Ship-based cloud remote sensing observations made onboard R/V Meteor during the ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte, EUREC4A, campaign are presented and used to calculate cloud droplet number concentrations. The calculation is based on cloud liquid water path LWP and droplet effective radius reff retrieved from spectral measurements of transmitted solar radiance. It is shown that measurement uncertainties and retrieval assumptions impact the accuracy of the results. A case study indicates that the retrieval of LWP and reff is most affected by 3D-radiative effects in case of shallow cumulus and drizzle, which violates the adiabatic theory and plan-parallel geometry on which the radiative transfer simulations of the retrieval are based. Depending on the cloud thickness, the retrieval of reff might suffers from ambiguity. These retrieval uncertainties and their implications on the estimated cloud droplet number concentration are investigated by a sensitivity study. The analysis showed that most of the uncertainty is introduced by reff, whereas LWP contributes significantly to the uncertainty only for thin clouds. Therefore, it is concluded that only selected cloud cases, which do not violate the retrieval assumption, such as stratiform cloud layers, are suited to apply the retrieval approach in further studies. / Fernerkundungsmessungen von Wolken auf dem Forschungsschiff R/V Meteor während der ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte, EUREC4A, Kampagnewerden vorgestellt und zur Berechnung der Tröpfchenanzahlkonzentration verwendet. Die Berechnung basiert auf Messungen des Flüssigwasserpfads LWP und dem effektiven Tröpfchenradius reff, welche aus spektralen Messungen der transmittierten solaren Strahldichte abgeleitet wurden. Es wird gezeigt, dass Messunsicherheiten und Annahmen bei der Ableitung der Wolkeneigenschaften die Genauigkeit der Ergebnisse beeinflussen. Eine Fallstudie zeigt, dass die Ableitung von LWP und reff am stärksten durch 3-dimensionale Strahlungseffekte von flachen Cumuli und Nieselregen beeinflusst wird. Beides wiederspricht den Idealisierungen von adiabatischen Wolken und einer planparallelen Geometrie, auf denen die Strahlungstransfersimulationen des Verfahrens beruhen. Abhängig von der Wolkendicke kann die Ableitung von reff zusätzlich durch Mehrdeutigkeiten beeinflusst sein.

In this work spectral airborne measurements of upward irradiance and a novel remote sensing technique for the cloud droplet number concentration are used to evaluate the representation of clouds in current operational weather prediction models.

Important aspects of the adjustments to aerosol-cloud interactions can be examined using the relationship between cloud droplet number concentration (Nd) and liquid water path (LWP). Specifically, this relation can constrain the role of aerosols in leading to thicker or thinner clouds in response to adjustment mechanisms. This study investigates the satellite retrieved relationship between Nd and LWP for a selected case of mid-latitude continental clouds using high-resolution Large-eddy simulations (LES) over a large domain in weather prediction mode. Since the satellite retrieval uses the adiabatic assumption to derive the Nd, we have also considered adiabatic Nd (NAd) from the LES model for comparison. The joint histogram analysis shows that the NAd-LWP relationship in the LES model and the satellite is in approximate agreement. In both cases, the peak conditional probability (CP) is confined to lower NAd and LWP; the corresponding mean LWP (LWP) shows a weak relation with NAd. The CP shows a larger spread at higher NAd (>50 cm–3), and the LWP increases non-monotonically with increasing NAd in both cases. Nevertheless, both lack the negative NAd-LWP relationship at higher NAd, the entrainment effect on cloud droplets. In contrast, the model simulated Nd-LWP clearly illustrates a much more nonlinear (an increase in LWP with increasing Nd and a decrease in LWP at higher Nd) relationship, which clearly depicts the cloud lifetime and the entrainment effect. Additionally, our analysis demonstrates a regime dependency (marine and continental) in the NAd-LWP relation from the satellite retrievals. Comparing local vs large-scale statistics from satellite data shows that continental clouds exhibit only a weak nonlinear NAd-LWP relationship. Hence a regime-based Nd-LWP analysis is even more relevant when it comes to warm continental clouds and their comparison to satellite retrievals.