The daytime cycle in dust aerosol direct radiative effects observed in the central Sahara during the Fennec campaign in June 2011

The direct clear-sky radiative effect (DRE) of atmospheric mineral dust is diagnosed over the Bordj Badji Mokhtar (BBM) supersite in the central Sahara during the Fennec campaign in June 2011. During this period, thick dust events were observed, with aerosol optical depth values peaking at 3.5. Satellite observations from Meteosat-9 are combined with ground-based radiative flux measurements to obtain estimates of DRE at the surface, top-of-atmosphere (TOA), and within the atmosphere. At TOA, there is a distinct daytime cycle in net DRE. Both shortwave (SW) and longwave (LW) DRE peak around noon and induce a warming of the Earth-atmosphere system. Toward dusk and dawn, the LW DRE reduces while the SW effect can switch sign triggering net radiative cooling. The net TOA DRE mean values range from -9 Wm-2 in the morning to heating of +59 Wm-2 near midday. At the surface, the SW dust impact is larger than at TOA: SW scattering and absorption by dust results in a mean surface radiative cooling of 145Wm-2. The corresponding mean surface heating caused by increased downward LW emission from the dust layer is a factor of 6 smaller. The dust impact on the magnitude and variability of the atmospheric radiative divergence is dominated by the SW cooling of the surface, modified by the smaller SW and LW effects at TOA. Consequently, dust has a mean daytime net radiative warming effect on the atmosphere of 153Wm-2

Sensitivity of radiative fluxes to aerosols in the ALADIN-HIRLAM numerical weather prediction system

The direct radiative effect of aerosols is taken into account in many limited-area numerical weather prediction models using wavelength-dependent aerosol optical depths of a range of aerosol species. We studied the impact of aerosol distribution and optical properties on radiative transfer, based on climatological and more realistic near real-time aerosol data. Sensitivity tests were carried out using the single-column version of the ALADIN-HIRLAM numerical weather prediction system, set up to use the HLRADIA simple broadband radiation scheme. The tests were restricted to clear-sky cases to avoid the complication of cloud–radiation–aerosol interactions. The largest differences in radiative fluxes and heating rates were found to be due to different aerosol loads. When the loads are large, the radiative fluxes and heating rates are sensitive to the aerosol inherent optical properties and the vertical distribution of the aerosol species. In such cases, regional weather models should use external real-time aerosol data for radiation parametrizations. Impacts of aerosols on shortwave radiation dominate longwave impacts. Sensitivity experiments indicated the important effects of highly absorbing black carbon aerosols and strongly scattering desert dust

Characterization of clouds and their radiative effects using ground-based instrumentation at a low-mountain site

The interaction of clouds with radiation and aerosols is the greatest source of uncertainty in future climate projections. Part of the reason is the limited amount of observations of clouds and hence the limited knowledge of cloud macro- and microphysical statistics in connection to their effects on the radiative budget and on the vertical redistribution of energy within the atmosphere. In 2007, the Atmospheric Radiation Measurement program�s (ARM) Mobile Facility (AMF) was operated for a nine-month period in the Murg Valley, Black Forest, Germany, in support of the Convective and Orographically-induced Precipitation Study (COPS). Based on the measurements of the AMF and COPS partner instrumentation, the present study aims at improving the data basis of cloud macro- and microphysical statistics and to assess the potential of the derived cloud properties to estimate the radiative effects of clouds. The synergy of various instruments is exploited to derive a data set of high quality thermodynamic and cloud property profiles with a temporal resolution of 30 s. While quality filters in the cloud microphysical retrieval techniques mostly affect the representativity of ice and mixed clouds in the data sample, water clouds are very well represented in the derived 364,850 atmospheric profiles. In total, clouds are present 72% of the time with multi-layer mixed phase (28.4%) and single-layer water clouds (11.3%) occurring most frequently. In order to evaluate the derived thermodynamic and cloud property profiles,radiative closure studies are performed with independent radiation measurements. In clear sky, average differences between calculated and observed surface fluxes are less than 2.1% and 3.6% for the shortwave and longwave, respectively. In cloudy situations, differences, in particular in the shortwave, are much larger, but most of these can be related to broken cloud situations. The cloud radiative effect (CRE), i.e. the difference of cloudy and clear-sky net fluxes, has been analyzed for the whole nine-month period. The largest surface (SFC) net CRE has been found for multi-layer water (-110 Wm-2) and mixed clouds (-116 Wm-2). The estimated uncertainties in the modeled SFC and top of atmopshere (TOA) net CRE are up to 39% and 26%, respectively. For overcast, single-layer water clouds, sensitivity studies reveal that the SW CRE uncertainty at the SFC and TOA is likewise determined by uncertainties in liquid water path (LWP) and effective radius, if the LWP is larger than 100 gm-2. For low LWP values, uncertainties in SFC and TOA shortwave CRE are dominated by the uncertainty in LWP. Uncertainties in CRE due to uncertainties in the shape of the liquid water content (LWC) profile are typically smaller by a factor of two compared to LWP uncertainties. For the difference between the cloudy and clear-sky net heating rates, i.e. the cloud radiative forcing (CRF), of water clouds, the LWP and its vertical distribution within the cloud boundaries are the most important factors. In order to increase the accuracy of LWC profiles and consequentially of the estimates of CRE and CRF, advanced LWC retrieval techniques, such as the Integrated Profiling Technique (IPT), are needed. The accuracy of a LWC profile retrieval using typical microwave radiometer brightness temperatures and/or cloud radar reflectivities is investigated for two realistic cloud profiles. The interplay of the errors of the a priori profile, measurements and forward model on the retrieved LWC error and on the information content of the measurements is analyzed in detail. It is shown that the inclusion of the microwave radiometer observations in the LWC retrieval increases the number of degrees of freedom, i.e. the independent pieces of information in the measurements, by about 1 compared to a retrieval using measuremets from the cloud radar alone. Assuming realistic measurement and forward model errors, it is further demonstrated, that the error in the retrieved LWC is 60% or larger, if no a priori information is available, and that a priori information is essential for a better accuracy. The results of the present work strongly suggest to improve the LWC a priori profile and the corresponding error estimates in the IPT. However, there are few observational datasets available to construct accurate a priori profiles of LWC, and thus more observational data are needed to improve the knowledge of the a priori profile and the corresponding error covariance matrix

Sensitivity of Mesoscale Modeling of Smoke Direct Radiative Effect to the Emission Inventory: a Case Study in Northern Sub-Saharan African Region

An ensemble approach is used to examine the sensitivity of smoke loading and smoke direct radiative effect in the atmosphere to uncertainties in smoke emission estimates. Seven different fire emission inventories are applied independently to WRF-Chem model (v3.5) with the same model configuration (excluding dust and other emission sources) over the northern sub-Saharan African (NSSA) biomass-burning region. Results for November and February 2010 are analyzed, respectively representing the start and end of the biomass burning season in the study region. For February 2010, estimates of total smoke emission vary by a factor of 12, but only differences by factors of 7 or less are found in the simulated regional (15degW-42degE, 13degS-17degN) and monthly averages of column PM(sub 2.5) loading, surface PM(sub 2.5) concentration, aerosol optical depth (AOD), smoke radiative forcing at the top-of-atmosphere and at the surface, and air temperature at 2 m and at 700 hPa. The smaller differences in these simulated variables may reflect the atmospheric diffusion and deposition effects to dampen the large difference in smoke emissions that are highly concentrated in areas much smaller than the regional domain of the study. Indeed, at the local scale, large differences (up to a factor of 33) persist in simulated smoke-related variables and radiative effects including semi-direct effect. Similar results are also found for November 2010, despite differences in meteorology and fire activity. Hence, biomass burning emission uncertainties have a large influence on the reliability of model simulations of atmospheric aerosol loading, transport, and radiative impacts, and this influence is largest at local and hourly-to-daily scales. Accurate quantification of smoke effects on regional climate and air quality requires further reduction of emission uncertainties, particularly for regions of high fire concentrations such as NSSA

Sensitivity of Mesoscale Modeling of Smoke Direct Radiative Effect to the Emission Inventory: A Case Study in Northern Sub-Saharan African Region

An ensemble approach is used to examine the sensitivity of smoke loading and smoke direct radiative effect in the atmosphere to uncertainties in smoke emission estimates. Seven different fire emission inventories are applied independently to WRF-Chem model (v3.5) with the same model configuration (excluding dust and other emission sources) over the northern sub-Saharan African (NSSA) biomass-burning region. Results for November and February 2010 are analyzed, respectively representing the start and end of the biomass burning season in the study region. For February 2010, estimates of total smoke emission vary by a factor of 12, but only differences by factors of 7 or less are found in the simulated regional (15°W–42°E, 13°S–17°N) and monthly averages of column PM2.5 loading, surface PM2.5 concentration, aerosol optical depth (AOD), smoke radiative forcing at the top-of-atmosphere and at the surface, and air temperature at 2 m and at 700 hPa. The smaller differences in these simulated variables may reflect the atmospheric diffusion and deposition effects to dampen the large difference in smoke emissions that are highly concentrated in areas much smaller than the regional domain of the study. Indeed, at the local scale, large differences (up to a factor of 33) persist in simulated smoke-related variables and radiative effects including semi-direct effect. Similar results are also found for November 2010, despite differences in meteorology and fire activity. Hence, biomass burning emission uncertainties have a large influence on the reliability of model simulations of atmospheric aerosol loading, transport, and radiative impacts, and this influence is largest at local and hourly-to-daily scales. Accurate quantification of smoke effects on regional climate and air quality requires further reduction of emission uncertainties, particularly for regions of high fire concentrations such as NSSA

Theoretical Foundations of Remote Sensing for Glacier Assessment and Mapping

The international scientific community is actively engaged in assessing ice sheet and alpine glacier fluctuations at a variety of scales. The availability of stereoscopic, multitemporal, and multispectral satellite imagery from the optical wavelength regions of the electromagnetic spectrum has greatly increased our ability to assess glaciological conditions and map the cryosphere. There are, however, important issues and limitations associated with accurate satellite information extraction and mapping, as well as new opportunities for assessment and mapping that are all rooted in understanding the fundamentals of the radiation transfer cascade. We address the primary radiation transfer components, relate them to glacier dynamics and mapping, and summarize the analytical approaches that permit transformation of spectral variation into thematic and quantitative parameters. We also discuss the integration of satellite-derived information into numerical modeling approaches to facilitate understandings of glacier dynamics and causal mechanisms

Aerosol-cloud-precipitation interaction based on remote sensing and cloud-resolving modeling over the Central Himalayas

The Central Himalayan region experiences pronounced orographic precipitation related to the South Asian summer monsoon, typically occurring from June to September. Atmospheric aerosols can influence regional and global climate through aerosol-radiation (ARI) and aerosol-cloud interactions (ACI). The study of the aerosol-precipitation relationship over the Central Himalayan region during the summer monsoon season is important due to extreme pollution over the upwind Indo-Gangetic Plains, enhanced moisture supply through monsoonal flow, and steep terrain of the Himalayas modulating the orographic forcing. This dissertation aims to study the impact of atmospheric aerosols, from natural and anthropogenic sources, in modulating the monsoonal precipitation, cloud processes, and freezing isotherm over the central Himalayas. The long-term (2002 – 2017) satellite-retrieved and reanalysis datasets showed regardless of the meteorological forcing, compared to relatively cleaner days, polluted days with higher aerosol optical depth is characterized by the invigorated clouds and enhanced precipitation over the southern slopes and foothills of the Himalayas. The mean freezing isotherm increased by 136.2 meters in a polluted environment, which can be crucial and significantly impact the hydroclimate of the Himalayas. Due to the limitations of satellite-retrieved observational data, these results underlined the need for state-of-the-art Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) in a cloud-resolving scale to better represent and study the impact of the aerosols from different sources through radiation and microphysics pathways over the complex terrain of the Central Himalayas. A cloud-resolving WRF-Chem simulation is performed to assess the impact of anthropogenic and remotely transported dust aerosols on the convective processes and elevation-dependent precipitation. Long-range transported dust aerosols significantly impacted cloud microphysical properties and enhanced the precipitation by 9.3% over the southern slopes of the Nepal Himalayas. The mid-elevation of the Central Himalayas, generally between 1000 and 3000 meters, acted as the region below and above which the diurnal variation and precipitation of various intensities (light, moderate, and heavy) responded differently for ARI, ACI, and the combined effect of aerosols. Due to the ARI effect of aerosols, the light precipitation is suppressed by 17% over the Central Himalayas. The ACI effect dominated and resulted in enhanced heavy precipitation by 12% below 2000 m ASL, which can potentially increase the risk for extreme events (floods and landslides). In contrast, above 2000 m ASL, the suppression of precipitation due to aerosols can be critical for the regional supply of water resources. The overview of the study suggests that the natural and anthropogenic aerosols significantly modulate the convective processes, monsoonal precipitation, and freezing isotherm over the Central Himalayan region, which could pose significant consequences to the changing Himalayan hydroclimate

Impact of High Concentrations of Saharan Dust Aerosols on Infrared-Based Land Surface Temperature Products

An analysis of three operational satellite-based thermal-infrared land surface temperature (LST) products is presented for conditions of heavy dust aerosol loading. The LST products are compared against ERA5’s skin temperature (SKT) across the Sahara Desert and Sahel region, where high concentrations of dust aerosols are prevalent. Large anomalous differences are found between satellite LST and ERA5’s SKT during the periods of highest dust activity, and satellite–ERA5 differences are shown to be strongly related to dust aerosol optical depth (DuAOD) at 550 nm, indicating an underestimation of LST in conditions of heavy dust aerosol loading. In situ measurements from two ground stations in the Sahel region provide additional evidence of this underestimation, showing increased biases of satellite LST with DuAOD, and no significant dependence of ERA5’s SKT biases on dust aerosol concentrations. The impact of atmospheric water vapor content on LST and SKT is also examined, but dust aerosols are shown to be the primary driver of the inaccurate LSTs observed. Based on comparisons with in situ data, we estimate an aerosol-induced underestimation of LST of approximately 0.9 K for every 0.1 increase in DuAOD. Analysis of brightness temperatures (BTs) in the thermal infrared atmospheric window reveals that dust aerosols have the opposite effect on BT differences compared to water vapor, leading to an underestimation of atmospheric correction by the LST retrieval algorithms. This article highlights a shortcoming of current operational LST retrieval algorithms that must be addressed

Surface energy budget at Curiosity through observations and column modeling

Diurnal ground surface temperatures (T-g) and the five major terms of the surface energy budget (SEB) are dis-played from hourly Mars Science Laboratory observations and from column model simulations in four contrasting cases along the Curiosity traverse. T(g )and the SEB terms are otherwise well simulated on regolith near the landing spot and on rocky Pahrump Hills, but the residual in observation-based SEB (-downwelling longwave radiation) shows unexplained peaks in the morning and evening and simultaneously model-T(g )is too cold. Enhanced or diurnally variable crater dust does not help but diurnally variable soil thermal inertia (suggested by Fourier analysis of observed T-g) reduces both defects at both sites. Sand on the steep Namib dune is instead homogeneous, defects here being reduced by taking into account slope effects. Regolith at the 2018 dust storm site appears inhomogeneous, with the SEB terms and T(g )relatively well simulated even in this case of extremely heavy dust load.Peer reviewe

Arctic low-level mixed-phase clouds and their complex interactions with aerosol and radiation: Remote sensing of the Arctic troposphere with the shipborne supersite OCEANET-Atmosphere

In the course of this thesis, Arctic low-level mixed-phase clouds and their interaction with aerosol and radiation have been investigated. To do so, measurements with the shipborne remote sensing supersite OCEANET-Atmosphere were conducted during the PS106 expedition in the Arctic summer 2017. OCEANET-Atmosphere comprises among other instruments a multiwavelength polarization lidar PollyXT and a microwave radiometer HATPRO. For PS106 the OCEANET-Atmosphere facility was complemented for the first time with a motion-stabilized vertically pointing Doppler cloud radar Mira-35. The cloud radar Doppler velocity was corrected for the ship’s vertical movement. The stabilization and the correction enabled, e.g., the derivation of eddy dissipation rates from the Doppler velocities. A data set of cloud microphysical and macrophysical properties was derived by applying the synergistic Cloudnet algorithm to the combined measurements of cloud radar, lidar, and microwave radiometer. Within this thesis, the set of the Cloudnet retrievals was improved to account for the complex structure of the Arctic cloud system. A new detection approach for the frequently observed low-level stratus clouds was developed based on the lidar signal-to-noise ratio. These clouds, which were below the lowest range gate of the cloud radar were observed during 50 % of the observational time. A new approach for the continuous determination of the ice crystal effective radius was introduced. This new retrieval made the data set suitable to perform high-resolved radiative transfer simulations. The retrieved data set was utilized to derive the first temperature relationship for heterogeneous ice formation in Arctic mixed-phase clouds. A strong dependence of the surface coupling state for high subzero ice-formation temperatures was found. For an ice-formation temperature above -15 °C, surface-coupled ice-containing clouds occur more frequently by a factor of 5 in numbers of observed clouds and by a factor of 2 in frequency of occurrence. Possible causes of the observed effect were discussed by sensitivity studies and a literature survey. Instrumental and methodological effects, and previously published similar observations of an increased ice occurrence at such high subzero temperatures have been ruled out as a possible explanation. The most likely cause of the observed effect was attributed to a larger reservoir of biogenic ice-nucleating particles in the surface-coupled marine boundary layer. This larger reservoir led to a higher freezing efficiency in these clouds which had at least their base in that layer. Finally, the importance of the detailed classification of the low-level clouds was highlighted by the evaluation of radiative transfer simulations. A difference in the cloud radiative effect of up to 100 W m-2 was calculated when these clouds were considered.:1 Introduction 2 Arctic — Amplified climate change 2.1 The Arctic climate system 2.2 Cloud radiation budget 2.3 Arctic mixed-phase clouds 2.4 Heterogeneous ice formation in Arctic mixed-phase clouds — constraints and previous findings 2.5 Motivating research questions 3 Data set — Applied instrumentation, processing, and retrievals 3.1 Introduction to ground-based active remote sensing of aerosol and clouds 3.1.1 Lidar principle 3.1.2 Radio Detection and Ranging — Radar 3.2 The Arctic expedition PS106 3.3 Instrumentation 3.3.1 The OCEANET-Atmosphere observatory 3.3.2 Other instruments used in this study 3.4 Data processing and synergistic retrievals 3.4.1 Correction of vertical-stare cloud radar observations for ship motion 3.4.2 Retrieval of eddy dissipation rate from Doppler radar spectra 3.4.3 Cloud macro- and microphysical properties from instrument-synergies 3.5 Summary of the data processing for PS106 4 Cloud and aerosol observations during PS106 4.1 Meteorological conditions during PS106 4.2 Case studies 4.3 Cloud and aerosol statistics during PS106 4.4 Discussion of the observational data sets 5 Contrasting surface-coupling effects on heterogeneous ice formation 5.1 Methodology 5.1.1 Ice-containing cloud analysis 5.1.2 Surface-coupling state 5.2 Results: influence of surface coupling on heterogeneous ice formation temperature 5.3 Discussion of the observed surface-coupling effects 5.3.1 Methodological and instrumental effects 5.3.2 Possible causes for increased ice occurrence in surface-coupled clouds 6 Application of the data set in collaborative studies and radiative transfer simulations within (AC)3 6.1 Radiative transfer simulations and cloud radiative effect 6.2 LLS treatment for improved radiative transfer simulations 6.3 Discussion 7 Summary and outlook Appendices A Determination of a volume depolarization threshold forlidar-based ice detection BibliographyIm Rahmen dieser Arbeit wurden niedrige arktische Mischphasenwolken und ihre Wechselwirkung mit Aerosolen und Strahlung untersucht. Dazu wurden Messungen mit der schiffsgestützten Fernerkundungs-Supersite OCEANET-Atmosphere während der PS106-Expedition im arktischen Sommer 2017 durchgeführt. OCEANET-Atmosphere vereint, u.a., ein Multiwellenlängen-Polarisations-Lidar PollyXT und ein Mikrowellen-Radiometer HATPRO. Für PS106 wurde OCEANET-Atmosphere erstmalig um ein stabilisiertes, vertikal ausgerichtetes Doppler-Wolkenradar Mira-35 erweitert. Die Doppler-Geschwindigkeit wurde in Bezug auf die Vertikalbewegung des Schiffes korrigiert. Dank Stabilisierung und Korrektur war, z.B., die Ableitung von Wirbeldissipationsraten aus den Doppler-Geschwindigkeiten möglich. Unter Anwendung des synergetischen Cloudnet-Algorithmus wurde aus den kombinierten Wolkenradar, Lidar und Mikrowellenradiometer Messungen ein Datensatz der mikro- und makrophysikalischen Wolkeneigenschaften für PS106 erstellt. Im Rahmen dieser Arbeit wurde Cloudnet verbessert, um der komplexen Struktur der arktischen Wolken Rechnung zu tragen. Ein neuer Ansatz zur Erkennung der häufig beobachteten niedrigen Stratuswolken wurde entwickelt, basierend auf dem Lidar-Signal-zu-Rausch-Verhältnis. Diese Wolken, die unterhalb des untersten Höhenlevels des Wolkenradars auftraten, wurden während 50% der Beobachtungszeit identifiziert. Ein neuer Ansatz für die kontinuierliche Bestimmung des effektiven Radius der Eiskristalle wurde eingeführt. Dank dieser neuen Methode eignet sich der erstellte Datensatz für die Durchführung von Strahlungstransfersimulationen. Zum ersten Mal wurde eine Temperaturbeziehung für heterogene Eisbildung in arktischen Mischphasenwolken in Abhängigkeit ihres Oberflächen-Kopplungsstatus abgeleitet. Bei Temperaturen über -15°C war die relative Häufigkeit von Eis beinhaltenden Wolken doppelt so hoch und die Anzahl fünf Mal höher wenn sie mxit der Oberfläche gekoppelt waren, als bei entkoppelte Wolken. Mögliche Ursachen für den beobachteten Effekt wurden anhand von Sensitivitätsstudien und einer Literaturanalyse diskutiert. Instrumentelle und methodische Effekte sowie früher veröffentlichte ähnliche Beobachtungen konnten als mögliche Erklärung ausgeschlossen werden. Die wahrscheinlichste Ursache für den beobachteten Effekt wurde auf ein größeres Reservoir an biogenen Eiskristallisationskeimen in der oberflächengekoppelten marinen Grenzschicht zurückgeführt. Dieses größere Reservoir hat zu einer höheren Gefriereffizienz in Wolken geführt, die zumindest ihre Basis in dieser Schicht hatten. Die Bedeutung der detaillierten Klassifizierung von tiefliegenden Wolken auf Strahlungstransfersimulationen wurde hervorgehoben. Der simulierte Effekt der Wolken auf den Strahlungshaushalt unterschied sich bis zu 100 W m-2, unter Berücksichtigung dieser Wolken.:1 Introduction 2 Arctic — Amplified climate change 2.1 The Arctic climate system 2.2 Cloud radiation budget 2.3 Arctic mixed-phase clouds 2.4 Heterogeneous ice formation in Arctic mixed-phase clouds — constraints and previous findings 2.5 Motivating research questions 3 Data set — Applied instrumentation, processing, and retrievals 3.1 Introduction to ground-based active remote sensing of aerosol and clouds 3.1.1 Lidar principle 3.1.2 Radio Detection and Ranging — Radar 3.2 The Arctic expedition PS106 3.3 Instrumentation 3.3.1 The OCEANET-Atmosphere observatory 3.3.2 Other instruments used in this study 3.4 Data processing and synergistic retrievals 3.4.1 Correction of vertical-stare cloud radar observations for ship motion 3.4.2 Retrieval of eddy dissipation rate from Doppler radar spectra 3.4.3 Cloud macro- and microphysical properties from instrument-synergies 3.5 Summary of the data processing for PS106 4 Cloud and aerosol observations during PS106 4.1 Meteorological conditions during PS106 4.2 Case studies 4.3 Cloud and aerosol statistics during PS106 4.4 Discussion of the observational data sets 5 Contrasting surface-coupling effects on heterogeneous ice formation 5.1 Methodology 5.1.1 Ice-containing cloud analysis 5.1.2 Surface-coupling state 5.2 Results: influence of surface coupling on heterogeneous ice formation temperature 5.3 Discussion of the observed surface-coupling effects 5.3.1 Methodological and instrumental effects 5.3.2 Possible causes for increased ice occurrence in surface-coupled clouds 6 Application of the data set in collaborative studies and radiative transfer simulations within (AC)3 6.1 Radiative transfer simulations and cloud radiative effect 6.2 LLS treatment for improved radiative transfer simulations 6.3 Discussion 7 Summary and outlook Appendices A Determination of a volume depolarization threshold forlidar-based ice detection Bibliograph