Carsten Rathke

Retrieval of Cloud Microphysical Properties from Thermal Infrared Observations by a Fast Iterative Radiance Fitting Method

Abstract An algorithm is presented for inferring the IR optical depth, effective radius, and liquid water path of clouds from multispectral observations of emitted thermal radiation, which takes advantage of the larger number of spectral channels of future satellite IR sounders. The proposed technique consists of fitting the measured radiances with theoretical results obtained from full radiative transfer simulations, to allow the correct description of vertically inhomogeneous cloud layers. The retrieval is made efficiently treatable by using a fast multiple scattering infrared radiative transfer model and a fast iterative fitting procedure. The theoretical basis of both parts of the algorithm is given in detail, together with the underlying equations. The sensitivity of the method to measurement and modeling errors is investigated systematically from synthetic spectra covering the range of variability of observations of moderately thick, low-water clouds. Instrument absolute calibration errors, as well ...

Direct thermal radiative forcing of tropospheric aerosol in the arctic measured by ground based infrared spectrometry

[1] We present, to our knowledge for the first time in the high Arctic, atmospheric thermal emission spectra recorded by ground-based FTIR spectrometry revealing an excess emission contribution, which can only be attributed to the presence of tropospheric aerosols. Consideration of direct thermal forcing in climatological sensitive and remote regions as the Arctic seems to be important for more realistic modelling. According to our measurements, the aerosols can alter the flux density of downward longwave radiation by up to +2.99 to +4.66 W m -2 , and the flux density of outgoing longwave radiation by -0.23 to +1.17 W m -2 .

Properties of coastal Antarctic aerosol from combined FTIR spectrometer and sun photometer measurements

Remotely sensing the physical and chemical properties of summertime aerosol at the Antarctic coastal station Neumayer has been accomplished for the first time by a combined analysis of atmospheric thermal emission spectra, measured by an FTIR spectrometer, and atmospheric visible-near infrared extinction spectra, measured by a sun photometer. From the synergy of both spectral ranges, we find that the aerosol is composed of 1.1-1.6 mg m -2 of sulfates, with the water component in the solid phase, having a bimodal size distribution with radii peaking at 0.04 and 0.64 μm. We also provide the first estimate of the direct thermal radiative forcing of this aerosol: +1.68 W m -2 at the surface, and +0.006 W m -2 at the top of the atmosphere.

Radiative and microphysical properties of Arctic stratus clouds from multiangle downwelling infrared radiances

[1] The information content of multiangle downwelling infrared radiance spectra of stratus clouds is investigated. As an example, 76 sets of spectra were measured at angles of 0, 15, 30 and 45� from zenith, using an interferometer based at the Surface Heat Budget of the Arctic Ocean (SHEBA) drifting ice camp. Exploiting the angular variation of radiance in infrared microwindows, a ‘‘geometric’’ algorithm is used to determine cloud temperature and optical depth without auxiliary information. For comparison, a spectral method allows us to infer cloud microphysical properties for each angle; each multiangle set therefore constitutes a microphysical characterization of horizontal inhomogeneity of the cloudy scene. We show that cloud temperatures determined with both approaches agree with temperatures obtained from lidar/radiosonde data. The multiangle radiance observations can also be used to calculate the longwave flux reaching the surface. We find that up to 14 W m � 2 of the overcast fluxes can be attributed to horizontal variations in cloud microphysical properties. INDEX TERMS: 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: infrared emission, Arctic clouds, multiangle FTIR Spectroscopy, horizontal inhomogeneity, longwave flux, SHEBA

Evaluation of four approximate methods for calculating infrared radiances in cloudy atmospheres

Abstract The goals of this study are the estimation of the accuracy of four approximations for the calculation of infrared radiances emerging at the top and bottom of a cloud layer and the identification of those useful for radiative transfer modelling and remote sensing. Therefore, results of two simple methods, the absorption approximation and the effective beam emissivity parameterization, as well as of two more elaborate methods, the multiple-stream absorption approximation with scattering effects and the delta-Eddington two-stream source function technique, are compared with “exact” results of the discrete ordinate radiative transfer method. The relative errors of the different approaches are evaluated at nine wavelengths located in the mid-infrared (3.99– 17.86 μm ), near the position of the channels of the MODIS and SEVIRI satellite spectrometers, over a wide range of cloud microphysical properties, observation angles, and cloud and clear-sky temperature contrasts. For the multiple-stream absorption approximation, the errors are presented as a function of the number of streams. The computational efficiencies of the four approximations are also compared. It is found that in several spectral ranges, the best accuracy is attained with the multiple-stream absorption approximation with scattering effects, and that the delta-Eddington two-stream source function technique offers the best overall applicability. Detailed error figures and tables are provided in order to facilitate the identification of particular pros and cons of each method.

Comparison of stratus cloud properties derived from coincident airborne visible and ground‐based infrared spectrometer measurements

Results of a remote sensing validation experiment are reported. Above cloud measurements of reflected solar radiation were made with a grating spectrometer, coincident below cloud measurements of emitted thermal radiation with a FTIR spectrometer. Two remote sensing algorithms based on radiative transfer simulations were employed to retrieve visible optical depths and effective droplet radii. The values derived from both measurements are consistent and indicative for the cloud vertical structure.

Infrared cloud properties retrieval algorithm accounting for cloud vertical inhomogeneity

A method is presented for inferring the microphysical and radiative properties of vertically inhomogeneous cloud layers from high spectral resolution measurements of emitted thermal radiation. The algorithm iteratively estimates vertical profiles of liquid water content and effective droplet radius by comparing spectral radiance observations with calculations from a multiple scattering IR radiative transfer model. Theoretically consistent spectral emissivity and effective temperature values are determined from the resulting cloud vertical structure. The influence of cloud vertical inhomogeneity on the retrieval is discussed.