Environmental effects on aerosol–cloud interaction in non-precipitating marine boundary layer (MBL) clouds over the eastern North Atlantic

Over the eastern North Atlantic (ENA) ocean, a total of 20 non-precipitating single-layer marine boundary layer (MBL) stratus and stratocumulus cloud cases are selected to investigate the impacts of the environmental variables on the aerosol–cloud interaction (ACIr) using the ground-based measurements from the Department of Energy Atmospheric Radiation Measurement (ARM) facility at the ENA site during 2016–2018. The ACIr represents the relative change in cloud droplet effective radius re with respect to the relative change in cloud condensation nuclei (CCN) number concentration at 0.2 % supersaturation (NCCN,0.2 %) in the stratified water vapor environment. The ACIr values vary from −0.01 to 0.22 with increasing sub-cloud boundary layer precipitable water vapor (PWVBL) conditions, indicating that re is more sensitive to the CCN loading under sufficient water vapor supply, owing to the combined effect of enhanced condensational growth and coalescence processes associated with higher Nc and PWVBL. The principal component analysis shows that the most pronounced pattern during the selected cases is the co-variations in the MBL conditions characterized by the vertical component of turbulence kinetic energy (TKEw), the decoupling index (Di), and PWVBL. The environmental effects on ACIr emerge after the data are stratified into different TKEw regimes. The ACIr values, under both lower and higher PWVBL conditions, more than double from the low-TKEw to high-TKEw regime. This can be explained by the fact that stronger boundary layer turbulence maintains a well-mixed MBL, strengthening the connection between cloud microphysical properties and the below-cloud CCN and moisture sources. With sufficient water vapor and low CCN loading, the active coalescence process broadens the cloud droplet size spectra and consequently results in an enlargement of re. The enhanced activation of CCN and the cloud droplet condensational growth induced by the higher below-cloud CCN loading can effectively decrease re, which jointly presents as the increased ACIr. This study examines the importance of environmental effects on the ACIr assessments and provides observational constraints to future model evaluations of aerosol–cloud interactions.

Addressing the difficulties in quantifying the Twomey effect for marine warm clouds from multi-sensor satellite observations and reanalysis

Aerosol–cloud interaction is the most uncertain component of the overall anthropogenic forcing of the climate, in which the Twomey effect plays a fundamental role. Satellite-based estimates of the Twomey effect are especially challenging, mainly due to the difficulty in disentangling aerosol effects on cloud droplet number concentration (Nd) from possible confounders. By combining multiple satellite observations and reanalysis, this study investigates the impacts of a) updraft, b) precipitation, c) retrieval errors, as well as (d) vertical co-location between aerosol and cloud, on the assessment of Nd-toaerosol sensitivity (S) in the context of marine warm (liquid) clouds. Our analysis suggests that S increases remarkably with both cloud base height and cloud geometric thickness (proxies for vertical velocity at cloud base), consistent with stronger aerosol-cloud interactions at larger updraft velocity. In turn, introducing the confounding effect of aerosol–precipitation interaction can artificially amplify S by an estimated 21 %, highlighting the necessity of removing precipitating clouds from analyses on the Twomey effect. It is noted that the retrieval biases in aerosol and cloud appear to underestimate S, in which cloud fraction acts as a key modulator, making it practically difficult to balance the accuracies of aerosol–cloud retrievals at aggregate scales (e.g., 1° × 1° grid). Moreover, we show that using column-integrated sulfate mass concentration (SO4C) to approximate sulfate concentration at cloud base (SO4B) can result in a degradation of correlation with Nd, along with a nearly twofold enhancement of S, mostly attributed to the inability of SO4C to capture the full spatio-temporal variability of SO4B. These findings point to several potential ways forward to account for the major influential factors practically by means of satellite observations and reanalysis, aiming at an optimal observational estimate of global radiative forcing due to the Twomey effect.

Balloon borne aerosol-cloud interaction studies (BACIS): New observational techniques to understand and quantify aerosol effects on clouds

Better understanding of aerosol-cloud interaction processes is an important aspect to quantify the role of clouds and aerosols in the climate system. There have been significant efforts to explain the ways aerosols modulate cloud properties. However, from the observational point of view, it is indeed challenging to observe and/or verify some of these processes because no single instrument or platform is proven sufficient. With this motivation, a unique set of observational field campaigns named Balloon borne Aerosol Cloud Interaction Studies (BACIS) is proposed and conducted using balloon borne in-situ measurements in addition to the ground-based (Lidars, MST radar, LAWP, MWR, Ceilometer) and space borne (CALIPSO) remote sensing instruments from Gadanki (13.45° N, 79.2° E). So far, 15 campaigns have been conducted as a part of BACIS campaigns from 2017 to 2020. This paper presents the concept of observational approach, lists the major objectives of the campaigns, describes the instruments deployed, and discusses results from selected campaigns. Consistency in balloon borne measurements is assessed using the data from simultaneous observations of ground-based, space borne remote sensing instruments. A good agreement is found among multi-instrumental observations. Balloon borne in-situ profiling is found to complement the information provided by ground-based and/or space borne measurements. A combination of the Compact Optical Backscatter AerosoL Detector (COBALD) and Cloud Particle Sensor (CPS) sonde is employed for the first time to discriminate cloud and aerosol in an in-situ profile. A threshold value of COBALD color index (CI) for ice clouds is found to be between 18 and 20 and CI values for coarse mode aerosol particle range between 11 and 15. Using the data from balloon measurements, the relationship between cloud and aerosol is quantified for the liquid clouds. A statistically significant slope (aerosol-cloud interaction index) of 0.77 (0.86) found between aerosol back scatter from 300 m (400 m) below the cloud base and cloud particle count within the cloud indicates the role of aerosol in the cloud activation process. In a nutshell, the results presented here demonstrate the observational approach to quantify aerosol-cloud interactions and paves the way for further investigations using the approach.

Beobachtung von Windprofilen mittels Wolkenradar und Dopplerlidar

<p>Die Messung des 3D-Windprofils erfordert – abgesehen von in-situ Messungen – ein aktives Fernerkundungsverfahren (meist Radar oder Lidar), welches mit geneigten Strahlen unter verschiedenen, mindestens drei Azimutwinkeln Pulse ausstrahlt („Doppler beam swinging“). Aus den gemessenen Doppler-Radialgeschwindigkeiten entlang der Strahlen kann dann das dreidimensionale Windfeld abgeleitet werden. Dies ist nur möglich, sofern Partikel vorhanden sind, die bei der gegebenen Wellenlänge ein Rückstreusignal erzeugen.</p> <p>Bereits seit mehreren Jahrzehnten sind Radar-Windprofiler im Einsatz, die bei Wellenlängen zwischen 50 und 1000 MHz arbeiten und mittels Bragg-Streuung an Fluktuationen des Brechungsindex ein Rückstreusignal erhalten. Durch die lange Wellenlänge sind große Antennen erforderlich, was dazu führt, dass die Geräte nicht flexibel einsetzbar sind.</p> <p>Innerhalb des Netzwerks der europäischen Forschungsinfrastruktur ACTRIS (Aerosol, Cloud and Trace Gas Research Infrastructure) sind mehrere Standorte für Wolkenbeobachtungen mit einem scannenden Wolkenradar und einem Doppler-Windlidar ausgestattet, die auch zur Beobachtung von Windprofilen in der Troposphäre herangezogen werden können. Diese Messgeräte ergänzen sich, da das Lidar besonders in der Grenzschicht bzw. unterhalb von Wolken messen kann. Das Wolkenradar hingegen liefert Signale hauptsächlich aus Wolkenschichten, von welchen das Lidar aufgrund der starken Extinktion der Strahlung in Wolken keine Information erhält. Zusätzlich können beim Wolkenradar in der warmen Jahreszeit auch Insekten als Tracer verwendet werden, die häufig bis in Höhen von 3-4 km beobachtet werden können. </p> <p>Diese Präsentation zeigt anhand von Beobachtungen über mehr als zwei Jahre an der Messplattform JOYCE (Jülich Observatory for Cloud Evolution) eine neue Methode zur Kombination der Windprofile aus Wolkenradar und Lidar. Neben einer Betrachtung der Genauigkeit, sowie möglicher Fehlerquellen, werden auch die generellen Bedingungen für die Anwendung der Methode diskutiert. Es werden Anwendungsbeispiele gezeigt, wie diese kombinierten Windprofile zur Validierung von Satellitenbeobachtungen (z.B. Aeolus) oder zur Evaluation von atmosphärischen Modellen genutzt werden können. </p> <p> </p>

Evaluation des EarthCare Cloud Profiling Radars durch bodengebundene Radare

<p>Radarmessungen liefern für die Erforschung von Niederschlag, Wolken und der involvierten Prozesse einen signifikanten Beitrag. Dazu tragen auch Netzwerke wie ACTRIS (Aerosol, Cloud and Trace Gases Research Infrastructure) bei, in welchen nicht nur die Zahl bodengebundener Wolkenradarsysteme stetig wächst, sondern auch deren Datennutzung, z. B. durch Anwendung im synergistischen Verbund mit anderen Messsystemen bei der Wolkenklassifizierung. Europa verfügt somit über ein dichtes bodengebundenes Netzwerk, um Wolken zu untersuchen, für die globale Betrachtung sind allerdings Satelliten notwendig. Mittels satellitengestützter Wolkenradarsysteme, wie z. B. CloudSat, ist es möglich, ein globales Bild zu erhalten. Satellitengestützte Cloud Profiling Radare (CPR) können allerdings hinsichtlich ihrer meist geringeren Sensitivität und aufgrund des sehr starken Bodenechos gegenüber bodengebundenen Systemen im Nachteil sein. Somit sind beispielsweise die Beobachtung bodennaher Wolken, z.B. Grenzschichtbewölkung, oder das Quantifizieren von bodennahem Niederschlag für CPR problematisch.</p> <p>In den kommenden Jahren wird die ESA/JAXA Mission EarthCare ein neues CPR mit verbesserter Performance in Umlauf bringen. Um bereits vor dem Start des Satelliten die Performance des CPR zu evaluieren, werden in dieser Arbeit bodengebundene Messdaten mit simulierten CPR-Daten verglichen. Hierzu werden Datensätze von bodengebundenen Radaren mittels Vorwärtsoperator in einen komplementären Radarsatellitendatensatz umgewandelt. Im Anschluss werden die Datensätze verglichen und ausgewertet.</p> <p>Die Datengrundlage für diese Arbeit liefern die W-Band-Radare des ACTRIS Netzwerks. Die zeitlich langen ACTRIS-Datensätze liefern eine optimale Datengrundlage für eine statistische Analyse der CPR-Performance. Diese Analyse macht es möglich, das neue CPR im Bezug auf die Beobachtung bodennaher Wolken und des bodennahen Niederschlags zu evaluieren.</p>

The Leipzig Aerosol and Cloud Remote Observations System LACROS – a mobile infrastructure for aerosol-cloud-interaction observations at hot spots of atmospheric research

<p>The large number of unsolved questions concerning the interaction between aerosol particles and clouds and corresponding indirect effects on precipitation and radiative transfer demand new measurement strategies and systems to resolve the atmospheric processes involved. Obtaining synergistic information about cloud and aerosol properties from multi–instrument and hence multi–sensor observations is a key approach to overcome the current lack of knowledge. Motivated by these needs, the mobile multi–instrument platform Leipzig Aerosol and Cloud Remote Observations System LACROS was set-up in 2011 by Leibniz Institute for Tropospheric Research (TROPOS). LACROS nowadays is the central component of a sophisticated framework of synergistic state-of-the-art measurement techniques and methodologies, embedded into an environment of comprehensive data management.</p> <p>The current setup of LACROS comprises a set of state-of-the-art remote-sensing instruments such as a 35-GHz scanning polarimetric cloud radar, multi-wavelength polarization Raman lidars, Doppler lidar, micro rain radar, microwave radiometer, laser disdrometer, as well as sensors for direct and diffuse downwelling solar and terrestrial radiation. All instruments are installed within customized sea-freight containers. This ensures a highest-possible mobility of the whole set of instruments. LACROS is a central mobile exploratory platform of the European Union Aerosol, Clouds, and Trace Gases Research Infrastructure (ACTRIS, http://www.actris.net). A variety of ways for physical, remote, and virtual access to the LACROS capabilities are provided via the European Union project ATMO-ACCESS (https://www.atmo-access.eu).</p> <p>LACROS measurements focus on three main tasks: (1) Investigation of mixed-phase cloud processes by exploiting co‐located remote-sensing observations of microphysical properties and radiative effects of aerosols and clouds and their interactions. (2) Instrument validation and development of algorithms and new measurement techniques for cloud and aerosol microphysics retrievals such as, i.e., dual‐field‐of‐view lidar to derive cloud droplet size information, or retrievals of aerosol microphysical properties from combined lidar and Sun photometer measurements. (3) Field deployments in key regions of atmospheric research, where the processes under investigation are already naturally constrained and observations can ideally be combined with in-situ observations or model simulations.</p> <p> </p> <p>This contribution will present the current setup of LACROS and its recent deployments in Leipzig, the Netherlands, Cyprus and southern Chile, results of aerosol-cloud-interaction studies by means of both, case studies and multi-site long-term statistics, as well as an overview on the current and future involvement of LACROS in cal/val activities of new methods and satellite missions. </p>