Cirrus Optical Depth Research Articles

Nighttime cirrus detection using Atmospheric Infrared Sounder window channels and total column water vapor

A method of cirrus detection at nighttime is presented that utilizes 3.8 and 10.4 μm infrared (IR) window brightness temperature differences (dBT) and total column precipitable water (PW) measurements. This technique is applied to the Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Sounding Unit A (AMSU‐A) instrument suite on board EOS‐Aqua, where dBT is determined from sets of carefully selected AIRS window channels, while PW is derived from the synergistic AIRS and AMSU‐A water vapor retrievals. Simulated and observed dBT for a particular value of PW are not constant; several physical factors impact dBT, including the variability in temperature and relative humidity profiles, surface emissivity, instrument noise, and skin/near‐surface air temperature differences. We simulate clear‐sky dBT over a realistic range of PWs using 8350 radiosondes that have varying temperature and relative humidity profiles. Thresholds between cloudy and uncertain sky conditions are derived once the scatter in the clear‐sky dBT is determined. Simulations of optically thin cirrus indicate that this technique is most sensitive to cirrus optical depth in the 10 μm window of 0.1–0.15 or greater over the tropical and subtropical oceans, where surface emissivity and skin/near‐surface air temperature impacts on the IR radiances are minimal. The method at present is generally valid over oceanic regions only, specifically, the tropics and subtropics. The detection of thin cirrus, and other cloud types, is validated using observations at the Atmospheric Radiation Measurement (ARM) program site located at Manus Island in the tropical western Pacific for 89 coincident EOS‐Aqua overpasses. Even though the emphasis of this work is on the detection of thin cirrus at nighttime, this technique is sensitive to a broad cloud morphology. The cloud detection technique agrees with ARM‐detected clouds 82–84% of the time, which include thin cirrus, as well as other cloud types. Most of the disagreements are well explained by AIRS footprint‐scale heterogeneity compared to ARM point measurements, cirrus overlying lower‐layer water clouds, possible mixed phase microphysics in midlevel clouds, and significant IR channel noise for cold BT scenes over deep convective towers.

Simultaneous retrieval of aerosol and thin cirrus optical depths using MODIS airborne simulator data during CRYSTAL‐FACE and CLAMS

We have developed a new methodology to simultaneously retrieve the optical depths of aerosols and thin cirrus clouds over the oceans by using the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) data. This methodology takes advantage of the fact that the visible (or a near‐infrared window) reflectance from cirrus can be characterized by its correlation to the reflectance from a near‐infrared band located in a strong water vapor absorption region. Once the reflectance due to cirrus has been removed, the residual reflectance in these bands is used to retrieve aerosol optical depth. Cirrus cloud properties are subsequently obtained by minimizing the difference between observed and calculated reflectance values stored in look‐up tables that are constructed a priori and correlated to the retrieved aerosol optical depth. Examination of the data gathered from the recent Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL‐FACE) and Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) field campaigns demonstrates that our retrievals are consistent with both in‐situ and ground based measurements.

A new concept on remote sensing of cirrus optical depth and effective ice particle size using strong water vapor absorption channels near 1.38 and 1.88 /spl mu/m

Techniques for retrieving cloud optical properties, i.e., the optical depths and particle size distributions, using atmospheric channels in the visible and near-infrared spectral regions are well established. For partially transparent thin cirrus clouds, these channels receive solar radiances scattered by the surface and lower level water clouds. Accurate retrieval of optical properties of thin cirrus clouds requires proper modeling of the effects from the surface and the lower level water clouds. In this paper, we describe a new concept using two strong water vapor absorption channels near 1.38 and 1.88 /spl mu/m, together with one window channel, for remote sensing of cirrus optical properties. Both the 1.38- and 1.88-/spl mu/m channels are highly sensitive in detecting the upper level cirrus clouds. Both channels receive little scattered solar radiances from the surface and lower level water clouds because of the strong water vapor absorption below cirrus. The 1.88-/spl mu/m channel is quite sensitive to changes in ice particle size distributions, while the 1.38-/spl mu/m channel is less sensitive. These properties allow for simultaneous retrievals of optical depths and particle size distributions of cirrus clouds with minimal contaminations from the surface and lower level water clouds. Preliminary tests of this new concept are made using hyperspectral imaging data collected with the Airborne Visible Infrared Imaging Spectrometer. The addition of a channel near 1.88 /spl mu/m to future multichannel meteorological satellite sensors would improve our ability in global remote sensing of cirrus optical properties.

Comparison of cirrus optical depths derived from GOES 8 and surface measurements

Ground‐based passive radiometer measurements are used to validate satellite‐derived cirrus optical depths over the Atmospheric Radiation Measurement Program Southern Great Plains site during March 2000. Optical depths derived from direct beam measurements by a multifilter rotating shadow band radiometer were well correlated with those determined from the Geostationary Operational Environmental Satellite, especially in relatively homogenous cloud fields. Compared to the multifilter rotating shadow band radiometer (MFRSR) results, on average, the satellite retrieval overestimated optical depth by ∼0.67 (29%), even though 75% of the GOES values were within ±1.0 of the MFRSR results. Some of the bias is attributable to cloud inhomogeneities, mismatches in observed clouds, errors in the surface albedo, and possible errors in the ice crystal scattering phase function. The results demonstrate the potential for using MFRSR data, available over many parts of the globe, for validating satellite cloud retrievals in many different surface and atmospheric conditions.

Radiative effects of simulated cirrus clouds on top of a deep convective storm in METEOSAT second generation SEVIRI channels

¶Cirrus clouds often form above intense convective storms due to several different mechanisms and affect the radiation field at the top of the atmosphere. Radiative transfer computations are performed to characterize these effects within the spectral bands of METEOSAT Second Generation’s (MSG) Spinning Enhanced Visible and InfraRed Imager (SEVIRI). Computations refer to five visible, near infrared and infrared MSG SEVIRI channels centered at 0.8, 1.6, 3.9, 10.8, and 12.0 µm. Reflectances and brightness temperatures are computed using the 1-D radiative transfer model STREAMER adopting simple parameterizations of the cloud layers and associated microphysical properties for the determination of the necessary optical properties. A sensitivity study is carried out by varying the cirrus ice crystal size and optical depth. The 1.6 and 3.9 µm channels reveal instrumental for the simultaneous detection of optical depth and crystal size of the cirrus layer. In particular, the results of the 3.9 µm channel show that the smaller the crystal size the higher the reflectance values. The computations provide interpretation clues on the phenomenon of ice crystal plumes on top of deep convective clouds, which are known to produce enhanced reflectivity signatures in the 3.7 µm channel of the Advanced Very High Resolution Radiometer (AVHRR). The sensitivity of the IR channels to cirrus cloud optical depth and ice crystal size is examined and the brightness temperature differences evaluated. Satellite observations and radiative transfer computations are at present the only way of studying such cloud features due to the unavailability of in situ aircraft measurements.

LIRAD Observations of Tropical Cirrus Clouds in MCTEX. Part I: Optical Properties and Detection of Small Particles in Cold Cirrus*

Abstract This paper presents further results on the optical properties of tropical and equatorial cirrus using the light detecting and ranging (lidar) radiometer (LIRAD) method. The results were obtained from observations in the Maritime Continent Thunderstorm Experiment (MCTEX). Values were obtained of cirrus cloud backscatter coefficient, infrared (IR) emittance, optical depth and absorption coefficient, cloud height and depth, and backscatter-to-extinction ratio. The values agree well with previous results obtained on equatorial cirrus in the Pilot Radiation Observation Experiment (PROBE) and extend those results to lower temperatures. Observations made of lidar linear depolarization ratio show similar trends to those observed in PROBE, extending those results to lower temperatures. Regressions of cloud IR emittance and absorption coefficients are performed as a preliminary tropical dataset for both cloud-resolving and climate models. These regressions are compared with previous regressions on midlatit...

Surface variability effects on the remote sensing of thin cirrus optical and microphysical properties

We have developed a methodology for the retrieval of the optical and microphysical properties of thin cirrus clouds with optical depths less than 0.5 using Moderate Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) measurements conducted during the Subsonic Aircraft Cloud and Contrail Special Study (SUCCESS). This methodology involves the use of correlated reflectance at three channels (0.65, 1.6, and 2.2 μm). We demonstrate the necessity of employing accurate values of the upwelling cloud base reflectance fields and present a method for the computation of these fields based on the atmospheric correction approach. For ocean surfaces the anisotropic reflectance is atmospherically corrected using appropriate radiative transfer calculations, along with the retrieved aerosol optical depths based on a simple aerosol microphysical model. For land surfaces a mosaic of ecosystems is used to compute the anisotropic reflectance associated with the surface terrain variability. We show that using these explicit computations of the emerging cloud base reflectance, thin cirrus optical depths and ice crystal size over ocean surfaces can be retrieved accurately. Uncertainties in the retrieved optical depth and ice crystal size are further reduced by 20 and 45%, respectively, over complex land surfaces.

Effect of Cloud Vertical Inhomogeneity on the Retrieval of Cirrus Cloud Temperature and Infrared Optical Depth Using the ASTR

Abstract A method is described for the retrieval of cirrus cloud temperature and optical depth using thermal infrared data from the Along-Track Scanning Radiometer. The method utilizes above-cloud and nearby clear-sky thermal infrared data at a single wavelength and two different viewing angles and assumes that the cirrus cloud is nonscattering, isothermal, and semitransparent. The sensitivity of the method to small uncertainties in the input parameters is calculated. The effect on the retrieval of vertical inhomogeneity is investigated using idealized models of cirrus cloud vertical structure. It is shown that a vertical temperature structure within the cloud, alone and in conjunction with vertical inhomogeneity in absorption coefficient, can cause large errors in the retrieved quantities for a wide range of cloud types. However, these investigations show that retrieved quantities remain within usable limits for the majority of expected cirrus clouds. For example, for clouds with a lapse rate of 9 K km−1...

Optical and geometrical properties of northern midlatitude cirrus clouds observed with a UV Raman lidar

Abstract Measurements of cirrus optical and geometrical properties are presented. Data have been taken at Geesthacht (53.5°N, 10.5°E) in northern Germany between May 1994 and March 1996. The instrument used for this purpose is a two-laser UV Raman DIAL for simultaneous and independent measurements of ozone, water vapor and clouds. The combination of the Raman lidar technique and the polarization lidar technique yields the most complete set of cirrus cloud parameters measured remotely so far. A statistical analysis of cirrus midcloud height and temperature, top height and temperature, and of cirrus geometrical and optical depth is presented and discussed. The cloud mean extinction, geometrical and optical depths, lidar ratio and depolarization ratio are given as a function of midcloud temperature. For the first time the dependence of the lidar ratio upon the depolarization ratio is investigated. The material presented is considered as an important data basis for the modeling of the effects of cirrus clouds on climate.

Airborne retrieval of cirrus cloud optical and microphysical properties using Airborne Remote Earth Sensing System 5.1–5.3 and 3.7‐μm channel data

We present an airborne retrieval algorithm to infer cirrus cloud temperature, optical depth, and mean effective sizes using the Airborne Remote Earth Sensing System (ARES) hyperspectral spectrometer data for the 5.1–5.3 and 3.7 μm channels. The algorithm, development and the selection of the channels are based on the principle and parameterization of radiative transfer involving cirrus clouds and the associated atmospheric and surface properties. It has been applied to a selected case of the ARES data collected over the western Boston area on September 16, 1995. Validation of the retrieved parameters was carried out using the collocated and coincident ground‐based 8.6‐mm radar data and ice crystal size distribution measurements obtained from the 2D‐P probe on board the high‐altitude reconnaissance platform (HARP). We show that the retrieved cirrus cloud temperature, mean effective ice crystal size, and optical depth match closely with those derived from the observations.

Light scattering by nonspherical particles: Remote sensing and climatic implications

Calculations of the scattering and adsorption properties of ice crystals and aerosols, which are usually nonspherical, require specific methodologies. There is no unique theoretical solution for the scattering by nonspherical particles. Practically, all the numerical solutions for the scattering of nonspherical particles, including the exact wave equation approach, integral equation method, and discrete-dipole approximation, are applicable only to size parameters less than about 20. Thus, these methods are useful for the study of radiation problems involving nonspherical aerosols and small ice crystals in the thermal infrared wavelengths. The geometric optics approximation has been used to evaluate the scattering, absorption and polarization properties of hexagonal ice crystals whose sizes are much larger than the incident wavelength. This approximation is generally valid for hexagonal ice crystals with size parameters larger than about 30. From existing laboratory data and theoretical results, we illustrate that nonspherical particles absorb less and have a smaller asymmetry factor than the equal-projected area/volume spherical counterparts. In particular, we show that hexagonal ice crystals exhibit numerous halo and arc features that cannot be obtained from spherical particles; and that ice crystals scatter more light in the 60° to 140° scattering angle regions than the spherical counterparts. Satellite remote sensing of the optical depth and height of cirrus clouds using visible and IR channels must use appropriate phase functions for ice crystals. Use of an equivalent sphere model would lead to a significant overestimation and underestimation of the cirrus optical depth and height, respectively. Interpretation of the measurements for polarization reflected from sunlight involving cirrus clouds cannot be made without an appropriate ice crystal model. Large deviations exist for the polarization patterns between spheres and hexagonal ice crystals. Interpretation of lidar backscattering and depolarization signals must also utilize the scattering characteristics of hexagonal ice crystals. Equivalent spherical models substantially underestimate the broadband solar albedos of ice crystal clouds because of stronger forward scattering and larger absorption by spherical particles than hexagonal ice crystals. We illustrate that the net cloud radiative forcing at the top of the atmosphere involving most cirrus clouds is positive, implying that the IR greenhouse effect outweighs the solar albedo effect. If the radiative properties of equivalent spheres are used, a significant increase in cloud radiative forcing occurs. Using a one-dimensional cloud and climate model, we further demonstrate that there is sufficient model sensitivity, in terms of temperature increase, to the use of ice crystal models in radiation calculations.

Comparison of Cirrus Height and Optical Depth Derived from Satellite and Aircraft Measurements

Abstract During the International Cirrus Experiment (ICE'89) simultaneous measurements of Cirrus cloud-top height and optical depth by satellite and aircraft have been taken. Data from the Advanced Very High Resolution Radiometer (AVHRR) onboard the NOAA polar-orbiting meteorological satellite system have been used together with the algorithm package AVHRR processing scheme over clouds, land and ocean (APOLLO) to derive optical depth. NOAA High-Resolution Infrared Radiation Sounder (HIRS) data have been used together with a bispectral technique to derive cloud-top height. Also, the optical depth of some contrails could be estimated. Airborne measurements have been performed simultaneously by using the Airborne Lidar Experiment (ALEX), a back-scatter lidar. Comparison of satellite data with airborne data showed agreement of the top heights to about 500 m and of the optical depths to about 30%. These uncertainties are within the limits obtained from error estimates.

The 27–28 October 1986 FIRE Cirrus Case Study: Retrieval of Cloud Particle Sizes and Optical Depths from Comparative Analyses of Aircraft and Satellite-based Infrared Measurements

Infrared radiance measurements were acquired from a narrow-field nadir-viewing radiometer based on the NASA ER-2 aircraft during a coincident Landsat 5 overpass on 28 October 1986 as part of the FIRE Cirrus IFO in the vicinity of Lake Michigan. The spectral bandpasses are 9.90–10.87 μm for the ER-2–based radiometer and 10.40–12.50 μm for the Landsat thematic mapper band. After adjusting for spatial and temporal differences, a comparative study using data from these two instruments is undertaken in order to retrieve cirrus cloud ice-crystal sizes and optical depths. Retrieval is achieved by analysis of measurement correlations between the two spectral bands and comparison to multistream radiative transfer model calculations. The results indicate that the equivalent sphere radii of the cirrus ice crystals were typically less than 30 μm. Such particles were too small to be measured by the available in situ instrumentation. Cloud optical depths at a reference wavelength of 11.4 µm ranged from 0.3 to 2.0 for this case study. Supplemental results in support of this study are described using radiation measurements from the King Air aircraft, which was also in near coincidence with the Landsat overpass.

Remote Sounding of the Tropical Cirrus Cloud Temperature and Optical Depth Using 6.5 and 10.5 μm Radiometers during STEP

A dual-channel retrieval technique involving the water vapor band at 6.5 microns and the window region at 10.5 microns has been developed to infer the temperature and emissivity of tropical anvils. This technique has been applied to data obtained from the ER-2 narrow field-of-view radiometers during two flights in the field observation of the Stratosphere-Troposphere Exchange Project near Darwin, Australia, January-February 1987. The retrieved cloud temperatures are between 190 and 240 K, while the cloud emissivities derived from the retrieval algorithm range from about 0.2 to 1. Moreover, the visible optical depths have been obtained from the cloud emissivity through a theoretical parameterization with values of 0.5-10. A significant portion of tropical cirrus clouds are found to have optical depths greater than about 6. Because of the parameterization, the present technique is unable to precisely determine the optical depth values for optically thick cirrus clouds.

Temperature dependence of cirrus extinction: Implications for climate feedback

The measured temperature dependence of infrared absorption in cirrus cloud obtained from previous extensive lidar and radiometer (LIRAD) observations of cirrus is used to investigate the sensitivities of changes in cirrus optical properties to changes in global temperature. Values of infrared absorption and ice water content calculated previously from observations of cirrus cloud microphysics are also used to investigate climate sensitivities. The values of calculated mean infrared absorption give a very similar temperature dependence, and thus climate sensitivity, to the LIRAD results. Values of mean ice water content calculated from the observations of cloud microphysics are compared with the available ice water content calculated from moist adiabatic ascent through a vertical cloud depth of 300 m, and a similar temperature dependence is found in both cases. However, taking into account the observed cloud depths of cirrus, considerable dilution of ice water content below adiabatic values is obviously occurring. The temperature sensitivities of extinction (or absorption) coefficients (calculated from a combination of LIRAD and microphysics results) are found to be generally less than the temperature sensitivities of ice water contents, due to an increase in cloud particle mode radius with temperature. The temperature sensitivity of cirrus extinction is found to vary from 0.033°C−1 at −22.5°C to 0.200°C−1 at −72.5°C. Equivalent sensitivities for cirrus optical depth are influenced by observed changes in cirrus cloud depth with temperature and vary from 0.003°C−1 at −22.5°C to 0.241°C−1 at −72.5°C. As high cold cirrus clouds are considered to cause a positive feedback with global temperature change, such a feedback is seen to become particularly strong at the lowest temperatures. A simple method of parameterization of cirrus visible and infrared optical properties in terms of cloud temperature is presented, thus making it possible to introduce cirrus optical properties into a numerical model without requiring knowledge of the ice water content.

Remote sounding of the cirrus optical depth and temperature from 3.7 and 11 micrometer windows

On the basis of forward IR radiation transfer analyses for an atmosphere containing semi-transparent, non-black cirrus, parameterization equations are derived for the retrieval of the cloud optical depth and cloud temperature utilizing AVHRR 3.7 and 10.8 μm channels. The retrieval techniques developed involve the use of either dual-frequency or dual-scanning angle radiance observations. We show that the cloud optical depth and cloud temperature may be inferred successively from the observed brightness temperature differences using these two techniques: Numerical experiments and error analyses demonstrate that the dual-frequency method is specifically appropriate for optically thin cirrus cases (τ<1). In case the optical depth of cirrus is close to 3, combination of dual-frequency and dual-scanning angle methods is shown to give reasonable accuracy for the cloud optical depth and temperature retrieval.

Determination of the cirrus particle single-scattering phase function from lidar and solar radiometric data

The single-scattering phase function of cirrus particles in the free atmosphere has been determined between angles of 15 and 90° in the forward direction and at 180° in the back direction. The technique involved measuring the solar radiation intensity scattered by thin cirrus with a lidar receiver and also determining the cirrus optical depth simultaneously from a lidar backscatter return. The solar scattered radiance appeared as a background signal on the lidar echo. The phase function at 180° was determined from the integrated cloud backscatter using standard techniques. Data were obtained during two experimental phases covering summer and winter periods. The values of the measured scattering functions fell within the range of various laboratory measurements. The results showed evidence for the 22° halo associated with scattering from hexagonal crystals.

The effect of cirrus of varying optical depth on the extraterrestrial net radiative flux

AbstractA simple model has been used to investigate the effects on the net radiation at the top of the atmosphere of a change in cirrus optical depth at various latitudes. the model employs relationships between the cloud solar albedo and the cloud infrared emittance which are based on theoretical considerations and also supported by recent observations. the model is used to predict the change in net radiant flux at the top of the atmosphere with a unit change in cloud amount and with a change in cloud emittance. Generally, tropical and mid‐latitude cirrus clouds cause a warming effect until the optical depths become large. For optical depths greater than about 12 for tropical cirrus, and about 5 for mid‐latitude cirrus, an increase in cloud will cause a cooling tendency.The importance of specifying the correct albedo‐emittance relationships is emphasized by a comparison of the solar reflection properties of cirrus using two different theoretical models; one model envisages cirrus as composed of ice (or water) spheres and the other as composed of long ice cylinders. Recent observations tend to support the cylinder model.