How well can brightness temperature differences of spaceborne imagers help to detect cloud phase? A sensitivity analysis regarding cloud phase and related cloud properties

This study investigates the sensitivity of two brightness temperature differences (BTDs) in the infrared (IR) window of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) to various cloud parameters in order to understand their information content, with a focus on cloud thermodynamic phase. To this end, this study presents radiative transfer calculations, providing an overview of the relative importance of all radiatively relevant cloud parameters, including thermodynamic phase, cloud-top temperature (CTT), optical thickness (τ), effective radius (Reff), and ice crystal habit. By disentangling the roles of cloud absorption and scattering, we are able to explain the relationships of the BTDs to the cloud parameters through spectral differences in the cloud optical properties. In addition, an effect due to the nonlinear transformation from radiances to brightness temperatures contributes to the specific characteristics of the BTDs and their dependence on τ and CTT. We find that the dependence of the BTDs on phase is more complex than sometimes assumed. Although both BTDs are directly sensitive to phase, this sensitivity is comparatively small in contrast to other cloud parameters. Instead, the primary link between phase and the BTDs lies in their sensitivity to CTT (or more generally the surface–cloud temperature contrast), which is associated with phase. One consequence is that distinguishing high ice clouds from low liquid clouds is straightforward, but distinguishing mid-level ice clouds from mid-level liquid clouds is challenging. These findings help to better understand and improve the working principles of phase retrieval algorithms

An estimation of the UV radiation inside the cockpits of large commercial jets

UV irradiances and UV doses inside the cockpit of large commercial jets are estimated. Results are based on radiative transfer calculations taking into account the spectral transmittances and the limited fields of view of large commercial jet windscreens. In a first step vertical profiles of UV irradiances for a cloud free atmosphere over snow free and snow covered surfaces and for an atmosphere containing a water cloud layer are simulated. It turns out that the windscreens block the UV-B radiation and transmit parts of the UV-A radiation. Comparing UV irradiances inside and outside the cockpit show that the intensity of UV radiation inside strongly depends on whether the direct sun is entering the cabin or not. Without direct sun the diffuse UV radiation inside the cockpit amounts to about 5% the ambient UV irradiance outside the aircraft. In cases of low sun when direct radiation can reach the pilot percentages grow from 50% to 100% with increasing solar zenith angle. A water cloud layer between 2 and 4 km increases the UV irradiances inside a cockpit by about 7% at 10 km altitude when compared to the cloud free atmosphere. A snow covered surface causes a similar increase. Finally, and by the aid of MOZAIC waypoint data UV doses were estimated for selected long-distance flights between Europe and the overseas continents North- and South America, South Africa, and East Asia. UV doses are affected by takeoff and landing time, by the sun position relative to the aircraft heading during flight, and by the day of the year. UV doses inside the cockpit amount to maximum 60% the UV doses outside at the same altitude, however, in most cases percentages are between about 10% and 40%

Diurnal evolution of cloud base heights in convective cloud fields from MSG/SEVIRI data

This study shows that it is possible to retrieve the temporal evolution of cloud base heights in convective broken cloud fields from data of the SEVIRI instrument onboard the geostationary satellite Meteosat-9. Presented and discussed are time dependent base heights with a temporal resolution of 15 min from morning to afternoon. Cloud base heights retrieved from SEVIRI data are also compared with independent measurements of a ceilometer, with condensation levels calculated from radiosonde data and with base heights obtained from an application of the method to NOAA/AVHRR data. The validation has been performed for three days in the year 2007 and for seven test areas distributed over Germany and neighbouring countries. The standard deviations of the absolute differences between cloud base heights from Meteosat-9 and radiosonde measurements as well as between NOAA/AVHRR and Meteosat-9 results are both of the order of ±290 m. The correlation coefficient is 0.53 for the comparison of satellite with radiosonde measurements and 0.78 for the intercomparison of the satellite measurements. Furthermore, it is shown that the method retrieves the temporal evolution of cloud base heights in very good agreement with time dependent ceilometer measurements

Satellite remote sensing of cloud base height for convective cloud fiels: A case study

A method is proposed for estimating base heights of convective clouds from satellite data. The approach takes advantage of the fact that convective water clouds appear as geometrically and optically thin clouds near an approximately constant condensation level in their earliest stage of growth and that deriving geometrical thicknesses for such thin clouds is less error prone. Striking is the fact that the method also provides the base height for clouds with large vertical extensions and high optical thicknesses. The method has been applied to NOAA/AVHRR data of 20 selected cloud scenes. For an evaluation satellite retrieved cloud base heights have been compared to surface ceilometer measurements at the same time. First results are encouraging. The standard deviation of the differences between satellite and ceilometer measurements is ± 369m with no systematic bias

Validation of a radiative transfer model with measurements of UV Radiation inside a commercial aircraft

A radiative transfer model for the determination of UV radiation on arbitrarily oriented surfaces is validated by spectral measurements taken directly on the inner surface of a cockpit window of an Airbus A321-231 during a flight from Frankfurt, Germany to Málaga, Spain on 23 August, 2018. The simulations consider the UV spectral range from 290 nm to 400 nm and take into account both the measured spectral transmittance of a cockpit window as well as its construction-related orientation. Comparisons are performed for selected route segments with largely cloud-free conditions. The cruising level of the Airbus on these segments was nearly constant between 11.27 km and 11.29 km. UV irradiance measurements at the cockpit window give values within a range of 19 W/m2 and 26 W/m2. The comparison of modelled and measured irradiances show a very good agreement, i.e. the relative differences between simulated and measured values range from -2.1 % to +4.3 %. In addition, horizontally and vertically oriented sensors are simulated for the same flight. The validation results generally underpin the application potential of the model. As an example of this, UV irradiances incident on differently oriented surfaces, as could occur inside and outside of a future flying taxi on a short-haul flight between Munich and Augsburg at low cruising level, are shown

Potential and limitations of space-based methods for the retrieval of surface UV-B daily doses: A numerical study

For a typical space-borne algorithm for the estimation of surface UV-B daily doses, the implications of a limited temporal sampling of cloud properties and of the neglect of three-dimensional radiative effects are addressed. By means of six synthetic diurnal cloud cycles, different temporal samplings of cloud properties have been investigated, while surface and atmospheric properties have been kept constant. The results have been compared with benchmark calculations of the daily doses using a 15 min time step. The neglect of the horizontal photon transport imposes limitations to the estimation of daily doses since it leads to statistical uncertainties up to 25% and biases ranging from 3% to 9%. Additionally, when a reduced temporal sampling is used, maximum uncertainties increase up to 85% when probing the cloud field only every 4 hours. Even in this case, however, 25 40% of the daily doses in the model domain have an accuracy between 20% and +20%. The time around noon turns out to be of crucial importance for a precise estimation of UV daily doses. Therefore usually a limited number of cloud probes around noon is particularly efficient, although in case of high temporal and spatial cloud variability throughout the day three cloud probes can still be too few to prevent the occurrence of large (>80%) deviations. Finally, by spatial averaging, a large improvement of the agreement between reference values and daily doses computed with a given time sampling can be achieved

Time sampling effects on UV daily doses derived from satellite data

The determination of ultraviolet daily doses from satellite data is a very helpful method for investigating the biological impact of UV radiation on biological species over wide areas. To achieve this aim, geostationary or polar orbiting satellites can be used, as was done for instance in the EU project UVAC. There, two distinct climatologies of UV radiation were produced, one based on the Meteosat and one on the NOAA satellites for the derivation of cloud optical properties. Geostationary satellites allow in principle to probe the cloud fields many times a day, thus better accounting for the diurnal cycle of the varying clouds than polar orbiting satellites. However, in Northern latitudes their spatial resolution is significantly reduced compared to that of polar orbiting satellites. Furthermore, considering the high demands on cpu and processing times as well as on disk space, the problem of finding a reasonable compromise between number of cloud probes per day and accuracy naturally arises. To address this question we simulate the surface UV radiation for different diurnal cycles of cloud fields by means of the 3- dimensional radiative transfer code SHDOM. The results illustrate how the accuracy in computing UV daily doses depends on the number of cloud probes and the smoothing effects due to temporal and spatial variability. We further compare model results obtained for the typical temporal sampling of the NOAA and Meteosat satellites and show the importance of sampling around noon time. Herewith, this study, that has been performed in the framework of UVAC, may represent a basic tool for evaluating every satellite based climatology

Some effects of ultraviolet radiation and climate on the reproduction of Calanus finmarchicus (Copepoda) and year class formation in Arcto-Norwegian cod (Gadus morhua)

Zooplankton sampling in 1997 identified the frontal zone of the Norwegian Coastal Current as a reproduction habitat for Calanus finmarchicus in June-August. This area is subject to considerable ultraviolet radiation (UVR), as calculated from satellite observations of ozone and cloudiness. While in situ experiments indicated UVR-induced mortality in reproducing C. finmarchicus, monthly UVR doses during the actual reproduction period did not appear to affect the abundance of the resulting generation of adolescent copepodites (CIV-V) that accumulated in a fjord habitat during October 1983-2000. Local UVR in the spawning grounds of Arcto-Norwegian cod at the Lofoten Islands in March-May was positively correlated with the stock's 0-group index, which resulted in the rejection of the hypothesis that local UVR leads to high mortality of cod eggs or reduces the abundance of prey for cod larvae. Rather, the result suggests an indirect positive effect of UVR on the survival of cod eggs and larvae, possibly by controlling harmful microbes

Earth’s radiation budget – the driver for weather and climate

Earth’s structure today is manifested in the known distribution of continents and oceans, in a variety of surface types, as well as in a surrounding atmosphere consisting of gaseous constituents, aerosols, and clouds. This complex composition defines a framework that determines the specific amounts of solar energy our planet currently absorbs, reflects, and emits back to space via radiation. The components of Earth's radiation budget and how they are measured are described, the primary energy flows within the Earth system in an equilibrium state are explained and the imbalances of the radiation budget when zooming into smaller temporal and spatial scales are discussed