Abstract.Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were performed at Tsukuba, Japan (36.1 • N, 140.1 • E), in November-December 2006. By analyzing the measured spectra of scattered sunlight with DOAS and optimal estimation methods, we first retrieve the aerosol optical depth (τ ) and the vertical profile of the aerosol extinction coefficient (k) at 476 nm in the lower troposphere. These retrieved quantities are characterized through comparisons with coincident lidar and sky radiometer measurements. The retrieved k values for layers of 0-1 and 1-2 km agree with lidar data to within 30% and 60%, respectively, for most cases, including partly cloudy conditions. Results similar to k at 0-1 km are obtained for the retrieved τ values, demonstrating that MAX-DOAS provides a new, unique aerosol dataset in the lower troposphere.

Demands for Monte Carlo radiative transfer modeling have grown with the increase in computational power in recent decades. This method provides realistic simulations of radiation processes for various types of application, including radiation budgets in cloudy conditions and remote measurements of clouds, aerosols, and gases. Despite many advantages, such as explicit treatment of three-dimensional radiative transfer, issues of numerical efficiency can make the method intractable, especially in radiance calculations. The commonly used local estimation method requires computationally intensive ray tracing at each collision. Furthermore, the realistic phase function of Mie scattering by cloud and aerosol particles has very sharp peaks in the forward direction. Radiance computations by Monte Carlo methods are inefficient for such spiky phase functions because of significant noise. Moreover, in optically thin regions, sampling of radiance contributions is so rare that long computing times are required to reduce noise. To solve these issues, several variance reduction methods have been proposed. This paper discusses a modified local estimation method, a truncation approximation for a highly anisotropic phase function, a collision-forcing method for optically thin media, a numerical diffusion technique, and several related topics. Numerical experiments demonstrated significant improvements in efficiency for solar radiance calculations in a limited number of cloudy cases.

Abstract. The results of a comparison exercise of radiative transfer models (RTM) of various international research groups for Multiple AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) viewing geometry are presented. Besides the assessment of the agreement between the different models, a second focus of the comparison was the systematic investigation of the sensitivity of the MAX-DOAS technique under various viewing geometries and aerosol conditions. In contrast to previous comparison exercises, box-air-mass-factors (box-AMFs) for different atmospheric height layers were modelled, which describe the sensitivity of the measurements as a function of altitude. In addition, radiances were calculated allowing the identification of potential errors, which might be overlooked if only AMFs are compared. Accurate modelling of radiances is also a prerequisite for the correct interpretation of satellite observations, for which the received radiance can strongly vary across the large ground pixels, and might be also important for the retrieval of aerosol properties as a future applicationCorrespondence to: T. Wagner (thomas.wagner@iup.uni-heidelberg.de) of MAX-DOAS. The comparison exercises included different wavelengths and atmospheric scenarios (with and without aerosols). The strong and systematic influence of aerosol scattering indicates that from MAX-DOAS observations also information on atmospheric aerosols can be retrieved. During the various iterations of the exercises, the results from all models showed a substantial convergence, and the final data sets agreed for most cases within about 5%. Larger deviations were found for cases with low atmospheric optical depth, for which the photon path lengths along the line of sight of the instrument can become very large. The differences occurred between models including full spherical geometry and those using only plane parallel approximation indicating that the correct treatment of the Earth's sphericity becomes indispensable. The modelled box-AMFs constitute an universal data base for the calculation of arbitrary (total) AMFs by simple convolution with a given trace gas concentration profile. Together with the modelled radiances and the specified settings for the various exercises, they can serve as test cases for future RTM developments.Published by Copernicus GmbH on behalf of the European Geosciences Union.

[1] The physical and optical properties of persistent contrails were studied with the measurements made by the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar. MODIS data were used to determine the contrail locations on the basis of their artificial shapes easily distinguished from natural cirrus, and the so-identified contrails were analyzed with collocated CALIPSO lidar data. Statistics of the geography, geometry, meteorology, and optical properties are reported for approximately 3400 persistent contrails observed over North America, the North Atlantic Ocean, and Europe. The majority of the detected contrails appear in ice-supersaturated air with temperatures lower than À40C. On average, contrails have significantly larger backscattering coefficients and slightly higher linear depolarization ratios (LDRs) than neighboring cirrus clouds. Depolarization tends to be strong when ice crystals are small, and LDR is approximately 0.4-0.45 for young contrails and contrail cores. The mean LDR for the detected contrails increases with decreasing temperature and is not strongly dependent on the lidar pointing angle. The backscattering properties suggest that contrails are primarily composed of small, randomly oriented ice crystals but may also contain a few horizontally oriented plates. Most contrails are optically thin with a mean (median) optical thickness of approximately 0.19 (0.14); however, optically thicker contrails do exist and tend to occur in warmer and more humid ambient air. The mean value and range of the observed LDR data are consistent with theoretical predictions based on a mixture of nonspherical ice crystals randomly oriented in the atmosphere.

No abstract