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At least 433 records · Page 24

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NASA NTRS1988

On the microphysics of microbursts

The effects on microbursts of precipitation loading, and cooling of the downdraft due to graupel/hail melting and rain evaporation, were studied, using numerical models. The results for the microburst index indicate a low value for the weak microburst and a higher value for the strong microburst. The relative magnitude of the various terms indicate the importance of the microphysical processes. For the dry microburst the evaporation and loading effects are comparable (1.56 and 1.75 respectively) but 5 to 6 times the magnitude of the melting effect. For a wet, tropical microburst, the loading and melting terms are most important. The intermediate Denver case shows loading as the largest term, followed by evaporation and melting.

Orville, H. D.↗

NASA NTRS1988

Theory on the uncertainty in cloud microphysical processes and climate

The interactive role of clouds and precipitation processes in a climate model is investigated. The basic model structure and parameterizations for condensation, evaporation, and precipitation are outlined. Results simulated from the cloud-climate model are discussed. The potential link between microphysical cloud processes and climate is theorized. The need to develop a climatology of the mean particle radius for clouds at the global scale, in order to understand the role of clouds in climate is stressed.

Liou, Kuo-Nan↗

NASA NTRS1988

Cirrus microphysics and infrared radiative transfer: A case study

Coincident measurements of cirrus cloud microphysical properties such as particle size distribution and particle shape and morphology, and measurements of infrared intensity and flux were made. Data was acquired nearly simultaneously in space and time by a KingAir in cloud and by an ER-2 at an altitude of 19 km. Upwelling infrared intensities and fluxes measured from the ER-2 and observations of cloud particle size distributions and particle phase and morphology made from the KingAir are discussed. Broad-band flux measurements were available both in and below the cirrus layer from the KingAir.

Ackerman, Thomas P.↗

NASA NTRS1989

Optical scattering and microphysical properties of subvisual cirrus clouds, and climatic implications

The optical and microphysical properties of subvisual cirrus clouds are derived from ground-based polarization lidar, shortwave radiation flux, and solar corona measurements of two approximately 0.75 km deep cirrus located near the tropopause. The first cloud produced no visual manifestations under excellent viewing conditions, and the second appeared to be a persistent aircraft contrail that was generally visible except in the zenith direction. Average lidar linear depolarization ratios and volume backscatter coefficients for the two clouds were 0.19 and 0.35, and 0.6 x 10 to the -3 and 1.4 x 10 to the -3 /km sr, respectively. It is estimated that the zenith-subvisual cirrus contained ice crystals of 25-micron effective diameter at a mean concentration of 25/1 and ice mass content of 0.2 mg/cu m. The threshold cloud optical thickness for visual-versus-invisible cirrus, derived from both broadband shortwave flux and 0.694 micrometer lidar data, is found to be tau sub c approx equal 0.03. Such tau values are comparable to those of 5 to 10 km deep stratospheric aerosol clouds of volcanic origin and polar stratospheric clouds, which are episodic in nature. Hence, we conclude that if these clouds are a fairly common feature of the upper troposphere, as recent SAGE satellite measurements would suggest, then the impact of natural and contrail subvisual cirrus on the planet's radiation balance may be relatively significant.

Sassen, Kenneth↗

NASA NTRS1989

The role of cloud microphysical processes in climate - An assessment from a one-dimensional perspective

A one-dimensional climate model with an interactive cloud formation and precipitation program is used to study the relationship between cloud microphysical processes and climate. It is shown that the rate of precipitation is directly proportional to the fourth power of the mean cloud particle radius. The effects of CO2 doubling on the climate are examined. It is suggested that smaller cloud droplets may be produced by condensation nuclei over the oceans, as a result of greenhouse warming and pollution over land. Larger cloud droplets could be caused by the removal of cloud-forming nuclei, resulting from enhance precipitation due to greenhouse perturbations.

Liou, Kuo-Nan↗

NASA NTRS1990

Cirrus microphysics and radiative transfer: Cloud field study on October 28, 1986

The radiative properties of cirrus clouds present one of the unresolved problems in weather and climate research. Uncertainties in ice particle amount and size and, also, the general inability to model the single scattering properties of their usually complex particle shapes, prevent accurate model predictions. For an improved understanding of cirrus radiative effects, field experiments, as those of the Cirrus IFO of FIRE, are necessary. Simultaneous measurements of radiative fluxes and cirrus microphysics at multiple cirrus cloud altitudes allows the pitting of calculated versus measured vertical flux profiles; with the potential to judge current cirrus cloud modeling. Most of the problems in this study are linked to the inhomogeneity of the cloud field. Thus, only studies on more homogeneous cirrus cloud cases promises a possibility to improve current cirrus parameterizations. Still, the current inability to detect small ice particles will remain as a considerable handicap.

Kinne, Stefan↗

NASA NTRS1990

Microphysical fundamentals governing cirrus cloud growth: Modeling studies

For application to Global Climate Models, large scale numerical models of cirrus cloud formation and maintenance need to be refined to more reliably simulate the effects and feedbacks of high level clouds. A key aspect is how ice crystal growth is initiated in cirrus, which has started a cloud microphysical controversy between camps either believing that heterogeneous or homogeneous drop freezing is predominantly responsible for cold cirrus ice crystal nucleation. In view of convincing evidence for the existence of highly supercooled cloud droplets in the middle and upper troposphere, however, it is concluded that active ice nuclei are rather scarce at cirrus cloud altitudes, and so a new understanding of cirrus cloud formation is needed. This understanding is sought through an examination of cirrus cloud growth models.

Sassen, Kenneth↗

NASA NTRS1990

The polar stratospheric cloud event of January 24, 1989. I - Microphysics

Rapid adiabatic cooling induced by synoptic forcing led to polar stratospheric cloud (PSC) formation on January 24, 1989, at altitudes sampled by the ER-2 aircraft. Particle characteristics measured by the Forward Scattering Spectrometer Probe (FSSP) on the ER-2 were compared to those calculated using a theoretical PSC microphysics model. Although calculations were sensitive to local changes in cooling rate, generally favorable agreement was found, that in particle surface area being especially important since this parameter dictates heterogeneous chemical rates. The overall model performance suggests that the current approach for simulating Type 1 (nitric acid trihydrate) PSC formation under rapid cooling conditions is well founded and can be used to study the effects of heterogeneous chemistry on stratospheric composition.

Poole, L. R.↗

NASA NTRS1990

Cloud microphysical properties from multispectral reflectance measurements

Images as well as statistical properties of the cloud optical thickness and effective particle radius derived from reflected solar radiation measurements at 0.75, 1.65, and 2.16 micron are presented. The remote-sensing-derived cloud optical thickness and effective radius are compared with the values inferred from nearly simultaneous in situ microphysical measurements obtained from an aircraft. A good spatial correlation is observed for both the optical thickness and effective radius, it is noted that the remote sensing has a tendency to overestimate the effective radius; the optical thickness is overestimated for small optical thicknesses and underestimated for large optical thicknesses. The marginal probability density functions of optical thickness and effective radius are derived from the remote-sensing results and discussed.

King, Michael D.↗

NASA NTRS1991

On the microphysical state of the surface of Triton

The microphysical processes involved in the pressureless sintering of particulate materials and the physical conditions likely to prevail on Triton are examined in order to investigate the processes leading to the frost metamorphism on Triton. It is argued that the presence of a well-annealed transparent nitrogen layer offers a natural explanation for most of what is seen on the surface of Triton; results of observations suggest that such a layer can form on Triton at 37 K on a seasonal time scale (about 100 earth years), provided the initial grain diameter is less than 1 micron. Grains up to 10 microns are allowed if grain growth does not hinder densification.

Eluszkiewicz, Janusz↗

NASA NTRS1990

Radiative transfer in cirrus clouds from airborne flux and microphysical measurements during FIRE 86

Microphysical and radiation measurements from three case studies during FIRE 86 are presented and analyzed. Calculated solar and infrared flux profiles are compared to measured flux values for the studies. The flux comparison shows that the modeled cirrus cloud underestimates cloud extinction, especially near the cloud base, where modeled extinctions are largest. The large downward solar attenuation near the cloud base cannot be matched, while the modeled solar reflection is too small. Most of this difference is attributed to an underestimation of cloud optical depth in the model. Modeling difficulties and difficulties in the measurements are discussed. It is proposed that, in order to assure a more useful flux comparison, field experiment set-ups and instrumentation as well as the modeling of cirrus clouds need to be improved.

Kinne, S.↗

NASA NTRS1989

Passive microwave remote sensing of precipitation from space - Impact of cloud microphysics

Numerical experiments to investigate the impact of time-dependent cloud microphysical structure on the transfer to space of passive microwave radiation are carried out, using a vertically and angularly detailed, plane-parallel radiative transfer model for unpolarized microwave radiation. The object is to explore the feasibility of multichannel passive-microwave retrieval techniques to estimate precipitation from space. Results on the time-dependent influence of large graupel-size ice particles on top-of-atmosphere brightness temperature - rainfall rate relationships at 6.6, 10.7, 18, 19.35, 22.235, 37, 89.5, 130, and 231 GHz are given. Frequency-dependent vertical profiles of a generalized brightness-temperature weighting function, introduced to vertically resolve the contributions by individual cloud and precipitation layers to the brightness temperatures, are also shown.

Mugnai, Alberto↗

NASA NTRS1993

On the microphysical state of the Martian seasonal caps

An analysis of the microphysical processes involved in pressureless sintering leads to the view that CO2 in the Martian seasonal polar caps can metamorphose into an annealed polycrystalline layer, if freshly condensed grains are smaller than 10 microns. Such large-area/low-porosity CO2 deposits may be associated with the mm-to-cm-scaled path lengths entailed by the explanation of several weak absorption features in the 1.5- and 2.3-micron spectral regions. Such an annealed layer would become transparent enough during the later stages of spring sublimation to establish a basis for the dichotomy between the north and south caps' albedos in the underlying substrate.

Eluszkiewicz, Janusz↗

NASA NTRS1993

Overview of microphysical and state parameter measurements from FIRE 2

In this article we present data collected by the NCAR King Air and Sabreliner aircraft in the FIRE 2 cirrus project over southeastern Kansas and northeastern Oklahoma in Nov. and Dec. of 1991. We present state parameter and microphysical measurements in summary form for the dates which have been selected by the FIRE Science Team for intensive analysis, 25 and 26 Nov. and 5 and 6 Dec. We will also evaluate the performance of 'key' aircraft instrumentation.

Heymsfield, Andrew J.↗

NASA NTRS1993

Investigation of the effects of the macrophysical and microphysical properties of cirrus clouds on the retrieval of optical properties: Results for FIRE 2

Due to the prevalence and persistence of cirrus cloudiness across the globe, cirrus clouds are believed to have an important effect on the climate. Stephens et al., (1990) among others have shown that the important factor determining how cirrus clouds modulate the climate is the balance between the albedo and emittance effect of the cloud systems. This factor was shown to depend in part upon the effective sizes of the cirrus cloud particles. Since effective sizes of cirrus cloud microphysical distributions are used as a basis of parameterizations in climate models, it is crucial that the relationships between effective sizes and radiative properties be clearly established. In this preliminary study, the retrieval of cirrus cloud effective sizes are examined using a two dimensional radiative transfer model for a cirrus cloud case sampled during FIRE Cirrus 11. The purpose of this paper is to present preliminary results from the SHSG model demonstrating the sensitivity of the bispectral relationships of reflected radiances and thus the retrieval of effective sizes to phase function and dimensionality.

Stackhouse, Paul W., Jr.↗

NASA NTRS1994

Microphysical growth state of ice particles and large-scale electrical structure of clouds

Cloud temperature, liquid water content, and vertical air velocity are all considered in evaluating the microphysical growth state of ice phase precipitation particles in the atmosphere. The large-scale observations taken together with in situ measurements indicated that the most prevalent growth condition for large ice particles in active convection is sublimation during riming, whereas the most prevalent growth condition in stratiform precipitation is vapor deposition. The large-scale electrical observations lend further support to the idea that particles warmed by riming into sublimation charge negatively and particles in vapor deposition charge positively in collisions with small ice particles.

Williams, Earle↗

NASA NTRS1997

Comparison of Modeled Backscatter using Measured Aerosol Microphysics with Focused CW Lidar Data over Pacific

During NASA's GLObal Backscatter Experiment (GLOBE) II flight mission over the Pacific Ocean in May-June 1990, extensive aerosol backscatter data sets from two continuous wave, focused CO2 Doppler lidars and an aerosol microphysics data set from a laser optical particle counter (LOPC) were obtained. Changes in aerosol loading in various air masses with associated changes in chemical composition, from sulfuric acid and sulfates to dustlike crustal material, significantly affected aerosol backscatter, causing variation of about 3 to 4 orders of magnitude. Some of the significant backscatter features encountered in different air masses were the low backscatter in subtropical air with even lower values in the tropics near the Intertropical Convergence Zone (ITCZ), highly variable backscatter in the ITCZ, mid-tropospheric aerosol backscatter background mode, and high backscatter in an Asian dust plume off the Japanese coast. Differences in aerosol composition and backscatter for northern and southern hemisphere also were observed. Using the LOPC measurements of physical and chemical aerosol properties, we determined the complex refractive index from three different aerosol mixture models to calculate backscatter. These values provided a well-defined envelope of modeled backscatter for various atmospheric conditions, giving good agreement with the lidar data over a horizontal sampling of approximately 18000 km in the mid-troposphere.

Srivastava, Vandana↗

NASA NTRS1998

Intercomparison of Pulsed Lidar Data with Flight Level CW Lidar Data and Modeled Backscatter from Measured Aerosol Microphysics Near Japan and Hawaii

Aerosol backscatter coefficient data were examined from two nights near Japan and Hawaii undertaken during NASA's Global Backscatter Experiment (GLOBE) in May-June 1990. During each of these two nights the aircraft traversed different altitudes within a region of the atmosphere defined by the same set of latitude and longitude coordinates. This provided an ideal opportunity to allow flight level focused continuous wave (CW) lidar backscatter measured at 9.11-micron wavelength and modeled aerosol backscatter from two aerosol optical counters to be compared with pulsed lidar aerosol backscatter data at 1.06- and 9.25-micron wavelengths. The best agreement between all sensors was found in the altitude region below 7 km, where backscatter values were moderately high at all three wavelengths. Above this altitude the pulsed lidar backscatter data at 1.06- and 9.25-micron wavelengths were higher than the flight level data obtained from the CW lidar or derived from the optical counters, suggesting sample volume effects were responsible for this. Aerosol microphysics analysis of data near Japan revealed a strong sea-salt aerosol plume extending upward from the marine boundary layer. On the basis of sample volume differences, it was found that large particles were of different composition compared with the small particles for low backscatter conditions.

Cutten, D. R.↗