Path-tracing Monte Carlo Library for 3D Radiative Transfer in Highly Resolved Cloudy Atmospheres

Interactions between clouds and radiation are at the root of many difficulties in numerically predicting future weather and climate and in retrieving the state of the atmosphere from remote sensing observations. The large range of issues related to these interactions, and in particular to three-dimensional interactions, motivated the development of accurate radiative tools able to compute all types of radiative metrics, from monochromatic, local and directional observables, to integrated energetic quantities. In the continuity of this community effort, we propose here an open-source library for general use in Monte Carlo algorithms. This library is devoted to the acceleration of path-tracing in complex data, typically high-resolution large-domain grounds and clouds. The main algorithmic advances embedded in the library are those related to the construction and traversal of hierarchical grids accelerating the tracing of paths through heterogeneous fields in null-collision (maximum cross-section) algorithms. We show that with these hierarchical grids, the computing time is only weakly sensitivive to the refinement of the volumetric data. The library is tested with a rendering algorithm that produces synthetic images of cloud radiances. Two other examples are given as illustrations, that are respectively used to analyse the transmission of solar radiation under a cloud together with its sensitivity to an optical parameter, and to assess a parametrization of 3D radiative effects of clouds.Comment: Submitted to JAMES, revised and submitted again (this is v2

An intercomparison of mesoscale simulations during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) experimental field campaign

The Convective (diurnal, CBL) and Stably stratified (nocturnal, SBL) Boundary Layers over land have been extensively observed and relatively successfully modeled. But the early morning transition, when the CBL emerges from the nocturnal boundary layer, and the late afternoon transition, when the CBL decays to an intermittently turbulent residual layer overlying a SBL, are difficult to observe and model due to the intermittency and anisotropy of turbulence, horizontal heterogeneity and rapid changes in timePeer ReviewedPostprint (published version

Boundary Layer Late Afternoon and Sunset Turbulence: the BLLAST 2011 experiment

Peer ReviewedPostprint (published version

Studying the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST)

At the end of the afternoon, when the surface heat fluxes start to sharply decrease, the CBL turns from a convective well-mixed layer to an intermittently turbulent residual layer overlying a stably-stratified boundary layer. This transition raises several observational and modelling issues. Even the definition of the boundary layer during this period is fuzzy, since there is no consensus on what criteria to use and no simple scaling laws to apply. Yet it plays an important role in such diverse atmospheric phenomena as transport and diffusion of trace constituents or wind energy production. This phase of the diurnal cycle remains largely unexplored, partly due to the difficulty of measuring weak and intermittent turbulence, anisotropy, horizontal heterogeneity, and rapid time changes. The Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) project is gathering about thirty research scientists from the European Union and the United States to work on this issue. A field campaign (BLLAST-FE) is planned for spring or summer 2011 in Europe. BLLAST will utilize these observations, as well as previous datasets, large-eddy and direct numerical simulations, and mesoscale modelling to better understand the processes, suggest new parameterisations, and evaluate forecast models during this transitional period. We will present the issues raised by the late afternoon transition and our strategy to study it.Peer ReviewedPostprint (published version

Studying the Boundary Layer Late Afternoon nd Sunset Turbulence (BLLAST)

At the end of the afternoon, when the surface heat fluxes start to sharply decrease, the CBL turns from a convective well-mixed layer to an intermittently turbulent residual layer overlying a stably-stratified boundary layer. This transition raises several observational and modeling issues. Even the definition of the boundary layer during this period is fuzzy, since there is no consensus on what criteria to use and no simple scaling laws to apply. Yet it plays an important role in such diverse atmospheric phenomena as transport and diffusion of trace constituents or wind energy production. This phase of the diurnal cycle remains largely unexplored, partly due to the difficulty of measuring weak and intermittent turbulence, anisotropy, horizontal heterogeneity, and rapid time changes. The Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) project is gathering about thirty research scientists from the European Union and the United States to work on this issue. A field campaign (BLLAST-FE) is planned for spring or summer 2011 in Europe. BLLAST will utilize these observations, as well as previous datasets, large-eddy and direct numerical simulations, and mesoscale modeling to better understand the processes, suggest new parameterizations, and evaluate forecast models during this transitional period. We will present the issues raised by the late afternoon transition and our strategy to study it.Peer ReviewedPostprint (published version

Six hundred years of South American tree rings reveal an increase in severe hydroclimatic events since mid-20th century

South American (SA) societies are highly vulnerable to droughts and pluvials, but lack of long-term climate observations severely limits our understanding of the global processes driving climatic variability in the region. The number and quality of SA climate-sensitive tree ring chronologies have significantly increased in recent decades, now providing a robust network of 286 records for characterizing hydroclimate variability since 1400 CE. We combine this network with a self-calibrated Palmer Drought Severity Index (scPDSI) dataset to derive the South American Drought Atlas (SADA) over the continent south of 12°S. The gridded annual reconstruction of austral summer scPDSI is the most spatially complete estimate of SA hydroclimate to date, and well matches past historical dry/wet events. Relating the SADA to the Australia–New Zealand Drought Atlas, sea surface temperatures and atmospheric pressure fields, we determine that the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are strongly associated with spatially extended droughts and pluvials over the SADA domain during the past several centuries. SADA also exhibits more extended severe droughts and extreme pluvials since the mid-20th century. Extensive droughts are consistent with the observed 20th-century trend toward positive SAM anomalies concomitant with the weakening of midlatitude Westerlies, while low-level moisture transport intensified by global warming has favored extreme rainfall across the subtropics. The SADA thus provides a long-term context for observed hydroclimatic changes and for 21st-century Intergovernmental Panel on Climate Change (IPCC) projections that suggest SA will experience more frequent/severe droughts and rainfall events as a consequence of increasing greenhouse gas emissions

The BLLAST field experiment: Boundary-Layer late afternoon and sunset turbulence

Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary layer, still has a number of unanswered scientific questions. This phase of the diurnal cycle is challenging from both modelling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state. These issues motivated the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of instrumented platforms including full-size aircraft, remotely piloted aircraft systems, remote-sensing instruments, radiosoundings, tethered balloons, surface flux stations and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observation periods that were conducted from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, such as new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere. Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary-layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the previous day's residual layer, as well as local, meso- or synoptic scale conditions. Ground-based measurements combined with tethered-balloon and airborne observations captured the turbulence decay from the surface throughout the whole boundary layer and documented the evolution of the turbulence characteristic length scales during the transition period. Closely integrated with the field experiment, numerical studies are now underway with a complete hierarchy of models to support the data interpretation and improve the model representations.publishedVersio

Contribution à l'analyse des processus de couche limite et à leur représentation dans les modèles à partir de simulations explicites

I will present some use of Large-Eddy simulations that correspond to numerical simulations of shallow clouds or clear boundary layer where most of the processes are resolved. After, a first description of such simulation and their main advantages and drawbacks, I will detail how they can be used both for the understanding of boundary-layer processes and for the improvement of their representation in models. I will in particular detail a simulation of initiation of deep convection observed during the AMMA field campaign that occurs over West Africa. With such a simulation, we show the different processes leading to initiation of deep convection in this very dry environment not favorable to convection. I will also show how these simulations can be used to derive parametrization-oriented diagnostics that have been used to improve boundary-layer parametrizations.Je présenterai quelques utilisations des simulations explicites type Large-Eddy Simulations qui correspondent à des simulations numériques de nuages ​​peu profonds ou de couche limite en ciel clair où la plupart des processus sont résolus. Après, une première description de ces simulations et de leurs principaux avantages et inconvénients, je détaillerai comment elles peuvent être utilisées à la fois pour la compréhension des processus de couche limite et pour l'amélioration de leur représentation dans les modèles. Je détaillerai notamment une simulation d'initiation de convection profonde observée lors de la campagne de terrain AMMA qui se déroule sur l'Afrique de l'Ouest. Avec une telle simulation, nous montrons les différents processus conduisant à l'initiation de la convection profonde dans cet environnement très sec peu favorable à la convection. Je montrerai également comment ces simulations peuvent être utilisées pour dériver des diagnostics orientés paramétrisation qui ont été utilisés pour améliorer les paramétrisations de la couche limite

Parameterization of the Dry Convective Boundary Layer Based on a Mass Flux Representation of Thermals

We present a mass flux parameterization of vertical transport in the convective boundary layer. The formulation of the new parameterization is based on an idealization of thermal cells or rolls. The parameterization is validated by comparison to "Large Eddy Simulations" (LES). It is also compared to classical boundary layer schemes on a documented case of a well developed convective boundary layer observed in the Paris area during the ESQUIF campaign. For both LES and observations, the new scheme performs better at simulating entrainment fluxes at the top of the convective boundary layer and near surface conditions. The explicit representation of mass fluxes allows a direct comparison with campaign observations and opens interesting possibilities for coupling with clouds and deep convection schemes

A short review of numerical cloud-resolving models

A cloud-resolving model (CRM) allows performing numerical simulations of convective clouds, such as shallow cumulus and stratocumulus, or storms and squall-lines with a resolution on the order of a few tens of metres to a few kilometres over a limited-area 4D (time and space) domain. The development of such models over the past decades is reviewed and their specific features are presented. The latter include a non-hydrostatic dynamic and parameterizations of sub-grid turbulence, microphysical and radiative processes. The capabilities of such models are discussed based on comparisons with observations and model-intercomparison studies. CRMs are used in a variety of ways, from the exploration of cloud phenomenology and process-understanding studies to the development of algorithms for satellite products, as well as to address climate issues and to develop convective and cloud parametrizations for large-scale weather and climate models. A few results illustrating this wide utilization are presented. The continuous increase of computer power induces rapid changes in modelling perspectives and therefore, influences the developments and applications of CRMs. This is discussed together with emerging scientific questions which will further benefit from CRM simulations