Verwendung von mikrophysikalischen Messungen zur Charakterisierung von Aerosol und Wolken für Strahlungsübertragungsrechnungen

In diesem Beitrag werden Ansätze beschrieben, wie man aus flugzeuggetragenen Messungen von Eigenschaften atmosphärischer Extingenten die für Strahlungstransportrechnungen notwendigen Größen und Parameter rekonstruieren kann. Angewendet wird das Programm zur ersten Auswertung für bezüglich der Strahlung verschiedene atmosphärische Situationen. Die Daten hierzu wurden auf der ersten Messkampagne des Projektes INSPECTRO gesammelt. Es zeigt sich für den Fall einer Atmosphäre mit stratiformer Bewölkung eine gute Übereinstimmung mit den Messungen. Eine genaue Berechnung der Extinktion durchWolken mittels Tropfenspektren bringt derzeit keine Vorteile gegenüber einer einfachen Parametrisierung. Für den Fall eines durchbrochenen Wolkenfeldes ergeben sich lokal große Unterschiede zwischen Simulation und Messung. Dennoch läßt sich zeigen, daß die gemessenen und berechneten Felder des aktinischen Flusses einander sehr ähnliche statistische Eigenschaften haben.In this article some methods to reconstruct an artificial three-dimensional atmosphere from flight measurement data are described. The artificial atmosphere shall accurately represent the scattering properties of the real atmosphere, that are necessary to perform radiative transfer simulations. The method is applied to conduct a preliminary analysis of several atmospheric situations corresponding to different sets of radiative properties. The analyzed measurement data was collected during the first measurement campaign within the INSPECTRO project. In the case of an atmosphere containing a stratiform cloud layer, the measurements and simulation results are in very good agreement. A more accurate calculation of the extinction of radiation within the clouds by way of accounting for measured droplet spectra does momentarily not lead to an improvement in comparison with the simple parameterization method. In the case of a broken cloud field the significant local discrepancies between the measurement data and simulation results are to be noted. Nevertheless it can be shown that the statistical properties of the measured and calculated actinic flux fields resemble each other very closely

The potential of increasing man-made air pollution to reduce rainfall over southern West Africa

Southern West Africa has one of the fastest-growing populations worldwide. This has led to a higher water demand and lower air quality. Over the last 3 decades, most of the region has experienced decreasing rainfall during the little dry season (LDS; mid-July to end of August) and more recently also during the second rainy season (SRS; September-October), while trends during the first rainy season (FRS; mid-May to mid-July) are insignificant. Here we analyse spatio-temporal variations in precipitation, aerosol, radiation, cloud, and visibility observations from surface stations and from space to find indications for a potential contribution of anthropogenic air pollution to these rainfall trends. The proposed mechanism is that the dimming of incoming solar radiation by aerosol extinction contributes to reducing vertical instability and thus convective precipitation. To separate a potential aerosol influence from large-scale climatic drivers, a multilinear-regression model based on sea-surface temperature (SST) indices is used. During both LDS and SRS, weakly statistically significant but accelerating negative rainfall trends unrelated to known climatic factors are found. These are accompanied by a strong increase in pollution over the upstream tropical Atlantic caused by fire aerosol from Central Africa, particularly during the LDS. Over southern West Africa, no long-term aerosol records are available, inhibiting a direct quantification of the local manmade effect. However, significant decreases in horizontal visibility and incoming surface solar radiation are strong indicators for an increasing aerosol burden, in line with the hypothesized pollution impact on rainfall. The radiation trend is further enhanced by an increase in low-level cloudiness. The large spatial extent of potentially aerosol-related trends during the LDS is consistent with the stronger monsoon flow and less wet deposition during this season. Negligible aerosol impacts during the FRS are likely due to the high degree of convective organization, which makes rainfall less sensitive to surface radiation. The overall coherent picture and the accelerating trends - some of which are concealed by SST effects - should alarm policymakers in West Africa to prevent a further increase in air pollution as this could endanger water supply and food and energy production for a large and growing population

Detektion interner Schwerewellen in der stabilen Grenzschicht mittels akustischer Fernerkundung

Der Einfluss interner Schwerewellen (IGW) auf die untere Atmosphäre bei stabiler Schichtung wurde mittels einer Kombination zweier akustischer Fernerkundungsmethoden untersucht. Im Juli 2015 wurde in Zvenigorod, Russland ein gemeinsames Experiment durchgeführt, an welchem sowohl eine Arbeitsgruppe des Oboukhov Institute of Atmospherie Physics (OLAP) als auch des Leipziger Instituts für Metrologie (LIM) teilnahmen. Bei der Feldkampagne wurde die sogenannte acoustic pulse sounding method des OLAPs und die aktustische Laufzeittomographie des LIM eingesetzt, SODAR und RASS Messungen kontrollierten dabei ständig den Zustand der Atmosphäre bezüglich der Wind- und Temperaturprofile. Die internen Schwerewellen wurden anschließend mittels Kreuzkorrelationsanalyse der Zeitserien der gemessenen akustischen Parameter (hier:Laufzeiten) detektiert. Die Empfängersysteme waren an verschiedenen Stellen im Messgebiet verteilt. Deswegen konnten zwei verschiedene Detektionsmethoden angewendet werden. Erstens die Detektion entlang gebrochener Schallstrahlen in der Nähe deren Umkehrpunkte zwischen 50m und 100m und zweitens die Detektion von nahezu horizontal verlaufenden Schallstrahlen welche Sender/Empfänger-Paare verbinden. Somit konnten sowohl vertikale als auch horizontale Informationen über den Zustand der Atmosphäre während des Experiments erfasst werden

An evaluation of operational and research weather forecasts for Southern West Africa using observations from the DACCIWA field campaign in June‐July 2016

Reliable and accurate weather forecasts have the potential to improve living conditions in densely populated southern West Africa (SWA), in particularly those of rainfall and its extremes. A limited availability of observations has long impeded a rigorous evaluation of current state‐of‐the‐art forecast models. The field campaign of the Dynamics‐Aerosol‐Chemistry‐Cloud Interactions in West Africa (DACCIWA) project in June‐July 2016 has created an unprecedentedly dense set of measurements from surface stations and radiosondes. Here we present results from a comprehensive evaluation of both numerical model forecasts and satellite products using these data on a regional and local level. Results reveal a substantial observational uncertainty showing considerable underestimations in satellite estimates of rainfall and low‐cloud cover with little correlation at the local scale. Models have a dry bias of 0.1–1.9 mmday−1 in rainfall and too low column relative humidity. They tend to underestimate low clouds, leading to excess surface solar radiation of 43 Wm−2. Remarkably, most models show some skill in representing regional modulations of rainfall related to synoptic‐scale disturbances, while local variations in rainfall and cloudiness are hardly captured. Slightly better results are found with respect to temperature and for the post‐onset rather than for the pre‐onset period. Delicate local features such as the Maritime Inflow phenomenon are also rather poorly represented, leading to too cool, dry and cloudy conditions at the coast. Differences between forecast days 1 and 2 are relatively small and hardly systematic, suggesting a relatively quick error saturation. Using explicit convection leads to more realistic spatial variability in rainfall, but otherwise no marked improvement. Future work should aim at improving the subtle balance between the diurnal cycles of low clouds, surface radiation, the boundary layer and convection. Further efforts are also needed to improve the observational system beyond field campaign periods

Key lessons from the DACCIWA project for operational meteorological services

This document describes the conclusions of the EU-funded project Dynamics- Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) directly relevant to operational meteorological services. DACCIWA produced the most comprehensive observational dataset of the atmosphere over densely populated southern West Africa to date and used this dataset to foster our understanding of atmospheric processes, and to evaluate dynamical models and satellite data. With this document DACCIWA aims to help improve atmospheric predictions across time-scales, which are important for the development of greater resilience of the West African population to hazardous weather and climate change

Combining the independent pixel and point-spread function approaches to simulate the actinic radiation field in moderately inhomogeneous 3D cloudy media

Field observations and three-dimensional (3D) radiative transfer (RT) simulations of the actinic flux density show strongly pronounced local variations in the presence of inhomogeneous cloud fields which depend on the spatial distribution of the clouds and their optical properties. While exact three-dimensional RT models like, for example, Monte-Carlo techniques or the Spherical Harmonics Discrete Ordinate Method (SHDOM), despite their large computational burden are fully capable of resolving the observed 3D structures of the actinic radiation field, the often employed independent pixel approximation (IPA) for RT can not treat this situation realistically. For inhomogeneous 3D optical property fields of clouds this lack of horizontal photon transport may lead to significant local errors with up to 30% in the present study. Therefore, a very fast method was developed which realistically reproduces the actinic radiation field for 3D cloud fields having moderate spatial variability. This method employs a combination of the IPA, including an exact 3D treatment of the direct sunlight, and a Gaussian point spread function (PSF) to mimic the smoothing properties caused by 3D diffuse radiative transport. As a post-processing step the effect of horizontal photon transport is approximated by means of a Gaussian smoothing filter which accounts for characteristic optical properties of the medium under consideration. Compared to exact 3D RT simulations, the new method delivers more realistic results than the pure IPA approach in all cases analysed. Local deviations of the actinic flux density from 3D RT simulations are significantly reduced in comparison to IPA RT simulations. Depending on the inhomogeneity of the treated cloud fields, the computing time is reduced by a factor of about 20 relative to a full 3D RT simulation. Due to the employment of the IPA in which the RT is handled independently in each column, the size of the simulated model domain is practically unlimited. We also illustrate how this method can improve IPA RT simulations over orographically structured terrain

Vergleich zweier numerischer Verfahren zur Impulsadvektion in einem dreidimensionalen mikroskaligen Strömungsmodell

Zwei numerische Verfahren, das Upstream- und das MacCormack-Verfahren, werden im Hinblick auf ihre Eigenschaften bei der Lösung unterschiedlicher Advektionsprobleme verglichen. Das Letzere, welches eine Genauigkeit zweiter Ordnung liefert, wird in den Impulsadvektionsteil des numerischen Strömungsmodells MISKAM implementiert und anhand von Sensitivitätsstudien mit dem Upstream-Verfahren verglichen. Anschließend findet eine Modellevaluierung mit Hilfe von Datensätzen, welche am Hamburger Grenzschichtwindkanal erzeugt wurden, statt. Das Verfahren zeigt bei stabiler thermischer Modellschichtung signifikant veränderte Ergebnisse, deutlich zeigt sich die verringerte numerische Diffusion, vor allem im Bereich von Ecken und Kanten eines Hindernisses.Two numerical schemes are compared concerning their numerical abilities when solving different advection problems. MacCormack\''s scheme which is of second order accuracy is implemented in the numerical flow model MISKAM in order to calculate the advection of momentum. It is compared to the upstream scheme with the help of sensitivity studies and with a model evaluation using wind tunnel data from the University of Hamburg. The scheme shows for stable thermical stratification significant differences near the edges of obstacles that result mainly from the reduced numerical diffusion which was the major problem when using the upstream scheme

Combining the independent pixel and point-spread function approaches to simulate the actinic radiation field in moderately inhomogeneous 3D cloudy media

A fast method is presented for gaining 3D actinic flux density fields, Fact, in clouds employing the Independent Pixel Approximation (IPA) with a parameterized horizontal photon transport to imitate radiative smoothing effects. For 3D clouds the IPA is an efficient method to simulate radiative transfer, but it suffers from the neglect of horizontal photon fluxes leading to significant errors (up to locally 30% in the present study). Consequently, the resulting actinic flux density fields exhibit an unrealistically rough and rugged structure. In this study, the radiative smoothing is approximated by applying a physically based smoothing algorithm to the calculated IPA actinic flux field