Estimating Cloud Optical Depth from Surface Radiometric Observations: Sensitivity to Instrument Noise and Aerosol Contamination

2005 ◽  
Vol 62 (11) ◽  
pp. 4095-4104 ◽  
Author(s):  
Alain Beaulne ◽  
Howard W. Barker ◽  
Jean-Pierre Blanchet
Keyword(s):  
Optical Depth ◽  
Surface Albedo ◽  
Transport Model ◽  
Cloud Base ◽  
Spectral Difference ◽  
Cloud Optical Depth ◽  
Sensitivity Tests ◽  
Cloud Resolving Model ◽  
Model Cloud ◽  
Instrument Noise

Abstract The spectral-difference algorithm of Barker and Marshak for inferring optical depth τ of broken clouds has been shown numerically to be potentially useful. Their method estimates cloud-base reflectance and τ using spectral radiometric measurements made at the surface at two judiciously chosen wavelengths. Here it is subject to sensitivity tests that address the impacts of two ubiquitous sources of potential error: instrument noise and presence of aerosol. Experiments are conducted using a Monte Carlo photon transport model, cloud-resolving model data, and surface albedo data from satellite observations. The objective is to analyze the consistency between inherent and retrieved values of τ. Increasing instrument noise, especially if uncorrelated at both wavelengths, decreases retrieved cloud fraction and increases retrieved mean τ. As with all methods that seek to infer τ using passive radiometry, the presence of aerosol requires that threshold values be set in order to discriminate between cloudy and cloud-free columns. A technique for estimating thresholds for cloudy columns is discussed and demonstrated. Finally, it was found that surface type and mean inherent τ play major roles in defining retrieval accuracy.

scholarly journals Effects of Surface Albedo on Cloud and Radiation Processes in Cloud-Resolving Model Simulations

2010 ◽  
Vol 67 (5) ◽  
pp. 1474-1491 ◽  
Author(s):  
Sunwook Park ◽  
Xiaoqing Wu
Keyword(s):  
Radiative Forcing ◽  
Surface Albedo ◽  
Critical Value ◽  
Cloud Base ◽  
Radiative Fluxes ◽  
Cloud Properties ◽  
Cloud Resolving Model ◽  
Albedo Effect ◽  
Evolving Surface ◽  
Radiation Processes

Abstract The relationship among the surface albedo, cloud properties, and radiative fluxes is investigated for the first time using a year-long cloud-resolving model (CRM) simulation with the prescribed evolving surface albedo. In comparison with the run using a fixed surface albedo, the CRM with the observed surface albedo represents the shortwave radiative budget closer to the observations in the winter. The greater surface albedo induces weaker instability in the low troposphere so that the amount of low clouds decreases during the winter. This reduces the shortwave and longwave cloud radiative forcing at the surface. The analysis of the CRM simulations with the evolving surface albedo reveals that there is a critical value (0.35) of the surface albedo. For albedos greater than the critical value, the upward shortwave flux at the top of the atmosphere (TOA) is positively proportional to the surface albedos when optically thin clouds exist, and is not much affected by reflection on the cloud top. If optically thick clouds occur and the surface albedo is greater than the critical value, the upward shortwave flux at the TOA is significantly influenced by the reflection of cloud top, but not much affected by the surface albedo. In addition, for albedos larger than the critical value, the downward shortwave flux at the surface is primarily influenced by the surface albedo and the reflection from the cloud base if optically thick clouds occur. However, the downward shortwave flux at the surface is not significantly affected by the surface albedo when optically thin clouds exist because the reflection on the cloud base is weak. When surface albedos are less than the critical value, those relationships among surface albedo, shortwave flux, and cloud properties are not obvious. The surface albedo effect on shortwave flux increases as solar zenith angle (SZA) decreases, but its dependence on the SZA is negligible when optically thick clouds exist.

scholarly journals Statistical Analyses of Satellite Cloud Object Data from CERES. Part III: Comparison with Cloud-Resolving Model Simulations of Tropical Convective Clouds

2007 ◽  
Vol 64 (3) ◽  
pp. 762-785 ◽  
Author(s):  
Yali Luo ◽  
Kuan-Man Xu ◽  
Bruce A. Wielicki ◽  
Takmeng Wong ◽  
Zachary A. Eitzen
Keyword(s):  
Physical Properties ◽  
Optical Depth ◽  
Weather Prediction ◽  
Cloud Microphysics ◽  
Retrieval Algorithm ◽  
Medium Size ◽  
Size Category ◽  
Cloud Optical Depth ◽  
Large Size ◽  
Cloud Resolving Model

Abstract The present study evaluates the ability of a cloud-resolving model (CRM) to simulate the physical properties of tropical deep convective cloud objects identified from a Clouds and the Earth’s Radiant Energy System (CERES) data product. The emphasis of this study is the comparisons among the small-, medium-, and large-size categories of cloud objects observed during March 1998 and between the large-size categories of cloud objects observed during March 1998 (strong El Niño) and March 2000 (weak La Niña). Results from the CRM simulations are analyzed in a way that is consistent with the CERES retrieval algorithm and they are averaged to match the scale of the CERES satellite footprints. Cloud physical properties are analyzed in terms of their summary histograms for each category. It is found that there is a general agreement in the overall shapes of all cloud physical properties between the simulated and observed distributions. Each cloud physical property produced by the CRM also exhibits different degrees of disagreement with observations over different ranges of the property. The simulated cloud tops are generally too high and cloud-top temperatures are too low except for the large-size category of March 1998. The probability densities of the simulated top-of-the-atmosphere (TOA) albedos for all four categories are underestimated for high albedos, while those of cloud optical depth are overestimated at its lowest bin. These disagreements are mainly related to uncertainties in the cloud microphysics parameterization and inputs such as cloud ice effective size to the radiation calculation. Summary histograms of cloud optical depth and TOA albedo from the CRM simulations of the large-size category of cloud objects do not differ significantly between the March 1998 and 2000 periods, consistent with the CERES observations. However, the CRM is unable to reproduce the significant differences in the observed cloud-top height while it overestimates the differences in the observed outgoing longwave radiation and cloud-top temperature between the two periods. Comparisons between the CRM results and the observations for most parameters in March 1998 consistently show that both the simulations and observations have larger differences between the large- and small-size categories than between the large- and medium-size, or between the medium- and small-size categories. However, the simulated cloud properties do not change as much with size as observed. These disagreements are likely related to the spatial averaging of the forcing data and the mismatch in time and space between the numerical weather prediction model from which the forcing data are produced and the CERES observed cloud systems.

scholarly journals Transmission of Solar Radiation by Clouds over Snow and Ice Surfaces. Part II: Cloud Optical Depth and Shortwave Radiative Forcing from Pyranometer Measurements in the Southern Ocean

2005 ◽  
Vol 18 (22) ◽  
pp. 4637-4648 ◽  
Author(s):  
Melanie F. Fitzpatrick ◽  
Stephen G. Warren
Keyword(s):  
Solar Radiation ◽  
Southern Ocean ◽  
Sea Ice ◽  
Optical Depth ◽  
Radiative Forcing ◽  
Surface Albedo ◽  
Cloud Droplet ◽  
Frequency Distributions ◽  
Cloud Optical Depth ◽  
Cloud Radiative Forcing

Abstract Downward solar irradiance at the sea surface, measured on several voyages of an icebreaker in the Southern Ocean, is used to infer transmittance of solar radiation by clouds. Together with surface albedo estimated from coincident hourly sea ice reports, instantaneous cloud radiative forcing and effective cloud optical depth are obtained. Values of “raw cloud transmittance” (trc), the ratio of downward irradiance under cloud to downward irradiance measured under clear sky, vary from 0.1 to 1.0. Over sea ice, few values of trc were observed between 0.8 and 1.0, possibly due to the threshold nature of the aerosol-to-cloud-droplet transition. This sparsely populated region of transmittances is referred to as the Köhler gap. The instantaneous downward shortwave cloud radiative forcing is computed, as well as the time-averaged net forcing. The net forcing at a solar zenith angle of 60° is typically −250 W m−2 over open ocean, but only half this value over sea ice because of the higher surface albedo and less frequent occurrence of clouds. “Effective” optical depths τ (for a radiatively equivalent horizontally homogeneous cloud) are classified by season and surface type. The frequency distributions of τ are well fitted by decaying exponentials, giving a characteristic optical depth of 15 at 47°S, increasing to 24 in the region of maximum cloud cover at 58°S, and decreasing to 11 at 67°S near the coast of Antarctica.

scholarly journals Statistics of vertical backscatter profiles of cirrus clouds

2011 ◽  
Vol 11 (24) ◽  
pp. 12925-12943 ◽  
Author(s):  
P. Veglio ◽  
T. Maestri
Keyword(s):  
Optical Depth ◽  
Satellite Observation ◽  
Cirrus Clouds ◽  
Cloud Base ◽  
Physical Parameters ◽  
Cloud Optical Depth ◽  
Cloud Temperature ◽  
The Mean ◽  
Depth Increase ◽  
The Impact

Abstract. A nearly global statistical analysis of vertical backscatter and extinction profiles of cirrus clouds collected by the CALIOP lidar, on-board of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, is presented. Statistics on frequency of occurrence and distribution of bulk properties of cirrus clouds in general and, for the first time, of horizontally homogeneous (on a 5-km field of view) cirrus clouds only are provided. Annual and seasonal backscatter profiles (BSP) are computed for the horizontally homogeneous cirri. Differences found in the day/night cases and for midlatitudes and tropics are studied in terms of the mean physical parameters of the clouds from which they are derived. The relationship between cloud physical parameters (optical depth, geometrical thickness and temperature) and the shape of the BSP is investigated. It is found that cloud geometrical thickness is the main parameter affecting the shape of the mean CALIOP BSP. Specifically, cirrus clouds with small geometrical thicknesses show a maximum in mean BSP curve located near cloud top. As the cloud geometrical thickness increases the BSP maximum shifts towards cloud base. Cloud optical depth and temperature have smaller effects on the shape of the CALIOP BSPs. In general a slight increase in the BSP maximum is observed as cloud temperature and optical depth increase. In order to fit mean BSPs, as functions of geometrical thickness and position within the cloud layer, polynomial functions are provided. The impact on satellite radiative transfer simulations in the infrared spectrum when using either a constant ice-content (IWC) along the cloud vertical dimension or an IWC profile derived from the BSP fitting functions is evaluated. It is, in fact, demonstrated that, under realistic hypotheses, the mean BSP is linearly proportional to the IWC profile.

scholarly journals A comprehensive numerical study of aerosol-cloud-precipitation interactions in marine stratocumulus

2011 ◽  
Vol 11 (5) ◽  
pp. 15497-15550 ◽  
Author(s):  
Y.-C. Chen ◽  
L. Xue ◽  
Z. J. Lebo ◽  
H. Wang ◽  
R. M. Rasmussen ◽  
...  
Keyword(s):  
Wind Speed ◽  
Optical Depth ◽  
Large Scale ◽  
Thickness Effect ◽  
Cloud Base ◽  
Dispersion Effect ◽  
Marine Stratocumulus ◽  
Cloud Optical Depth ◽  
Aerosol Cloud ◽  
Cloud Thickness

Abstract. Three-dimensional large-eddy simulations (LES) with detailed bin-resolved microphysics are performed to explore the diurnal variation of marine stratocumulus (MSc) clouds under clean and polluted conditions. The sensitivity of the aerosol-cloud-precipitation interactions to variation of sea surface temperature, free tropospheric humidity, large-scale divergence rate, and wind speed is assessed. The comprehensive set of simulations corroborates previous studies that (1) with moderate/heavy drizzle, an increase in aerosol leads to an increase in cloud thickness; and (2) with non/light drizzle, an increase in aerosol results in a thinner cloud, due to the pronounced effect on entrainment. It is shown that for higher SST, stronger large-scale divergence, drier free troposphere, or lower wind speed, the cloud thins and precipitation decreases. The sign and magnitude of the Twomey effect, droplet dispersion effect, cloud thickness effect, and overall cloud optical depth susceptibility to aerosol perturbations are evaluated by LES experiments and compared with analytical formulations. The Twomey effect emerges as dominant in total cloud susceptibility to aerosol perturbations. The dispersion effect, that of aerosol perturbations on the cloud droplet size spectrum, is positive (i.e., increase in aerosol leads to spectral narrowing) and accounts for 3 % to 10 % of the total cloud susceptibility at nighttime, with the largest influence in heavier drizzling clouds. The cloud thickness effect is negative (i.e., increase in aerosol leads to thinner cloud) for non/light drizzling cloud and positive for moderate/heavy drizzling clouds; the cloud thickness effect contributes 5 % to 22 % of the nighttime cloud susceptibility. The range of magnitude for each effect is more variable in the daytime owing to cloud thinning and decoupling. Overall, the cloud susceptibility is ~0.28 to 0.53 at night; an increase in aerosol concentration enhances cloud optical depth, especially with heavier precipitation and in a more pristine environment. The good agreement between LES experiments and analytical formulations suggests that the latter may be useful in evaluations of cloud susceptibility. The ratio of the magnitude of the cloud thickness effect to that of the Twomey effect depends on cloud base height and cloud thickness in unperturbed (clean) clouds.

scholarly journals Statistics of vertical backscatter profile of cirrus clouds

2011 ◽  
Vol 11 (9) ◽  
pp. 25813-25870
Author(s):  
P. Veglio ◽  
T. Maestri
Keyword(s):  
Optical Depth ◽  
Satellite Observation ◽  
Cirrus Clouds ◽  
Cloud Base ◽  
Physical Parameters ◽  
Cloud Optical Depth ◽  
Cloud Temperature ◽  
The Mean ◽  
Depth Increase ◽  
The Impact

Abstract. A nearly global statistical analysis of vertical backscatter and extinction profiles of cirrus clouds collected by the CALIOP lidar, on-board of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, is presented. Statistics on frequency of occurrence and distributions of bulk properties of cirrus clouds in general and, for the first time, of horizontally homogeneous (on a 5-km field of view) cirrus clouds only are provided. Annual and seasonal backscatter profiles (BSP) are computed for the horizontally homogeneous cirri. Differences found in the day/night cases and for midlatitudes and tropics are studied in terms of the mean physical parameters of the clouds from which they are derived. The relation between cloud physical parameters (optical depth, geometrical thickness and temperature) and the shape of the BSP is investigated. It is found that cloud geometrical thickness is the main parameter affecting the shape of the mean CALIOP BSP. Specifically, cirrus clouds with small geometrical thicknesses show a maximum in mean BSP curve placed near cloud top. As the cloud geometrical thickness increases the BSP maximum shifts towards cloud base. Cloud optical depth and temperature have smaller effect on the shape of the CALIOP BSPs. In general a slight increase in the BSP maximum is observed as cloud temperature and optical depth increase. In order to fit mean BSPs, as functions of geometrical thickness and position within the cloud layer, polynomial functions are provided. The impact on satellite radiative transfer simulations in the infrared spectrum when using either a constant ice-content (IWC) along the cloud vertical dimension or an IWC profile derived from the BSP fitting functions is evaluated. It is, in fact, demonstrated that, under realistic hypotheses, the mean BSP is linearly proportional to the IWC profile.

scholarly journals Interdependence of tropical cirrus properties and their variability

2008 ◽  
Vol 26 (3) ◽  
pp. 413-429 ◽  
Author(s):  
S. V. Sunilkumar ◽  
K. Parameswaran ◽  
Bijoy V. Thampi
Keyword(s):  
Optical Depth ◽  
Particle Orientation ◽  
Asymmetry Factor ◽  
Cloud Base ◽  
Short Term ◽  
Cloud Particle ◽  
Cloud Optical Depth ◽  
Opposite Phase ◽  
Cloud Dynamics ◽  
Cloud Evolution

Abstract. The mean properties of tropical cirrus, such as cloud top, cloud base, optic centre, cloud strength/optical depth, asymmetry factor and cloud depolarization, as well as their heterogeneities are examined using lidar observations over 281 nights from a tropical station Gadanki (13.5° N, 79.2° E) during the period 1998–2002. This study shows that as the cloud optical depth (τc) increases the cloud becomes more asymmetric in its scattering property. The amount of asymmetry is less than 2% for very low values of (τc and increases nonlinearly with an increase in (τc. The physical properties of these clouds also show significant variation with different time scales during the course of each night. On average, while the short-term variations in (τc are in opposite phase with those of the asymmetry factor (ξ) and volume depolarization ratio (δ), the long-term variation in (τc extending over a night are found to be in opposite phase with that of ξ and in-phase with that of δ. The short-term variations in δ and (τc were attributed to possible changes in the cloud particle orientation and the long period variations to cloud evolution process. The value of δ shows a pronounced variation along the vertical, with low values near the cloud top and cloud base and high values in the middle, which is attributed to the cloud dynamics.

scholarly journals Long-range transport of mineral aerosols and its absorbing and heating effects on cloud and precipitation: a numerical study

2007 ◽  
Vol 7 (1) ◽  
pp. 3203-3228 ◽  
Author(s):  
Y. Yin ◽  
L. Chen
Keyword(s):  
Optical Depth ◽  
Mineral Dust ◽  
Numerical Study ◽  
Cloud Condensation Nuclei ◽  
Dust Particles ◽  
Cloud Base ◽  
Condensation Nuclei ◽  
Dust Layer ◽  
Cloud Optical Depth ◽  
Mineral Aerosols

Abstract. There have been numerous recent publications showing that mineral dust might be a good absorber for solar radiation in addition to its capability as cloud condensation nuclei (CCN) and ice forming nuclei (IFN), and could lead to reduced cloud cover and precipitation in the region it presents. This effect is investigated using a cloud model with detailed microphysics of both warm and ice phase processes. The model is initialized using measured distributions and concentration of mineral dust particles. Our results show that when the dust layer with peak concentration appears at the cloud-base height and below 3 km, where the temperature is warmer than –5°C, inhibits the development of cloud particles and precipitation, and together with early activation of larger cloud droplets on giant cloud condensation nuclei, which accelerates drizzle formation through collision coalescence process, reduces the cloud optical depth and albedo. It is also found that only when the dust layer locates at altitudes with temperature colder than –5°C, mineral aerosols can act as effective ice nuclei and intensify the ice-forming processes. Under this condition, the existence of dust layer can either increase or decrease cloud optical depth and albedo, depending on the concentration and chemical composition of the absorbing components, or the time the mineral aerosols suspended in the atmosphere.

scholarly journals Satellite observations of peroxyacetyl nitrate from the Aura Tropospheric Emission Spectrometer

2014 ◽  
Vol 7 (6) ◽  
pp. 5347-5379 ◽  
Author(s):  
V. H. Payne ◽  
M. J. Alvarado ◽  
K. E. Cady-Pereira ◽  
J. R. Worden ◽  
S. S. Kulawik ◽  
...  
Keyword(s):  
Detection Limit ◽  
Transport Model ◽  
A Priori ◽  
Initial Guess ◽  
Chemical Transport Model ◽  
Peroxyacetyl Nitrate ◽  
Cloud Optical Depth ◽  
Spatial Features ◽  
Instrument Noise ◽  
Global Chemical Transport Model

Abstract. We present a description of the algorithm used to retrieve peroxyacetyl nitrate (PAN) concentrations from the Aura Tropospheric Emission Spectrometer (TES). We describe the spectral microwindows, error analysis and the utilization of a priori and initial guess information provided by the GEOS-Chem global chemical transport model. The TES PAN retrievals contain up to one degree of freedom for signal. Estimated single-measurement uncertainties are 30 to 50%. The detection limit for a single TES measurement is dependent on the atmospheric and surface conditions as well as on the instrument noise. For observations where the cloud optical depth is less than 0.5, we find that the TES detection limit for PAN is in the region of 200 to 300 pptv. We show that PAN retrievals over the Northern Hemisphere Pacific in springtime show spatial features that are qualitatively consistent with the expected distribution of PAN in outflow of Asian pollution.

Venus: Cloud optical depth and surface albedo from Venera 8

Icarus ◽  
1975 ◽  
Vol 24 (1) ◽  
pp. 19-27 ◽  
Author(s):  
C. Devaux ◽  
M. Herman
Keyword(s):  
Optical Depth ◽  
Surface Albedo ◽  
Cloud Optical Depth
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