Abstract

Remote sensing lidar and imaging spectral radiometer observations were obtained from the ER-2 high-altitude research aircraft during the 1986 FIRE cirrus missions. The dual polarization lidar measurements were nadir directed with 7.5 m vertical and 40 m horizontal resolution, and clearly depicted structure at the top and within the cirrus. Simultaneous radiometric cloud top images were acquired with 5 mrd resolution at ten visible channels, three infrared window channels, and four near-infrared channels. The combined lidar and radiometer data were analyzed for the cirrus structure, radiative parameters, and inferred microphysical properties. On 28 October 1986 a cirrus formation crossed Wisconsin. The results indicate that for the eastern edge of the formation there was a cirrus layer at 9 to 11 km altitude, and a separate lower cloud at 7 to 8 km. The lidar depolarization indicated the upper layer was ice crystals, the lower layer was ice in some areas, and water or possibly mixed phase in others. Split window thermal brightness measurements indicated the upper layer was principally particles of an effective radius less than 25 μm. To the west, the cirrus formation was a denser layer extending between 6 to 11 km altitude. An equivalent height for the thermal IR emission of cirrus was defined. The equivalent height was found to be as much as 4 km below the true cloud top height. The average vertical structure of radiation parameters was derived. For the upward infrared radiance the strongest contribution was from 7 to 8 km altitude but higher cirrus were significant. Cloud visible reflectance approached 0.6 and the 10.84 μm emittance ranged to 0.9. Distinct local vacations in the relation between reflectance and emittance were found, while a significant dispersion of the emittance to reflectance relation for the entire dataset was present. The dispersion was principally due to variations in surface albedo. An overall parameterization for the average measured relation between emittance and visible albedo is given.

Abstract

A summary of experimental observations and analysis of cirrus from high-altitude aircraft remote sensing is presented. The vertical distribution of cirrus optical and infrared cross-section parameters and the relative effective emittance and visible reflectance are derived from nadir-viewing lidar and multispectral radiometer data for observations during the 1986 and 1991 FIRE cirrus experiments. Statistics on scattering and absorption cross sections in relation to altitude and temperature are given. The emittance and reflectance results are considered as a function of solar zenith angle. Comparative radiative transfer calculations based on the discrete-ordinate method were carried out for three representative cloud phase function models: a spherical water droplet, an ice column crystal cloud, and a Henyey-Greenstein function. The agreements between observations of the effective emittance and shortwave reflectance and the model calculations were a function of the solar zenith angle. At angles between 54° and 60° a Henyey-Greenstein (HG) function with an asymmetry factor of 0.6–0.7 produced the best comparison. At 66°–72° the ice column model was equally comparable to observations. Comparisons to the water cloud model wore poor in all cases. The effects of ice crystal microphysical variations on the observed results were not generally apparent, but one dramatic example of difference was found. In order to explain the variations noted for solar zenith angle, an instrument–the Tilt Scan CCD Camera radiometer–was developed to directly observe the shortwave bidirectional reflectance function for 1991 measurements. The results indicate a characteristic angular function of the visible reflectance of cirrus that is flatter than predicted by the ice column scattering model, but the overall asymmetry factor is comparable. The good agreement with values from an HG function at some angles is not generally applicable. The characteristics of the observed cirrus angular reflectance pattern correlate well with, and are explained by, the results that were found for the solar zenith angle dependence of the eminence and reflectance.

Abstract

A Nd:YAG lidar system was flown aboard NASA's ER-2 high altitude aircraft. Observations of cloud top height were made with 70 m along-track and 7.5 m vertical-height resolution. The lidar data observed from an East Pacific stratocumulus cloud height deck revealed large cloud variability on 1–5 km scales. The cloud deck sloped upward from 700 to 1000 m in a northeast-southwest direction over a distance of 120 km. Vertical cloud top distributions were negatively skewed indicating flat-topped clouds. The dominant spectral peak of the cloud top variations was found at 4.5 km, which is 5 to 7 times the depth of the local boundary layer. No other peaks were significant in the average spectrum, The cloud layer was stable with respect to cloud top entrainment instability. The southwestern region of the study area was more prone to shear instability at cloud top than the northeastern region. The results of this study show that a lidar system is ideal to provide the topography of clouds and local boundary layer depth. This information is useful in the study of cloud top radiation and parameterization of clouds in numerical models.

Abstract

The temperature and windfield structure and hydrometeor composition of cirrus clouds sampled by the NCAR King Air and Sabreliner aircraft on 28 October 1986 near Madison, Wisconsin are described as part of a case study that examines cirrus cloud radiative and microphysical properties. Two cloud layers were sampled from top to base. The upper layer was found at altitudes between 8.5 and 11.5 km and the lower between 6.0 and 8.5 km. Vertical velocities calculated from the increase in ice mass flux with height were typical of synoptic scale lifting. Stronger vertical velocities were measured in convective cells at the top of the lower layer.

The total ice particle concentration was dominated by particles <200 μm. Mean particle size and ice water content increased with decreasing altitude. The largest particles, exceeding 1000 μm in the upper layer and 1500 μm in the lower layer, probably resulted from aggregation, even at cold temperatures. Cloud emissivity and optical depth were calculated from the ice particle size spectra.

The distribution of ice mass was narrow at cloud top and broadened with decreasing altitude. At the highest levels of the upper cloud, half the mass was in particles <150 μm. In this region, we underestimate the mass by a significant fraction presumably contained in particles too small to detect. In the lower levels, half the mass was in particles <200–400 μm. In the cloud sampled by the King Air, half the mass was in particles <400–600 μm. Up to 10% of the mass in the higher cloud and up to 30% in the lower cloud was contained in particles >500 μm.

We relate the microstructure of a shallow liquid water layer associated with an altocumulus to lidar observations. Thirteen separate episodes of liquid water were sampled at about −30°C. Mean droplet dimensions were <9 μm, and the liquid water contents were low. Virtually no ice particles were detected within and below the layer. We surmised that under such conditions these liquid water clouds remained colloidally stable. Kelvin-Helmholz waves may have produced the undulations observed at cloud top.

Abstract

Optical depths in the visible to infrared spectral region were obtained from solar extinction measurements with two sun photometers during the First ISCCP Regional Experiment Phase II Cirrus Intensive Field Observation in Kansas.

A method is described to correct sun photometry for gaseous absorption and is extended to estimate the water vapor amount. The approach uses a prior computation of gaseous absorption for the narrowband-pass sun photometry, parameterized with the slant-path absorber amount. These produce correction coefficients for gaseous absorption, as determined by LOWTRAN 7 models. Near-infrared channels were calibrated by modified Langley plots taking account of gaseous absorption.

After the correction and calibration, the aerosol optical depths at the wavelength of 0.4–4 µm were obtained for clear sky conditions. The aerosol optical depth at the wavelength λ = 0.5 µm was 0.1–0.2 during the campaign. The cloud optical depth at λ = 0.5 µm was obtained for cirrus events on 26 November and 5 December 1991 correction of multiple scattering effects involved in sun photometry. The column amount of water vapor was estimated from the 0.94-µm-channel measurement and compared with results from radiosonde measurements. The comparison has shown a good agreement within a 10% difference during the campaign when the equivalent water vapor amount ranges from 0.3 to 1.2 g cm⁻².

Abstract

The instrumented NASA ER-2 aircraft overflew severe convection with infrared (IR) V features for the first time in the Midwest United States during May 1984. Measurements taken by the ER-2 were: visible and IR imagery, high-frequency passive microwave (92, 183 GHz) imagery, nadir lidar backscattered return, and flight altitude information. The 7 May and 13 May 1984 cases are analyzed in detail and the various data sources are combined and compared with GOES imagery. Topics addressed in the paper are 1) relation of thermal couplets and V features in aircraft IR measurements to previous findings from GOES data, 2) examination of the cloud radiative hypothesis for the V feature, and 3) stratospheric perturbations above severe thunderstorms and mesoscale convective systems.

The high resolution aircraft IR imagery shows that thermal couplets are considerably more pronounced than in GOES imagery. In one of the cases (7 May 1984) the minimum cloud-top IR temperature was located upshear of the overshooting cloud top in the lidar height field. This was suggested in previous papers to result from cloud top mixing with the stratospheric environment and subsidence. The IR temperatures in the downshear anvils were as much as 5°C warmer than the ambient air temperatures, implying that the upwelling IR radiance comes from about 0.5–1.0 km below the cloud top. Finally, the in situ ER-2 measurements of temperature and air velocity 3–4 km above the overshooting tops showed very intense temperature and vertical velocity perturbations. These perturbations are suggestive of 1) lee waves generated by the overshooting tops, or 2) a cold dome above the squall line possibly due to tropopause lifting by the storms.

Abstract

The cloud dataset from the Geoscience Laser Altimeter System (GLAS) lidar on the Ice, Cloud, and Land Elevation Satellite (ICESat) spacecraft is compared to the cloud analysis of the Wisconsin NOAA High Resolution Infrared Radiation Sounder (HIRS) Pathfinder. This is the first global lidar dataset from a spacecraft of extended duration that can be compared to the HIRS climatology. It provides an excellent source of cloud information because it is more sensitive to clouds that are difficult to detect, namely, thin cirrus and small boundary layer clouds. The second GLAS data collection period from 1 October to 16 November 2003 was used for this comparison, and a companion dataset of the same days were analyzed with HIRS. GLAS reported cloud cover of 0.70 while HIRS reported slightly higher cloud cover of 0.75 for this period. The locations where HIRS overreported cloud cover were mainly in the Arctic and Antarctic Oceans and parts of the Tropics.

GLAS also confirms that upper-tropospheric clouds (above 6.6 km) cover about 0.33 of the earth, similar to the reports from HIRS data. Generally, the altitude of the cloud tops reported by GLAS is, on average, higher than HIRS by 0.4 to 4.5 km. The largest differences were found in the Tropics, over 4 km, while in midlatitudes average differences ranged from 0.4 to 2 km. Part of this difference in averaged cloud heights comes from GLAS finding more high cloud coverage in the Tropics, 5% on average but >13% in some areas, which weights its cloud top average more toward the high clouds than the HIRS. The diffuse character of the upper parts of high clouds over tropical oceans is also a cause for the difference in reported cloud heights.

Statistics on cloud sizes also were computed from GLAS data to estimate the errors in cloud cover reported by HIRS from its 20-km field-of-view (FOV) size. Smaller clouds are very common with one-half of all clouds being <41 km in horizontal size. But, clouds <41 km cover only 5% of the earth. Cloud coverage is dominated by larger clouds with one-half of the coverage coming from clouds >1000 km. GLAS cloud size statistics also show that HIRS possibly overreports some cloud forms by 2%–3%. Looking at groups of GLAS data 21 km long to simulate the HIRS FOV, the authors found that ∼5% are partially filled with cloud. Since HIRS does not account for the part of the FOV without cloud, it will overreport the coverage of these clouds. However, low-altitude and optically thin clouds will not be reported by HIRS if they are so small that they do not affect the upwelling radiation in the HIRS FOV enough to trigger the threshold for cloud detection. These errors are partially offing.

Abstract

Multiangle remote sensing provides a wealth of information for earth and climate monitoring, such as the ability to measure the height of cloud tops through stereoscopic imaging. Further, as technology advances so do the options for developing spacecraft instrumentation versatile enough to meet the demands associated with multiangle measurements. One such instrument is the infrared spectral imaging radiometer, which flew as part of mission STS-85 of the space shuttle Columbia in 1997 and was the first earth-observing radiometer to incorporate an uncooled microbolometer array detector as its image sensor. Specifically, a method for computing cloud-top height with a precision of ±620 m from the multispectral stereo measurements acquired during this flight has been developed, and the results are compared with coincident direct laser ranging measurements from the shuttle laser altimeter. Mission STS-85 was the first space flight to combine laser ranging and thermal IR camera systems for cloud remote sensing.

Abstract

A multispectral scanning radiometer has been used to obtain measurements of the reflection function of marine stratocumulus clouds at 0.75, 1.65 and 2.16 μm. These observations were obtained from the NASA ER-2 aircraft as part of the First ISCCP [International Satellite Cloud Climatology Project] Regional Experiment (FIRE), conducted off the coast of southern California during July 1987. Multispectral images of the reflection function were used to derived the optical thickness and effective particle radius of stratiform cloud layers on four days. In addition to the radiation measurements, in situ microphysical measurements were obtained from the University of Washington Convair C-131A aircraft. In this paper we compare remote sensing results with in situ observations, which show a good spatial correlation for both optical thicknesses and effective radius. These comparisons further show systematic differences between remote sensing and in situ values, with a tendency for remote sensing to overestimate the effective radius by ∼2–3 μm, independent of particle radius. The optical thickness, in contrast, is somewhat overestimated for small optical thickness and underestimated for large optical thicknesses. An introduction of enhanced gaseous absorption at a wavelength of 2.16 μm successfully explains some of these observed discrepancies.

Marginal probability density functions of optical thickness, liquid water path and effective radius have been derived from our remote sensing results. The joint probability density function of liquid water path and effective radius shows that the effective radius increases as the liquid water path increases for optically thin clouds, in contrast to optically thick clouds for which the effective radius decreases with increasing liquid water path.