Radiative Properties Of Cirrus Research Articles

The Impacts of Single-Scattering and Microphysical Properties of Ice Particles Smaller Than 100 µm on the Bulk Radiative Properties of Tropical Cirrus

There are large uncertainties in the single-scattering (i.e., morphologies) and microphysical (i.e., concentrations) properties of ice particles whose size are less than ~100 µm. Insufficient resolutions of the most advanced cloud probes (e.g., cloud particle imager) cannot resolve the micrometer-scale morphologies of small ice particles. Further, the shattering of large ice particles on probes’ inlets or tips causes uncertainties in the measurement of the concentrations of small ice particles. These uncertainties have large impacts on the single-scattering and microphysical properties of small ice particles that are utilized to quantify the bulk radiative properties of cirrus. In this study, the impacts of uncertainties in the morphologies and concentrations of small ice particles on the bulk radiative properties of tropical cirrus were calculated using measurements acquired during the Tropical Warm Pool-International Cloud Experiment. Five different models (i.e., budding Buckyball, Chebyshev particle, droxtal, Gaussian random sphere, and sphere) that represent the shapes of small ice particles were used to calculate the single-scattering properties. The bulk radiative properties, average phase-function (P11¯), and average asymmetry parameter (g¯) were computed by combining the measured size/habit distributions and the calculated single-scattering properties of ice particles. The impacts of the selection of varying morphologies of small particles on the bulk radiative properties were quantified. For these calculations, the possible range of the concentrations of small ice particles which depend on the degree of shattered large particles were also used. The impacts of varying the single-scattering properties of small ice particles on the bulk radiative properties were the largest in the upper parts of cirrus (T < −60 °C), while they were the smallest in the lower parts of cirrus (−45 < T < −30 °C). The impacts of uncertainties in the concentrations of small ice particles on the bulk radiative properties were largest in the lower parts of cirrus (−45 < T < −30 °C), whereas they were smallest in the upper parts of cirrus (T < −60 °C). The effect of shattering was maximum in the lower parts of cirrus, whilst it was minimum in the upper parts of cirrus. The combined impacts of uncertainties in the single-scattering (i.e., morphologies) and microphysical (i.e., concentrations) properties of small ice particles revealed variations of up to 11.2% (127.1%; 67.3%) of the integrated intensity in the forward (sideward; backward) angles in P11¯ and a corresponding change in g¯ by up to 12.61%.

Cirrus Clouds

Abstract The goal of this chapter is to synthesize information about what is now known about one of the three main types of clouds, cirrus, and to identify areas where more knowledge is needed. Cirrus clouds, composed of ice particles, form in the upper troposphere, where temperatures are generally below −30°C. Satellite observations show that the maximum-occurrence frequency of cirrus is near the tropics, with a large latitudinal movement seasonally. In situ measurements obtained over a wide range of cirrus types, formation mechanisms, temperatures, and geographical locations indicate that the ice water content and particle size generally decrease with decreasing temperature, whereas the ice particle concentration is nearly constant or increases slightly with decreasing temperature. High ice concentrations, sometimes observed in strong updrafts, result from homogeneous nucleation. The satellite-based and in situ measurements indicate that cirrus ice crystals typically differ from the simple, idealized geometry for smooth hexagonal shapes, indicating complexity and/or surface roughness. Their shapes significantly impact cirrus radiative properties and feedbacks to climate. Cirrus clouds, one of the most uncertain components of general circulation models (GCM), pose one of the greatest challenges in predicting the rate and geographical pattern of climate change. Improved measurements of the properties and size distributions and surface structure of small ice crystals (about 20 μm) and identifying the dominant ice nucleation process (heterogeneous versus homogeneous ice nucleation) under different cloud dynamical forcings will lead to a better representation of their properties in GCM and in modeling their current and future effects on climate.

Uncertainties in synthetic Meteosat SEVIRI infrared brightness temperatures in the presence of cirrus clouds and implications for evaluation of cloud microphysics

Synthetic brightness temperatures of five infrared Meteosat SEVIRI channels are investigated for their sensitivities on cirrus radiative properties. The operational SynSat scheme of the regional German weather prediction model COSMO-DE is contrasted to a revised scheme with a special emphasis on consistency between the model-internal ice-microphysics and infrared radiation in convective situations. In particular, the formulation of generalized effective diameters of ice, snow and graupel as well as subgrid-scale cloud cover has been improved. Based on the applied modifications, we first show that changed assumptions on the cirrus radiative properties can lead to 10K warmer brightness temperatures. Second, we demonstrate that prescribed relative changes of 20% in cloud cover and particle size induce maximum changes of around 4 to 5K. The maximum sensitivity appears for semi-transparent cirrus having brightness temperatures around 240 and 260K and total frozen water path around 30gm−2 for viewing geometries over Central Europe. We further consider the known COSMO-DE cold bias to discuss the problem of inconsistencies in model-internal and external formulations of cloud microphysical and radiative properties. We demonstrate that between 35% and 70% of the cold bias can be attributed to the radiative representation of cirrus clouds. We additionally discuss the use of window-channel brightness temperature differences for evaluation of model microphysics and hypothesize that the amount of COSMO-DE ice is overestimated in convective situations.

Ice supersaturations and cirrus cloud crystal numbers

Abstract. Upper tropospheric observations outside and inside of cirrus clouds indicate water vapour mixing ratios sometimes exceeding water saturation. Relative humidities over ice (RHice) of up to and more than 200% have been reported from aircraft and balloon measurements in recent years. From these observations a lively discussion continues on whether there is a lack of understanding of ice cloud microphysics or whether the water measurements are tainted with large uncertainties or flaws. Here, RHice in clear air and in ice clouds is investigated. Strict quality-checked aircraft in situ observations of RHice were performed during 28 flights in tropical, mid-latitude and Arctic field experiments in the temperature range 183–240 K. In our field measurements, no supersaturations above water saturation are found. Nevertheless, super- or subsaturations inside of cirrus are frequently observed at low temperatures (<205 K) in our field data set. To explain persistent RHice deviating from saturation, we analysed the number densities of ice crystals recorded during 20 flights. From the combined analysis – using conventional microphysics – of supersaturations and ice crystal numbers, we show that the high, persistent supersaturations observed inside of cirrus can possibly be explained by unexpected, frequent very low ice crystal numbers that could scarcely be caused by homogeneous ice nucleation. Heterogeneous ice formation or the suppression of freezing might better explain the observed ice crystal numbers. Thus, our lack of understanding of the high supersaturations, with implications for the microphysical and radiative properties of cirrus, the vertical redistribution of water and climate, is traced back to the understanding of the freezing process at low temperatures.

Midlatitude Cirrus Clouds Derived from Hurricane Nora: A Case Study with Implications for Ice Crystal Nucleation and Shape

Hurricane Nora traveled up the Bala Peninsula coast in the unusually warm El Nino waters of September 1997, until rapidly decaying as it approached Southern California on 24 September. The anvil cirrus blowoff from the final surge of tropical convection became embedded in subtropical flow that advected the cirrus across the western US, where it was studied from the Facility for Atmospheric Remote Sensing (FARS) in Salt Lake City, Utah. A day later, the cirrus shield remnants were redirected southward by midlatitude circulations into the Southern Great Plains, providing a case study opportunity for the research aircraft and ground-based remote sensors assembled at the Clouds and Radiation Testbed (CART) site in northern Oklahoma. Using these comprehensive resources and new remote sensing cloud retrieval algorithms, the microphysical and radiative cloud properties of this unusual cirrus event are uniquely characterized. Importantly, at both the FARS and CART sites the cirrus generated spectacular optical displays, which acted as a tracer for the hurricane cirrus, despite the limited lifetimes of individual ice crystals. Lidar polarization data indicate widespread regions of uniform ice plate orientations, and in situ particle masticator data show a preponderance of pristine, solid hexagonal plates and columns. It is suggested that these unusual aspects are the result of the mode of cirrus particle nucleation, presumably involving the lofting of sea-salt nuclei in thunderstorm updrafts into the upper troposphere. This created a reservoir of haze particles that continued to produce halide-saltcontaminated ice crystals during the extended period of cirrus cloud maintenance. The reference that marine microliters are embedded in the replicas of ice crystals collected over the CART site points to the longevity of marine effects. Various nucleation scenarios proposed for cirrus clouds based on this and other studies, and the implications for understanding cirrus radiative properties or a global scale, are discussed.

Retrieval of Cirrus Cloud Radiative and Backscattering Properties Using Combined Lidar and Infrared Radiometer (LIRAD) Measurements

Abstract A method for retrieval of cirrus macrophysical and radiative properties using combined ruby lidar and infrared radiometer measurements is explained in detail. The retrieval algorithm includes estimation of a variable backscatter-to-extinction ratio for each lidar profile, which accounts for changes in cloud microphysical properties with time. The technique also utilizes a correlated K distribution radiative transfer model, where absorption coefficients K have been tabulated specifically for the bandwidth and filter function of the infrared radiometer. The radiative transfer model allows for estimation of infrared emission due to atmospheric water vapor, ozone, and carbon dioxide, which is essential for deriving cirrus radiative properties. Also described is an improved technique for estimation of upwelling IR radiation that is emitted by the surface of the earth and reflected by the cloud into the radiometer field of view. Derived cirrus cloud properties include base and top height and temperatur...

Parameterization of Tropical Cirrus Ice Crystal Size Distributions and Implications for Radiative Transfer: Results from CEPEX

Average ice crystal size distributions are parameterized as functions of temperature and ice water content (IWC), based on observations in cirrus produced as outflows of deep convection made during the Central Equatorial Pacific Experiment (CEPEX), as the sum of a first-order gamma function, describing ice crystals with melted equivalent diameters (Dm) less than 100 mm, and a lognormal function, describing larger ice crystals. The fit parameters are chosen to minimize the chi-squared function describing the difference between observed and parameterized distribution functions. The parameterization is mass conserving, accurately represents small ice crystals, and is easily integrable. The parameterization gives accurate estimates of mass, area, and number contained in different size ranges. The radiative properties estimated from midlatitude parameterizations are compared with those estimated from this parameterization using anomalous diffraction theory. As opposed to some previous studies, small crystals do not dominate the mass and radiative properties of cirrus. Comparison with midlatitude observations shows that size distribution shape can vary substantially depending on where, when, and how the cirrus is measured and on how it forms.

A Model Predicting the Evolution of Ice Particle Size Spectra and Radiative Properties of Cirrus Clouds. Part II: Dependence of Absorption and Extinction on Ice Crystal Morphology

This study builds upon the microphysical modeling described in Part 1 by deriving formulations for the extinction and absorption coefficients in terms of the size distribution parameters predicted from the micro-physical model. The optical depth and single scatter albedo of a cirrus cloud can then be determined, which, along with the asymmetry parameter, are the input parameters needed by cloud radiation models. Through the use of anomalous diffraction theory, analytical expressions were developed describing the absorption and extinction coefficients and the single scatter albedo as functions of size distribution parameters, ice crystal shapes (or habits), wavelength, and refractive index. The extinction coefficient was formulated in terms of the projected area of the size distribution, while the absorption coefficient was formulated in terms of both the projected area and mass of the size distribution. These properties were formulated as explicit functions of ice crystal geometry and were not based on an 'effective radius.' Based on simulations of the second cirrus case study described in Part 1, absorption coefficients predicted in the near infrared for hexagonal columns and rosettes were up to 47% and 71% lower, respectively, than absorption coefficients predicted by using equivalent area spheres. This resulted in single scatter albedos in the near-infrared that were considerably greater than those predicted by the equivalent area sphere method. Reflectances in this region should therefore be underestimated using the equivalent area sphere approach. Cloud optical depth was found to depend on ice crystal habit. When the simulated cirrus cloud contained only bullet rosettes, the optical depth was 142% greater than when the cloud contained only hexagonal columns. This increase produced a doubling in cloud albedo. In the near-infrared (IR), the single scatter albedo also exhibited a significant dependence on ice crystal habit. More research is needed on the geometrical properties of ice crystals before the influence of ice crystal shape on cirrus radiative properties can be adequately understood. This study provides a way of coupling the radiative properties of absorption, extinction, and single scatter albedo to the microphysical properties of cirrus clouds. The dependence of extinction and absorption on ice crystal shape was not just due to geometrical differences between crystal types, but was also due to the effect these differences had on the evolution of ice particle size spectra. The ice particle growth model in Part 1 and the radiative properties treated here are based on analytical formulations, and thus represent a computationally efficient means of modeling the microphysical and radiative properties of cirrus clouds.

Role of small ice crystals in radiative properties of cirrus: A case study, FIRE II, November 22, 1991

Aircraft observations of cirrus cloud were made near Coffeyville, Kansas, during November 1991 as part of the FIRE II project. Cloud ice particle spectra measurements were made using both a PMS 2DC probe and an ice particle replicator. Particles larger than 200 μm were column rosettes. The replicator shows the presence of large numbers of ice crystals smaller than 66 μm (two PMS size bins) that are not recorded by the PMS 2DC probe. Calculations based on the replicator data of the geometrical blocked area and absorption cross section of the cloud per unit volume show that small particles can contribute significantly to and sometimes dominate both the solar extinction and the infrared emission. Intercomparison is made of the ice particle size, area, and mass distributions determined by these different instruments. Power law relationships for area occluded by a crystal as a function of crystal maximum dimension were computed from the PMS 2DC data. The wavelength‐dependent infrared absorption cross section per volume was computed using a simple model based on anomalous diffraction and area and mass dimensional relationships for the ice crystals.

Inference of Cirrus Cloud Properties Using Satellite-observed Visible and Infrared Radiances. Part II: Verification of Theoretical Cirrus Radiative Properties

The study presents a methodology developed to apply a parameterization of radiative transfer calculations to satellite analyses of cirrus clouds. Cloud heights and optical depths are derived from visible and IR window measurements taken during FIRE when cirrus clouds were present. Geostationary satellite retrievals are compared to lidar-derived cloud heights and retrievals from a polar-orbiting satellite taken at different angles to determine which theoretical models of scattering phase function and single-scattering albedo best represent actual cirrus clouds. Models using small scattering phase function and single-scattering albedo best represent actual cirrus clouds. It is concluded that interpretation of cirrus reflectance with water-droplet models leads to biased results. The cloud-height and optical depth biases can be minimized with the aid of the C20 or cirrostratus models.

A Theoretical and Observational Study of the Radiative Properties of Cirrus: Results from FIRE 1986

A two-stream radiative transfer model is used to examine the radiative properties of cirrus clouds and compare simulations with the observations made during the cirrus FIRE IFO. The sensitivity of cirrus cloud radiative properties to altitude and size distribution changes are examined. The net radiative effect of cirrus in the infrared is largely determined by the amount of ice in the cloud and the surface—cloud base temperature difference (and thus altitude). Increases (decreases) of this temperature difference produce a net radiative heating (cooling). Cloud-top solar heating increases (decreases) with increasing (decreasing) altitude as the optical path of the atmosphere above the cloud layer decreases (increases). The impact of varying concentrations of ice particles less than 100 μm in diameter is also examined. The addition of these particles greatly enhances the longwave absorption and shortwave albedo of cirrus clouds in a manner that is spectrally dependent. Model simulations using observed microphysical and environmental conditions are compared to measured cirrus cloud radiative properties. Although cloud inhomogeneties are shown to be quite large, broad agreement in the cloud emittance is found between the highly uncertain observations of FIRE, other aircraft observations, and model simulations. Similar comparisons of the solar albedo reveal cirrus clouds to be significantly brighter than predicted by the model. Possible explanations of this brightening anomaly suggest that it may not be possible to use Mie scattering to model the cloud albedo.

A Two-Dimensional Radiation-Turbulence Climate Model. I: Sensitivity to Cirrus Radiative Properties

Based on the thermodynamic energy balance between radiation and vertical plus horizontal dynamic transports, a two-dimensional radiation-turbulence climate model is developed. This model consists of a broadband solar and IR radiation transfer scheme previously presented by the authors and vertical and horizontal dynamic eddy transports utilizing the elementary turbulent theory. In the model, three kinds of feedback mechanisms are considered: the humidity feedback via the constant relative humidity assumption, the ice-albedo feedback via a preliminary correlation between the surface albedo and the surface temperature and the dynamic transport feedback through the parameterization of eddy transports and the prescribed mean wind field. A standard temperature field, which differs from the climatological data by no more than 0.1°C, is first obtained by solving the coupled thermodynamic and surface flux budget equations using climatological distributions of H2O, CO2, O3, surface albedo and cloud properties. The model-derived atmospheric radiation budget, surface energy balance and horizontal transport patterns compare reasonably well with available observational data. Further validation of the model includes sensitivity studies on the effects of doubling of CO2 and a 2% increase in the solar constant. The temperature changes relative to the standard field on these experiments agree closely with those presented by Manabe and Wetherald utilizing a general circulation model. To investigate the two-dimensional cirrus–radiation interaction, a relationship between the cirrus IR emissivity and solar reflectance (and transmittance) is established based on the parameterization equations. On the basis of a number of experiments involving various couplings and feedbacks, we find that 1) the humidity and albedo feedbacks are most active in the tropics and arctic area, respectively, 2) the dynamic transport is a negative feedback in the equatorial and polar regions but a positive one in midlatitudes and 3) the relative importance of each feedback depends only slightly on the radiative properties of cirrus. Finally, we demonstrate that slight variations in the cirrus IR emissivity lead to significant temperature perturbations in the arctic surface and tropical troposphere.

On the Radiative Properties of Cirrus in the Window Region and Their Influence on Remote Sensing of the Atmosphere

Abstract Transmission, emission and reflection characteristics of cirrus clouds in the 800–1200 cm−1 window region are obtained by means of solving the transfer of infrared radiation in the realistic moist and dry model atmospheres. These model atmospheres include the water vapor within, above and below the cirrus clouds which are assumed to be composed of randomly oriented cylinders. Results of the radiation parameters are presented in terms of the geometrical thickness of circus as well as the model atmospheres. The discrepancies between the transmission and emission for flux and those for the vertically emergent intensity are pointed out and discussed. It is shown that the radiative properties of cirrus depend strongly upon the particle concentration. For a typical cirrus with a thickness of 1 km whose concentration is 0.05 cm−3, the intensity transmission is about 0.65 with an emissivity of about 0.35. Further, based upon the theoretical analyses, the reductions in the effective temperature when cirru...