for FIRE. Clear fractions during ASTEX are generally of shorter duration than those in FIRE, as are those in the 8-day subset. The diurnal mean fractal parameter is about 0.6 in ASTEX compared to 0.5 in FIRE, while the 8-day subset has nearly the same mean but a wider range. The diurnal cycle in cloud albedo mid and albedo bias is computed from the cloud parameters for both sets, assuming zero clear-sky albedo. The total absolute albedo bias rises to values above 0.3 at sunrise and sunset, but since there is little incident energy at that time, the reflected flux is more affected by the midday bias. The total albedo bias has a 10OO LST maximum of about 0.3, largely due to a cloud fraction contribution of 0.2, absent in FIRE because in that case cloud frontier remains near 100% until after 1000 LST. The albedo bias has a second maximum of about 0.2 at noon, again mainly from cloud fraction and then drops to a minimum of about 0.1 at 1400 LST, when cloud fraction and fractal structure contribute about equally. Finally, a third maximum due to cloud fraction occurs at 1600 LST.

In the, 8-day subset the 1000 LST maximum becomes dominated by the frontal structure, since the cloud fraction remains near 100% until 1000 LST, as in FIRE. The noon maximum receives roughly equal contributions, while the 1400 LST minimum bias is mainly due to fractal structure. Finally, the 1600 LST maximum and the evening limb bias are similar to those of the full 28-day set. These results show lids cloud fractal and radiative properties can vary considerably from one site and time to another mid at different times within the same site, as meterological conditions change.

Full access
Roger F. Reinking, Jack B. Snider, and Janice L. Coen

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 the cloud bands, vapor influxes (precipitable water P w) cycled through large changes in magnitude, and prefrontal peaks in P w coincided with the gravity waves in a succession of episodes during a five-day period. Thus, the cyclic trend in P w and the magnitudes of peak P w were simple indicators of wave cloud development. The first two cycles, with minor peak P w, were precursors. Significant wave clouds first appeared during the second episode. During the final two episodes with large vapor influxes, very deep, precipitating wave clouds were coupled with underlying clouds formed in flow up the mountain slopes to create the prefrontal storms. Rain fell on an existing snowpack on the main recipient ridge and, in the end, produced rapid runoff and flash flooding.

The gravity waves persistently condensed CLW that averaged 0.5 mm and reached 1.0 mm in the first of the main storm episodes, and averaged 1.0 mm and reached 2.0 mm and more in the second (column-integrated values). These values equaled or exceeded the larger of those represented in liquid water climate datasets for orographic cloud systems in other locations in the West, where only the upslope and not the wave component had been examined. The effect of shifts between cross-barrier and barrier-parallel flows was reflected in abrupt buildups and declines in wave CLW, but the gravity wave clouds persisted for a total of 22 h during the two storm periods. In the wave updrafts, the condensation rate regularly exceeded the consumption rate by ice, even though ice was usually present. Conversion to ice consumed and precipitated wave CLW. Pulses of available P w and wave CLW on a 2- to 4-h timescale, cyclically followed by partial glaciation, produced the precipitation from the wave clouds. Their seeder effect on the upslope feeder clouds was to enhance the total precipitation from the coupled system. Estimates of the liquid water fluxes in comparison with the precipitation rates suggest precipitation efficiencies in the 11%–33% range from the seeder–feeder couplets. The periods of gravity wave forcing contributed some 80% or more of the total precipitation, and trailing fronts produced the remainder.

Several factors derived from the observed availability of CLW determine the potential for precipitation enhancement by seeding wave clouds; these are enumerated. Given demands for improved water supply, the challenge often presented in mountain watersheds of separating seeding opportunities from potential flash flood situations is examined. The results here show that storms that could threaten flash floods can be readily identified by continuous monitoring with polarization radar and in real-time simulations as those with the altitude of the melting level above the elevation of the highest terrain with existing snowpack.

In the sense that orographically generated gravity waves will significantly influence cloud water and precipitation, geographic transferability of the results is indicated by the existence of wave-generating and precipitation-generating parallel ridges in many places throughout the world. The quantitative effects will, of course, depend on particulars of the locale such as nature of the prevalent forcing, available moisture, and physical stature of the ridges.

Full access
Kenneth Sassen, Arlen W. Huggins, Alexis B. Long, Jack B. Snider, and Rebecca J. Meitín

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 clouds or propagating mesoscale cloud systems. The orographic cloud component consisted of weakly (−3° to −10°C) supercooled liquid water (SLW) clouds in the form of an extended barrier-wide cap cloud that contained localized SLW concentrations. The spatial SLW distribution was linked to topographical features surrounding the midbarrier site, such as abrupt terrain rises and nearby ridges. This orographic cloud contributed to precipitation primarily through the riming of particles sedimenting from aloft, and also to some extent through an ice multiplication process involving graupel growth. In contrast, mesoscale precipitation bands associated with a slowly moving cold front generated much more significant amounts of snowfall. These precipitation bands periodically disrupted the shallow orographic SLW clouds. Mesoscale vertical circulations appear to have been particularly important in SLW and precipitation production along the leading edges of the bands. Since the SLW clouds during the latter part of the storm were based at the frontal boundary, SLW and precipitation gradually diminished as the barrier became submerged under the cold front.

Based on a winter storm conceptual model, we conclude that low-level orographic SLW clouds, when decoupled from the overlying ice cloud layers of the storm, are generally inefficient producers of precipitation due to the typically warm temperatures at these altitudes in our region.

Full access
Joseph A. Shaw, Jack B. Snider, James H. Churnside, and Mark D. Jacobson

Abstract

Increased interest in using atmospheric brightness temperature measurements from simple infrared radiometers combined with radars and lidars has prompted the investigation of their accuracy for various sky conditions. In comparisons of atmospheric brightness temperatures (Tb) measured by a Fourier transform infrared (FTIR) spectrometer and a single-band filter radiometer (PRT5), the authors establish that the PRT5 measurements agree with those from the more sophisticated and accurate FTIR within 1.5 K rms over the range where both instruments are calibrated. Below the PRT5's cold calibration cutoff of 205 K, the PRT5 measures too warm. The FTIR, which is calibrated over the entire measurement range, provides a calibration for the erroneous PRT5 measurements, enabling quantitative use of the simple and inexpensive PRT5 over a larger, more useful range. The corrected data agree within 1.5 K rms, with over 90% differing by less than one temporal standard deviation. The calibration correction technique is applicable to a variety of radiometers and most importantly shows that PRT5-type radiometers are indeed capable of accurately measuring clear-sky and cirrus emission.

Full access
Robert F. Cahalan, William Ridgway, Warren J. Wiscombe, Thomas L. Bell, and Jack B. Snider

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, of the wide variety of earth's cloud types, they are most nearly plane parallel, so that they have the least albedo bias. The fractal model employed here reproduces both the probability distribution and the wavenumber spectrum of the stratocumulus liquid water path, as observed during the First ISCCP Regional Experiment (FIRE). The model distributes the liquid water by a cascade process, related to the upscale cascade of energy transferred from the cloud thickness scale to the mesoscale by approximately 2D motions. For simplicity, the cloud microphysical parameters are assumed homogeneous, as is the geometrical cloud thickness; and the mesoscale-averaged vertical optical thickness is kept fixed at each step of the cascade. A single new fractal parameter, 0 ≤ f ≤ 1, is introduced and determined empirically by the variance of the logarithm of the vertically integrated liquid water. In the case of conservative scattering, the authors are able to estimate the albedo bias analytically as a function of the fractal parameter f, mean vertical optical thickness Tν, and sun angle θ. Typical observed values are f = 0.5, Tν = 15, and θ = 60°, which give an absolute bias of 0.09, or a relative bias equal to 15% of the plane-parallel albedo of 0.60. The reduced reflectivity of fractal stratocumulus clouds is approximately given by the plane-parallel reflectivity evaluated at a reduced “effective optical thickness,” which when f = 0.5 is T eff ≈ 10.

Study of the diurnal cycle of stratocumulus liquid water during FIRE leads to a key unexpected result: the plane-parallel albedo bias is largest when the cloud fraction reaches 100%, that is, when any bias associated with the cloud fraction vanishes. This is primarily due to the variability increase with cloud fraction. Thus, the within-cloud fractal structure of stratocumulus has a more significant impact on estimates of its mesoscale-average albedo than does the cloud fraction.

Full access
Taneil Uttal, Sergey Y. Matrosov, Jack B. Snider, and Robert A. Kropfli

Abstract

A vertically pointing 3.2-cm radar is used to observe altostratus and cirrus clouds as they pass overhead. Radar reflectivities are used in combination with an empirical Zi-IWC (ice water content) relationship developed by Sassen (1987) to parameterize IWC, which is then integrated to obtain estimates of ice water path (IWP). The observed dataset is segregated into all-ice and mixed-phase periods using measurements of integrated liquid water paths (LWP) detected by a collocated, dual-channel microwave radiometer. The IWP values for the all ice periods are compared to measurements of infrared (IR) downward fluxes measured by a collocated narrowband (9.95 − 11.43 um) IR radiometer, which results in scattergrams representing the observed dependence of [R fluxes on IWP. A two-strum model is used to calculate the infrared fluxes expected from ice clouds with boundary conditions specified by the actual clouds, and similar curves relating IWP and infrared fluxes are obtained. The model and observational results suggest that IWP is one of the primary controls on infrared thermal fluxes for ice clouds.

Full access
Brooks E. Martner, Jack B. Snider, Robert J. Zamora, Gregory P. Byrd, Thomas A. Niziol, and Paul I. Joe

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 storm's 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.

Full access
Sergey Y. Matrosov, Andrew J. Heymsfield, Robert A. Kropfli, Brooks E. Martner, Roger F. Reinking, Jack B. Snider, Paivi Piironen, and Edwin W. Eloranta

Abstract

Ice cloud microphysical parameters derived from a remote sensing method that uses ground-based measurements from the Environmental Technology Laboratory’s Ka-band radar and an IR radiometer are compared to those obtained from aircraft sampling for the cirrus priority event from the FIRE-II experiment. Aircraft cloud samples were taken not only by traditional two-dimensional probes but also by using a new video sampler to account for small particles. The cloud parameter comparisons were made for time intervals when aircraft were passing approximately above ground-based instruments that were pointed vertically. Comparing characteristic particle sizes expressed in terms of median mass diameters of equal-volume spheres yielded a relative standard deviation of about 30%. The corresponding standard deviation for the cloud ice water content comparisons was about 55%. Such an agreement is considered good given uncertainties of both direct and remote approaches and several orders of magnitude in natural variability of ice cloud parameters. Values of reflectivity measured by the radar and calculated from aircraft samples also showed a reasonable agreement; however, calculated reflectivities averaged approximately 2 dB smaller than those measured. The possible reasons for this small bias are discussed. Ground-based and aircraft-derived particle characteristic sizes are compared to those available from published satellite measurements of this parameter for the cirrus priority case from FIRE-II. Finally, simultaneous and collocated, ground-based measurements of visible (0.523 nm) and longwave IR (10–11.4 μm) ice cloud extinction optical thickness obtained during the 1995 Arizona Program are also compared. These comparisons, performed for different cloud conditions, revealed a relative standard deviation of less than 20%;however, no systematic excess of visible extinction over IR extinction was observed in the considered experimental events.

Full access