Large-eddy simulation of a nocturnal stratocumulus-topped marine atmospheric boundary layer: An uncertainty analysis

A large-eddy simulation (LES) model has been used to study a nocturnal stratocumulus-topped marine atmospheric boundary layer. The main objectives of our study have been first to investigate the statistical significance of LES-derived data products. Second, to test the sensitivity of our LES results with respect to the representation of subgrid-scale mixing and microphysical processes, and third to evaluate and to quantify the parametric uncertainty arising from the incomplete knowledge of the environmental parameters that are required to specify the initial and boundary conditions of a particular case study. Model simulations were compared with observations obtained in solid stratocumulus during the third flight of the first 'Lagrangian' experiment of the Atlantic Stratocumulus Transition Experiment (ASTEX). Based on these simulations the following conclusions could be drawn. Resolution (50 x 50 x 25 m(3)) and domain size (3.2 x 3.2 x 1.5 km(3)) of the LES calculations were adequate from a numerical point of view to represent the essential features of the stratocumulus-topped boundary layer. However, the ensemble runs performed in our study to investigate the statistical significance of LES-derived data products demonstrate that the area-time averaging procedure for the second-order moments produces only a low degree of statistical reliability in the model results. This illustrates the necessity of having LES model results that are not only of adequate resolution but also of sufficiently large domain. The impact of different subgrid schemes was small, but the primary effects of drizzle were found to influence the boundary-layer structure in a climatologically significant way. The parametric uncertainty analysis revealed that the largest contribution to the variance of the LES-derived data products is due to the uncertainties in the cloud-top jump of total water mixing ratio and the net radiative forcing. The differences between the model and measurements for most of the simulated quantities were within the modelling uncertainties, but the calculated precipitation rate was found to differ significantly from that derived in the observations

Grobstruktursimulation - eine Methode zur Berechnung turbulenter atmosphärischer Strömungen

Bei der Grobstruktursimulotion hondelt es sich um eine Methode zur Be- rechnung turbulenter Strömungen, bei der nur die großen, energiereichen Wirbel erfoßt werden und der Einfluß der nicht oufgelösten, subskoligen Dynomik durch eine einfoche Porometerisierung berucksichtigt wird, Die- se Methode wird in zunehmendem Moße eingesefzt, um die Eigenschof- Ten bestimmter lurbulenter Strömungen zu untersuchen, Die mit Grobstruktursimulotionsmodellen produzierten Dotensötze können dor- über hinous ols Grundloge zum Test, zur Weiterentwicklung und zum Ent- wurf von komplexen Schließungsonsötzen für gróßerskolige Modelle verwendet werden. ln der vorliegenden Arbeit werden der Stond der For- schung ouf diesem Gebiet doçelegt, die theoretischen Grundlogen die- ser Methode beschrieben und die dieser Technik zugrunde liegenden Annohmen kritisch gewurdigt. Weiterhin werden dos zur Berechnung zweiphosiger, turbulenter otmosphörischer Strömungen entwickelte und hier eingeselzte Grobstruktursimulotionsmodell vorgestellt und einige nu- merische Aspekte der verwendeten Lösungsolgorithmen behondelt, Die Möglichkeiten und Grenzen der Grobstruktursimulotionsmethode zur Un- tersuchung olmosphörischer Strömungen werden durch Anwendung die- ses Berechnungsverfohrens ouf drei Strömungsregime oufgezeigt, Die Anwendungsfölle umfossen die Simulolion der konvektiven Grenzschicht wöhrend eines Koltluftousbruches, die strotocumulusbedeckte Grenz- schicht und die Wirbelstruktur in Kondensstreifen, Die Houptresultole lou- ten: L Die sich unter Bedingungen eines Koltluftousbruches vollziehende Grenzschicht- und Wolkenentwicklung konn mit dem Modell reoli- tötsnoh nochvollzogen werden. 2. Mil Ausnohme des Niederschlogsflusses weisen die modellierten dynomischen, thermodynomischen und mikrophysikolischen Eigen- schoften der strotocumulusbedeckten Grenzschicht eine gute Über- einstimmung mit den wöhrend des ASTEX-Experiments gewonnenen Beobochtungsdoten ouf. 3, Die relotive Feuchte, die Temperotur und die Dichteschichtung der u mgebenden Atmosphöre stel len die entscheidenden E influßg rößen dor, die die Entwicklung eines Kondensstreifens im Dispersionsregime beeinflussen, Die ezielten Resultote lossen den Schluß zu, doß mon mit Grobstruktursi- mulotionsmodellen ein effektives numerisches Werkzeug on der Hond hot, um Grundlogenfrogen zur Turbulenz zu beorbeiten und die Erkenntnisse uber lurbulente Strömungen zu vertiefen.Lorge-eddy-simulotion (LES) is o method of simuloting o turbulent flow by copturing only the lorgest, most energeïic structures in the flow ond poro- meterizing the effect of the smoll subgrid-scole eddies, This Technique is being increosingly used os o meons of both predicting the properties of specific turbulent flows ond providing flow detoils which con be used like doto to test, to refine ond to develop complex turbulence-closures for lor- ger-scole models, ln this poper, we provide o scientific overview of the to- pic ond we review the theoreticol bockground ond the underlying ossumptions of this technique, Moreover, the current implementotion of the LES-model thot hos been developed to study two-phose turbulent ot- mospheric flows is introduced ond some issues of the numericol schemes ore described, Limitotions ond credibility of the LES-method in investigo- ting the chorocteristics of turbulent flows ore ossessed by opplying this technique to three different flow regimes. ln porticulor, the convective boundory loyer during cold oir outbreoks, the strotocumulus-topped boundory loyer ond the turbulent eddy structure within controils ore simu- loted. The moin results ore: L Boundory loyer ond cloud development which occur under conditi- ons during o cold oir outbreok ore reolisticolly reproduced by the model. 2, With the exception of the precipitotion flux the model predictions of dynomic, thermodynomic ond microphysicol properties in o stroto- cumulus topped boundory loyer ore generolly in o reosonoble ogreement with meosurements obtoined during the ASTEX-experi- ment, 3, During the dispersion phose controil evolution is controlled primorily by temperoture, humidity ond stotic stobility of the ombient oir, The results obïoined leod us to conclude thot LES-models estoblish on excellent reseorch tool for studying turbulent phenomeno ond for odvon- cing our understonding obout turbulent flows

The climatic role of interactive leaf phenology in the vegetation-atmosphere system of radiative-convective equilibrium storm-resolving simulations

Storm-resolving simulations where deep convection can be explicitly resolved are performed in the idealized radiative-convective equilibrium framework to explore the climatic role of interactive leaf phenology. By initializing the system with different initial soil moisture and leaf area index (LAI) conditions, we find three categories of potential equilibrium climatic and vegetation states: a hot desert planet without vegetation, an intermediate sparsely vegetated planet, and a wet fully vegetated planet. The wet fully vegetated equilibrium category occurs over the widest range of initial soil moisture as it occurs as soon as soil saturation is 19 higher than the permanent wilting point (35). This indicates that a quite harsh environment is needed in our modeling system to force leaves to be shed. The attained equilibrium states are only dependent upon the initial soil moisture, not the initial LAI. However, interactive leaves do allow an earlier transition from the intermediate to the wet vegetated equilibrium category. Hence, interactive leaves make the vegetation-atmosphere system more stable and more resilient to drying. This effect could be well approximated by just prescribing the LAI to its maximum value. Finally, our sensitivity experiments reveal that leaves influence the climate equally through their controls on canopy conductance and vegetation cover, whereas albedo changes play a negligible role. © 2022 The Author(s)

The effect of soil moisture and atmospheric conditions on the development of shallow cumulus convection: A coupled large-eddy simulation-land surface model study

Many processes and feedback mechanisms are involved in land-atmosphere interactions that play an important role in determining the boundary layer structure throughout the diurnal cycle. Here, the effect of soil moisture on the development of shallow cumulus convection is investigated using a coupled large-eddy simulation (LES)-land surface model (LSM) framework. First, the coupled model is run for an idealised case based on measurements at the ARM Southern Great Plain site on 21 June 1997 to demonstrate that many characteristics of the subcloud layer turbulence and of the cumulus layer can be modelled successfully. Moreover, an extensive sensitivity study is performed with different amounts of soil moisture and varying atmospheric conditions. Our results support the hypothesis that the response of shallow cumulus clouds due to a change of soil moisture severely depends on the thermal stability conditions. Furthermore, they also point out that the atmospheric moisture content is as important as the static stability in determining the boundary layer characteristics and in particular the fractional cloud cover. The results demonstrate that the soil moisture-cloud cover coupling is positive in most of the cases. However, we show that under specific conditions (a less stably stratified moist atmosphere) convective activity and cloud formation is stronger over dry soils, where the principle driving mechanism for cloud development is the boundary layer growth that tends to increase relative humidity by adiabatic cooling of the air at the top of the boundary layer. This leads to a soil moisture cloud cover relationship in which the cloud cover fraction decreases with an increase of soil moisture. Moreover, our findings suggest that in the limiting case of a water saturated soil the mean cloud cover is independent of static stability, but only depends on the vertical integrated atmospheric moisture content

A single-column model intercomparison of a heavily drizzling stratocumulus-topped boundary layer

This study presents an intercomparison of single-column model simulations of a nocturnal heavily drizzling marine stratocumulus-topped boundary layer. Initial conditions and forcings are based on nocturnal flight observations off the coast of California during the DYCOMS-II field experiment. Differences in turbulent and microphysical parameterizations between models were isolated by slightly idealizing and standardizing the specification of surface and radiative fluxes. For most participating models, the case was run at both typical operational vertical resolution of about 100 m and also at high vertical resolution of about 10 m. As in prior stratocumulus intercomparisons, the simulations quickly develop considerable scatter in liquid water path (LWP) between models. However, the simulated dependence of cloud base drizzle fluxes on LWP in most models is broadly consistent with recent observations. Sensitivity tests with drizzle turned off show that drizzle substantially decreases LWP for many models. The sensitivity of entrainment rate to drizzle is more muted. Simulated LWP and entrainment are also sensitive to the inclusion of cloud droplet sedimentation. Many models underestimate the fraction of drizzle that evaporates below cloud base, which may distort the simulated feedbacks of drizzle on turbulence, entrainment, and LWP