Evelyn Jäkel

ACRIDICON–CHUVA Campaign: Studying Tropical Deep Convective Clouds and Precipitation over Amazonia Using the New German Research Aircraft HALO

AbstractBetween 1 September and 4 October 2014, a combined airborne and ground-based measurement campaign was conducted to study tropical deep convective clouds over the Brazilian Amazon rain forest. The new German research aircraft, High Altitude and Long Range Research Aircraft (HALO), a modified Gulfstream G550, and extensive ground-based instrumentation were deployed in and near Manaus (State of Amazonas). The campaign was part of the German–Brazilian Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems–Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modeling and to the GPM (Global Precipitation Measurement) (ACRIDICON– CHUVA) venture to quantify aerosol–cloud–precipitation interactions and their thermodynamic, dynamic, and radiative effects by in situ and remote sensing measurements over Amazonia. The ACRIDICON–CHUVA field observations were carried out in cooperation with the second intensive operating period...

Airborne system for fast measurements of upwelling and downwelling spectral actinic flux densities

An airborne system for fast measurements of spectral actinic flux densities in the wavelength range 305-700 nm is introduced. The system is called the Actinic Flux Density Meter (AFDM). The AFDM utilizes the diode array technique and measures downwelling and upwelling spectral actinic flux densities separately with a time resolution of less than 1 s. For airborne measurements this means a spatial resolution of approximately 60 m, assuming an average aircraft velocity of 60 m/s. Thus the AFDM resolves fast changes in the actinic radiation field, which are of special importance for conditions of inhomogeneous clouds or surface reflection. Laboratory characterization measurements of the AFDM are presented, and a method to correct the nonideal angular response of the optical inlets is introduced. Furthermore, exemplar field data sampled simultaneously with spectral irradiance measurements are shown. The horizontal variability of the measured spectra of actinic flux density is quantified, and profile measurements for overcast situations are presented. Finally, the effects of clouds on the spectral actinic flux density are discussed.

A CCD Spectroradiometer for Ultraviolet Actinic Radiation Measurements

Abstract A new spectroradiometer for spectral measurements of ultraviolet (UV) atmospheric radiation (290–400 nm) using a charge coupled device (CCD) as a detector is introduced. The instrument development is motivated by the need for measurements with (a) high accuracy in the UV-B spectral range (290–315 nm) for photochemistry applications and (b) high temporal resolution in quickly changing atmospheric conditions such as partial cloud cover. The new CCD instrument is mainly intended for airborne use. It allows fast data collection (<300 ms time resolution for each spectrum) with improved sensitivity in the UV spectral range. The instrumental setup and its characterization in terms of stray light, dark current, noise, and detection limits are described and compared to a spectroradiometer with a photodiode array (PDA) detector. The new CCD spectroradiometer has a one order of magnitude greater sensitivity than the PDA-based spectroradiometer. However, the stray light of the CCD instrument is wavelength de...

Improving solar ultraviolet irradiance measurements by applying a temperature correction method for Teflon diffusers

To establish trends in surface ultraviolet radiation levels, accurate and stable long-term measurements are required. The accuracy level of todays measurements has become high enough to notice even smaller effects that influence instrument sensitivity. Laboratory measurements of the sensitivity of the entrance optics have shown a decrease of as much as 0.07-0.1%/deg temperature increase. Since the entrance optics can heat to greater than 45 degrees C in Dutch summers, corrections are necessary. A method is developed to estimate the entrance optics temperatures from pyranometer measurements and meteorological data. The method enables us to correct historic data records for which temperature information is not available. The temperature retrieval method has an uncertainty of less than 2.5 degrees C, resulting in a 0.3% uncertainty in the correction to be performed. The temperature correction improves the agreement between modeled and measured doses and instrument intercomparison as performed within the Quality Assurance of Spectral Ultraviolet Measurements in Europe project. The retrieval method is easily transferable to other instruments.

Influence of spatial heterogeneity of surface albedo on its retrieval from airborne irradiance measurements

Spectral airborne upward and downward irradiance measurements are used to derive the area-averaged surface albedo. In contrast, ground-based measurements deliver the local surface albedo. This work studies the effect of the heterogeneity of the local surface albedo on the area-averaged surface albedo. This helps to understand, how well the surface albedo can be spatially-resolved by airborne measurements. To quantify this effect as a function of heterogeneity, aerosol optical depth (AOD) and flight altitude, spatially heterogeneous surface albedo maps were input into a 3-dimensional (3D) Monte Carlo radiative transfer model to simulated 3D irradiance fields at artificial flight altitudes. The calculated up-and downward irradiances are used to derive the area-averaged surface albedo using an iterative retrieval that removes the atmospheric masking of the surface. For the case of adjacent land and sea surfaces a parameterization is presented which quantifies the horizontal distance to the coastline that is ...

Airborne measurements of urban surface reflectivity and albedo

Airborne measurements of spectral upward radiance and irradiance over the city of Zhongshan, China, were used to examine the heterogeneity of reflection properties of urban surfaces. After the surface reflectivity was derived from the airborne measurements, ratios of surface reflectivity at 466 and 644 nm wavelengths were calculated for the present case study. Implementing the derived ratios into the operational aerosol algorithm over land for the moderate resolution imaging spectroradiometer, enabled us to quantify the influence of the assumptions made for the estimation of the surface reflectivity at 466 nm on the retrieved aerosol optical depth.