Abstract

A method is proposed to estimate the optical thickness of cirrus clouds from ground-based sun photometry. Transfer calculations of solar radiation in ice clouds were made by the Monte Carlo method. A scattering phase function presented by Takano and Liou was employed for ice clouds. Simulations of sun photometry, which include strong forward scattering into the instrument's field of view, give a simple relationship between the true and apparent optical thicknesses. The correction method was applied to Sun photometer measurements for cirrostratus clouds observed at Tsukuba, Japan. The relationship between the visible optical thickness and the broadband solar flux transmittance obtained from observations agreed well with that theoretically expected for cloud optical thickness up to about 10.

Abstract

A new method is proposed to retrieve various cloud physical parameters of water clouds from the solar-flux reflectances at four wavelengths measured by using the airborne Multi-channel Cloud Pyranometer (MCP) system. The MCP system was designed to measure the spectral reflectances at nine wavelengths in the visible and near-infrared region. The estimation procedure assumes a locally plane-parallel and vertically homogeneous water-cloud layer with monomodal particle size distributions of a fixed width. The cloud optical thickness τ₅₀₀ and the effective particle radius r_e , of the water-cloud layer can be simultaneously retrieved from the MCP reflectances measured at the visible channel of λ=500 nm and at the near-infrared channel of λ=1650 nm. Under the assumption that cloud pressure height is known, the cloud liquid water content (LWC) can then be retrieved from the MCP reflectance at the oxygen absorption-band channel cantered at λ=760 nm. Finally, the in-cloud water vapor amount u _H2O can be estimated from the MCP reflectance at the water vapor absorption-band channel centered at λ=938 nm. Using these directly retrieved parameters, we can estimate byproduct parameters such as integrated liquid water path, cloud particle concentration, and geometrical thickness of the homogeneous cloud layer. A reliably applicable range of the present method was estimated to be 1≤&tau₅₀₀<100, 4 µm ≤r_e <25 µm, LWC≤1 g m⁻³, and u _H2O≤10 g m⁻³.

The present retrieval method was applied to the MCP spectral reflectance data obtained through aircraft observations for wintertime stratocumulus clouds over the ocean south of Tokyo, made as part of the Western North Pacific Cloud-Radiation Experiment/Meteorological Research Institute program in Decembers of 1989 and 1990. Reasonable values of the cloud physical parameters were successfully retrieved for the stratocumulus clouds. However, a comparison with the simultaneous in situ cloud measurement showed that the estimated effective particle radii and liquid water content were lager than the in situ measured values.

Statistical microphysical features of the marine stratocumulus clouds have been investigated by taking various correlations among the contemporaneous cloud physical parameters estimated from the MCP spectral reflectances. It was found that, for thin water clouds with τ₅₀₀<∼20, the effective particle radius was positively correlated with cloud optical thickness, but for thicker clouds with τ₅₀₀<∼20, there was a negative correlation between them. For both cases, however, liquid water content was positively correlated with cloud optical thickness. It is suggested that, for thin clouds in the dissipating stage, evaporation might be dominant process of cloud particle formation; on the other hand, coalescence might be dominant in thick clouds in the mature stage with precipitation.

Abstract

The design and performance of a spectral radiometer system are described for airborne measurements of solar flux reflectance by clouds. The system consists of a pair of identical multichannel pyranometers: one installed on the top and the other on the bottom of an aircraft fuselage to measure the downward and upward solar irradiances, respectively. This measurement scheme has an advantage in that reflectances derived from ratios between the upward and downward irradiances can avoid the need for absolute radiometric calibrations. The multichannel cloud pyranometer (MCP) system measures near-monochromatic solar irradiances at nine discrete wavelengths between 420 and 1650 nm by using interference filters with very narrow bandwidths. Included among these wavelengths are 760 and 938 nm in the oxygen and water vapor absorption bands, respectively. Solar radiation passing through the filters is instantly detected by a silicon photodiode for wavelength λ<1 µm and by a germanium photodiode for λ>1 µm. Good performance of the MCP system was confirmed through laboratory calibrations and airborne tests. The MCP system is suitable for remote sensing application to retrieve cloud physical parameters of water clouds from airborne spectral reflectance measurements.

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

A Coulter Multisizer, which is based on the electrical sensing zone (ESZ) or the Coulter principle, was used to measure the size distribution of Asian dust. Coulter Multisizer analysis provides high-resolution size measurements of water-insoluble aerosol particles (WIPs) and the number concentration at each size bin. Aerosol filter sampling was conducted at four sites in Japan during spring 2003. The measured volume size distributions fit fairly well with a lognormal distribution. The results show that the WIP size distributions of the same Asian dust air mass varied at each sampling site and the volume mode diameter at the sites reduced from west to east. The derived volume mode diameter ranged from 1.4 to 2.2 μm and was comparatively smaller than those in previous studies on Asian dust. This can be explained by the possible internal mixing of Asian dust with other components and by the breaking of particles and dispersion of aggregations by ultrasonification during extraction. The analysis method was improved by washing the aerosol particles collected on a filter using a magnetic stirrer, instead of an ultrasonic cleaner. As a result, the WIP size distribution can be measured in the range of 1–10 μm. The measured mode diameters were 2.6–3.1 and 4.3–5.6 μm in 2 Asian dust phenomena at Kofu, Japan, in 2004. It is demonstrated that the Coulter Multisizer method can furnish detailed information regarding the spatial and temporal variations in the mineral dust size distribution.

Abstract

Cloud observations over the past decade from six Arctic atmospheric observatories are investigated to derive estimates of cloud occurrence fraction, vertical distribution, persistence in time, diurnal cycle, and boundary statistics. Each observatory has some combination of cloud lidar, radar, ceilometer, and/or interferometer for identifying and characterizing clouds. By optimally combining measurements from these instruments, it is found that annual cloud occurrence fractions are 58%–83% at the Arctic observatories. There is a clear annual cycle wherein clouds are least frequent in the winter and most frequent in the late summer and autumn. Only in Eureka, Nunavut, Canada, is the annual cycle shifted such that the annual minimum is in the spring with the maximum in the winter. Intersite monthly variability is typically within 10%–15% of the all-site average. Interannual variability at specific sites is less than 13% for any given month and, typically, is less than 3% for annual total cloud fractions. Low-level clouds are most persistent at the observatories. The median cloud persistence for all observatories is 3–5 h; however, 5% of cloud systems at far western Arctic sites are observed to occur for longer than 100 consecutive hours. Weak diurnal variability in cloudiness is observed at some sites, with a daily minimum in cloud occurrence near solar noon for those seasons for which the sun is above the horizon for at least part of the day.

Abstract

With the aim of improving the consistency of terrestrial and atmospheric longwave radiation measurements within the Baseline Surface Radiation Network, five Eppley Precision Infrared Radiometer (PIR) pyrgeometers and one modified Meteorological Research Flight (MRF) pyrgeometer were individually calibrated by 11 specialist laboratories. The round-robin experiment was conducted in a “blind” sense in that the participants had no knowledge of the results of others until the whole series of calibrations had ended. The responsivities C(μV/W m⁻²) determined by 6 of the 11 institutes were within about 2% of the median for all five PIR pyrgeometers. Among the six laboratories, the absolute deviation around the median of the deviations of the five instruments is less than 1%. This small scatter suggests that PIR pyrgeometers were stable at least during the two years of the experiment and that the six different calibration devices reproduce the responsivity C of PIR pyrgeometers consistently and within the precision required for climate applications. The results also suggest that the responsivity C can be determined without simultaneous determination of the dome correction factor k, if the temperature difference between pyrgeometer body and dome is negligible during calibration. For field measurements, however, k has to be precisely known. The calibration of the MRF pyrgeometer, although not performed by all institutes, also showed satisfactory results.