Dynamics of the Cloud-Environment Interface and turbulence effects in a LES of a growing cumulus congestus

A giga-large-eddy simulation of a cumulus congestus has been performed with a 5-m resolution in order to examine the fine-scale dynamics and mixing on its edges. At 5-m resolution, the dynamical production of subgrid turbulence clearly dominates over the thermal production, while the situation is reversed for resolved turbulence, the tipping-point occurring near the 250-m scale. Concerning cloud dynamics, the toroïdal circulation already obtained in previous observational and numerical studies remains, with a strong signature on the resolved turbulent fluxes, the most important feature for the exchanges between the cloud and its environment even though numerous smaller eddies are also well resolved. The environment compensates for the upward mass flux through a large-scale compensating subsidence and the so-called “subsiding shell” composed of cloud-edge downdrafts, both having a significant contribution. A partition is used to characterize the dynamics, buoyancy and turbulence of the inner and outer edges of the cloud, the cloud interior and the far environment. On the edges of the cloud, downdrafts caused by the eddies and by evaporative cooling effects coexist with a buoyancy reversal while the cloud interior is mostly rising and positively buoyant. An alternative simulation, where evaporative cooling is suppressed, indicates that this process reinforces the downdrafts near the edges of the cloud and causes a general decrease of the convective circulation. Evaporative cooling has also an impact on the buoyancy reversal and on the fate of the engulfed air inside the cloud.

Meteorites that produce K-feldspar-rich ejecta blankets correspond to mass extinctions

Meteorite impacts load the atmosphere with dust and cover the Earth's surface with debris. They have long been debated as a trigger of mass extinctions through Earth's history. Impact winters generally last <100 years, whereas ejecta blankets persist for 103-105 years. Here we show that only meteorite impacts that emplaced ejecta blankets rich in K-feldspar (Kfs) correlate to Earth system crises (n=11, p<0.000005). Kfs is a powerful ice-nucleating aerosol yet is normally rare in atmospheric dust mineralogy. Ice nucleation plays an important role in cloud microphysics, which modulates global albedo. A conceptual model is proposed whereby the anomalous prevalence of Kfs is posited to have two key effects on cloud dynamics: 1) reducing the average albedo of mixed-phase cloud, which effected a hotter climate; 2) weakening of the cloud albedo feedback, which increased climate sensitivity. These mechanisms offer an explanation as to why this otherwise benign mineral is correlated so strongly with mass extinction events: every K-feldspar-rich ejecta blanket corresponds to a severe extinction episode over the past 600 Myr. This model may also explain why many kill mechanisms only variably correlate with extinction events through geological time: they coincide with these rare periods of climate destabilization by atmospheric Kfs.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5690646

Automatic Edge Detection Applied to Cavitating Flow Analysis: Cavitation Cloud Dynamics and Properties Measured through Detected Image Regions

In a cavitating water jet, cavity clouds emerge and collapse with an unsteady, but periodic tendency where the frequencies depend on the working conditions. The presented work aims at examining and analyze the dynamic behavior and properties of the clouds under different circumstances. Computer vision and image processing were introduced as tools to define the cavitation clouds based on the Contour Recognition technique. A Canny operator and Otsu threshold fragmenting methods were used. The use of these methods allows for a better understanding of the cavitating jet clouds' behavior based on the pixel intensities and shows that for an arbitrary cloud the surface itself has a dynamic feature and depends on the cavity composition. The clouds' properties could be measured and correlated to the applied working conditions. Also, the oscillation frequencies of the area of the clouds could be determined. The analysis shows that the quality of the obtained results depends mainly on the input threshold values separating the foreground and background pixels. The difficulty of defining the threshold value is discussed in the paper, as well as the validity of using the Contour Recognition technique in this field.

Automatic detection of atmospheric boundary layer heights at European scale (ABL testbed)

&lt;p&gt;A detailed understanding of atmospheric boundary layer (ABL) processes is key to improve forecasting of pollution dispersion and cloud dynamics in the context of future climate scenarios. International networks of automatic lidars and ceilometers (ALC) are gathering valuable data that allow for ABL layers to be derived in near real time. A new generation of advanced methods to automatically detect the ABL heights now exist. However, diversity in ALC models means these algorithms need to be tailored to instrument-specific capabilities. Initial evaluation of the advanced algorithms STRATfinder (for application to high signal-to-noise ratio (SNR) ALC observations) and CABAM (low-SNR measurements) to automatically derive ABL heights indicates promising performances (Kotthaus et al. 2020).&lt;/p&gt;&lt;p&gt;In the framework of the ABL testbed project (funded by ICOS, ACTRIS and E-PROFILE) the two algorithms are now being assessed for application in an operational network setting, such as EUMETNET E-PROFILE. A subset of 11 E-PROFILE sites in a range of geographical and land cover settings across Europe are selected where data from low-SNR and/or high-SNR ALC are available for multiple years. Automatic layer detection is implemented, including instrument-specific corrections and calibrations. Algorithm performance for layer height detection is being evaluated via comparison of results from different ALC and by including reference data from thermodynamic- and turbulence derived layer heights from radiosondes and other ground-based profiling sensors where available. Recommendations are formulated for implementation of automatic ABL height retrievals across a diverse sensor network. A prime example of collaborations within the EU COST action PROBE on profiling the atmospheric boundary layer, the ABL testbed is a crucial step towards harmonised ABL height products at the European scale.&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Kotthaus, S, M Haeffelin, MA Drouin, JC Dupont, S Grimmond, A Haefele, M Hervo, Y Poltera, M Wiegner, 2020: Tailored Algorithms for the Detection of the Atmospheric Boundary Layer Height from Common Automatic Lidars and Ceilometers (ALC). &lt;em&gt;Remote Sens&lt;/em&gt;, 12, 3259, DOI: 10.3390/rs12193259.&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Cavitation Bubble Cloud Break-Off Mechanisms at Micro-Channels

This paper provides new physical insight into the coupling between flow dynamics and cavitation bubble cloud behaviour at conditions relevant to both cavitation inception and the more complex phenomenon of flow “choking” using a multiphase compressible framework. Understanding the cavitation bubble cloud process and the parameters that determine its break-off frequency is important for control of phenomena such as structure vibration and erosion. Initially, the role of the pressure waves in the flow development is investigated. We highlight the differences between “physical” and “artificial” numerical waves by comparing cases with different boundary and differencing schemes. We analyse in detail the prediction of the coupling of flow and cavitation dynamics in a micro-channel 20 m high containing Diesel at pressure differences 7 MPa and 8.5 MPa, corresponding to cavitation inception and "choking" conditions respectively. The results have a very good agreement with experimental data and demonstrate that pressure wave dynamics, rather than the “re-entrant jet dynamics” suggested by previous studies, determine the characteristics of the bubble cloud dynamics under “choking” conditions.

Assessment of the spectral downward irradiance at the surface of the Mediterranean Sea using the radiative Ocean-Atmosphere Spectral Irradiance Model (OASIM)

A multiplatform assessment of the Ocean–Atmosphere Spectral Irradiance Model (OASIM) radiative model focussed on the Mediterranean Sea for the period 2004–2017 is presented. The BOUée pour l'acquiSition d'une Série Optique à Long termE (BOUSSOLE) mooring and biogeochemical Argo (BGC-Argo) float optical sensor observations are combined with model outputs to analyse the spatial and temporal variabilities in the downward planar irradiance at the ocean–atmosphere interface. The correlations between the data and model are always higher than 0.6. With the exception of downward photosynthetic active radiation and the 670 nm channel, correlation values are always higher than 0.8 and, when removing the inter-daily variability, they are higher than 0.9. At the scale of the BOUSSOLE sampling (15 min temporal resolution), the root mean square difference oscillates at approximately 30 %–40 % of the averaged model output and is reduced to approximately 10 % when the variability between days is filtered out. Both BOUSSOLE and BGC-Argo indicate that bias is up to 20 % for the irradiance at 380 and 412 nm and for wavelengths above 670 nm, whereas it decreases to less than 5 % at the other wavelengths. Analysis of atmospheric input data indicates that the model skill is strongly affected by cloud dynamics. High skills are observed during summer when the cloud cover is low.

The role of updrafts in the scaling of extreme precipitation in mid-latitudes

&lt;p&gt;We investigate the hypothesis that invigoration of convective updrafts under warming conditions contributes to the stronger than Clausius-Clapeyron (CC) scaling. Focus is on a mid-latitude case of extreme precipitation, based on idealised forcing conditions derived for the Netherlands, with strong surface forcing as well as strong forcing from large-scale rising motion associated with the passage of a synoptic scale low pressure or frontal system. Various Large Eddy Simulations (LES) of this composite case have been performed on a 192x192 km domain. By systematically perturbing the atmospheric temperature profile, a large response of cloud dynamics to warming with larger and more vigorous cloud structures in the warmer runs has been found.&lt;sup&gt;*&lt;/sup&gt;&lt;/p&gt;&lt;p&gt;Here, we study these cloud dynamics further by investigating the vertical wind velocity in the cloud (cores). Updrafts play a key role in rain formation by transporting moisture upward in the clouds. We will demonstrate how the distributions of these vertical velocities near the surface and at different levels in the clouds respond to warming in this mid-latitude setting and how they relate to cloud properties as cell size and buoyancy.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;&lt;em&gt;*&lt;/em&gt;&lt;/sup&gt;&lt;em&gt;Lochbihler, K., Lenderink, G., and Siebesma, A. P. (2019). Response of extreme precipitating cellstructures to atmospheric warming. Journal of Geophysical Research: Atmospheres&lt;/em&gt;&lt;/p&gt;