Fabio D'Andrea

Reducing systematic errors by empirically correcting model errors

A methodology for the correction of systematic errors in a simplified atmospheric generalcirculationmodel is proposed. First, a method for estimating initial tendency model errors isdeveloped, based on a 4-dimensional variational assimilation of a long-analysed dataset ofobservations in a simple quasi-geostrophic baroclinic model. Then, a time variable potentialvorticity source term is added as a forcing to the same model, in order to parameterize subgridscaleprocesses and unrepresented physical phenomena. This forcing term consists in a (largescale) flow dependent parametrization of the initial tendency model error computed by thevariational assimilation. The flow dependency is given by an analogues technique which relieson the analysis dataset. Such empirical driving causes a substantial improvement of the modelclimatology, reducing its systematic error and improving its high frequency variability. Lowfrequencyvariability is also more realistic and the model shows a better reproduction of Euro-Atlantic weather regimes. A link between the large-scale flow and the model error is found onlyin the Euro-Atlantic sector, other mechanisms being probably the origin of model error in otherareas of the globe.

Multiple equilibria on planet Dune: Climate-vegetation dynamics on a sandy planet

ABSTRACT We study the interaction between climate and vegetation on an ideal water-limited planet, focussing on the influence of vegetation on the global water cycle. We introduce a simple mechanistic box model consisting in a two-layer representation of the atmosphere and a two-layer soil scheme. The model includes the dynamics of vegetation cover, and the main physical processes of energy and water exchange among the different components. With a realistic choice of parameters, this model displays three stable equilibria, depending on the initial conditions of soil water and vegetation cover. The system reaches a hot and dry state for low values of initial water content and/or vegetation cover, while we observe a wet, vegetated state with mild surface temperature when the system starts from larger initial values of both variables. The third state is a cold desert, where plants transfer enough water to the atmosphere to start a weaker, evaporation-dominated water cycle before they wilt. These results indicate that in this system vegetation plays a central role in transferring water from the soil to the atmosphere and trigger a hydrologic cycle. The model adopted here can also be used to conceptually illustrate processes and feedbacks affecting the water cycle in water-limited continental areas on Earth. †Now at: International Max Planck Research School on Earth System Modelling, Hamburg, Germany

A Probabilistic Bulk Model of Coupled Mixed Layer and Convection. Part II: Shallow Convection Case

The probabilistic bulk convection model (PBCM) developed in a companion paper is here extended to shallow nonprecipitating convection. The PBCM unifies the clear-sky and shallow convection boundary layer regimes by obtaining mixed-layer growth, cloud fraction, and convective inhibition from a single parameterization based on physical principles. The evolution of the shallow convection PBCM is based on the statistical distribution of the surface thermodynamic state of convective plumes. The entrainment velocity of the mixed layer is related to the mass flux of the updrafts overshooting the dry inversion capping the mixed layer. The updrafts overcoming the convective inhibition generate active cloudbase mass flux, which is the boundary condition for the shallow cumulus scheme. The subcloud-layer entrainment velocity is directly coupled to the cloud-base mass flux through the distribution of vertical velocity and fractional cover of the updrafts. Comparisons of the PBCM against large-eddy simulations from the Barbados Oceanographic and MeteorologicalExperiment (BOMEX) andfromthe SouthernGreat PlainsAtmospheric RadiationMeasurement Program (ARM) facility demonstrate good agreement in terms of thermodynamic structure, cloud-base mass flux, and cloud top. The equilibrium between the cloud-base mass flux and rate of growth of the mixed layer determines the equilibrium convective inhibition and cloud cover. This process is an important new insight on the coupling between the mixed-layer and cumulus dynamics. Given its relative simplicity and transparency, the PBCM represents a powerful tool for developing process-based understanding and intuition about the physical processes involved in boundary layer–convection interactions, as well as a test bed for diagnosing and validating shallow convection parameterizations.

Weather regime prediction using statistical learning

Abstract Two novel statistical methods are applied to the prediction of transitions between weather regimes. The methods are tested using a long, 6000-day simulation of a three-layer, quasigeostrophic (QG3) model on the sphere at T21 resolution. The two methods are the k nearest neighbor classifier and the random forest method. Both methods are widely used in statistical classification and machine learning; they are applied here to forecast the break of a regime and subsequent onset of another one. The QG3 model has been previously shown to possess realistic weather regimes in its northern hemisphere and preferred transitions between these have been determined. The two methods are applied to the three more robust transitions; they both demonstrate a skill of 35%–40% better than random and are thus encouraging for use on real data. Moreover, the random forest method allows one, while keeping the overall skill unchanged, to efficiently adjust the ratio of correctly predicted transitions to false alarms. A l...

Extratropical low‐frequency variability as a low‐dimensional problem. II: Stationarity and stability of large‐scale equilibria

Stationarity and stability properties of large-scale persistent anomalies in the northern hemisphere are addressed. The low-order model developed in Part I is used for this purpose. It was obtained as the projection of a three-level quasi-geostrophic system on the ten leading empirical orthogonal functions. Three global quasi- stationary states are identified, which represent the Arctic high and the positive and negative phases of the main teleconnections (Pacific North American and North Atlantic oscillation). The quasi-stationary solutions have only a partial correspondence to the weather regimes found in Part I. Stability analysis shows a growing mode that describes an oscillation between the two phases of the teleconnections. The negative teleconnection state is also shown to be much more stable than the other two. Possible decay mechanisms are also discussed. Copyright

A Probabilistic Bulk Model of Coupled Mixed Layer and Convection. Part I: Clear-Sky Case

A new bulk model of the convective boundary layer, the probabilistic bulk convection model (PBCM), is presented. Unlike prior bulk approaches that have modeled the mixed-layer-top buoyancy flux as a constant fractionofthesurfacebuoyancyflux,PBCMimplementsanewmixed-layer-topentrainmentclosurebasedon the mass flux of updrafts overshooting the inversion. This mass flux is related to the variability of the surface state (potential temperature u and specific humidity q) of an ensemble of updraft plumes. The authors evaluatethemodelagainstobservedclear-skyweakandstronginversioncasesandshowthatPBCMperforms well. The height, state, and timing of the boundary layer growth are accurately reproduced. Sensitivitystudies are performed highlighting the role of the main parameters (surface variances, lateral entrainment). The model is weakly sensitive to the exact specification of the variability at the surface and is most sensitive to the lateral entrainment of environmental air into the rising plumes. Apart from allowing time-dependent top-ofthe-boundary-layer entrainment rates expressed in terms of surface properties, which can be observed in situ, PBCM naturally takes into account the transition to the shallow convection regime, as described in a companion paper. Thus, PBCM represents an important step toward a unified framework bridging parameterizations of mixed-layer entrainment velocity in both clear-sky and moist convective boundary layers.

Tree‐grass competition for soil water in arid and semiarid savannas: The role of rainfall intermittency

Arid and semiarid savannas are characterized by the coexistence of trees and grasses in water limited conditions. As in all dry lands, also in these savannas rainfall is highly intermittent. In this work, we develop and use a simple implicit-space model to conceptually explore how precipitation intermittency influences tree-grass competition and savanna occurrence. The model explicitly includes soil moisture dynamics, and life-stage structure of the trees. Assuming that water availability affects the ability of both plant functional types to colonize new space and that grasses outcompete tree seedlings, the model is able to predict the expected sequence of grassland, savanna, and forest along a range of mean annual rainfall. In addition, rainfall intermittency allows for tree-grass coexistence at lower mean annual rainfall values than for constant precipitation. Comparison with observations indicates that the model, albeit very simple, is able to capture some of the essential dynamical processes of natural savannas. The results suggest that precipitation intermittency affects savanna occurrence and structure, indicating a new point of view for reanalyzing observational data from the literature.

A validation of heat and carbon fluxes from high‐resolution land surface and regional models

The performance of Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE), a model of surface hydrology, plant phenology, and vegetation dynamics, in reproducing field measurements of heat and carbon fluxes at various spatial and temporal scales is assessed. The model is forced by two high-resolution (30 km) regional climate models (RegCM3 and WRF) output in the Euro-Mediterranean region for the period 2002-2007. First a validation of the regional models surface climatology is conducted in comparison to gridded meteorological station data and to other state of the art regional models. Then annual cycles and interannual variability of latent and sensible heat, gross primary production, and net ecosystem exchange are compared to in situ experimental data provided from the CARBOEUROPE network. Six sites were chosen across the Euro-Mediterranean region, representing different forest environments. Results show that ORCHIDEE is able to reproduce the annual cycle of heat and carbon fluxes, with errors comparable with the measurement uncertainties. The interannual variations appear more problematic. While the variations of sensible heat are at least of the same sign as observed, latent heat is less well reproduced, while there is hardly any skill in reproducing carbon fluxes. The main difference between the two regional models used for forcing is an excessive precipitation produced on average by RegCM3. This causes errors in the latent heat and gross primary production of ORCHIDEE. Copyright

Low‐frequency variability in the Southern Ocean region in a simplified coupled model

[1] Patterns of interannual variability of the ocean-atmosphere coupled system in the Southern Hemisphere extratropics are studied with a simple dynamical model in order to determine the basic physical processes of interaction independently of tropical forcing. The model used is an atmospheric quasi-geostrophic model coupled to a ‘‘slab’’ oceanic mixed layer, which includes mean geostrophic advection by the Antarctic Circumpolar Current (ACC). The ocean-atmosphere coupling occurs through surface heat fluxes and Ekman current heat advection. In a fully coupled simulation, the atmospheric part of the model, which includes high-frequency transient eddies at midlatitudes, exhibits a strong Southern Annular Mode (SAM) as the first mode of variability at interannual timescales. The SAM-related wind anomalies induce Ekman currents in the mixed layer which produce sea surface temperature anomalies. These are then advected along by the ACC. A forced mechanism where the ocean role is reduced to advect the sea surface temperature (SST) appears sufficient to reproduce the main features of the variability. Nevertheless, a positive feedback of the ocean was also found. It operates through anomalous Ekman currents heat advection and contributes to the maintenance of the SST anomaly.

Correction to “Low‐frequency variability in the Southern Ocean region in a simplified coupled model”

[1] In the paper ‘‘Low-frequency variability in the Southern Ocean region in a simplified coupled model’’ by Guillaume Maze, Fabio D’Andrea, and Alain Colin de Verdière (Journal of Geophysical Research, 111, C05010, doi:10.1029/2005JC003181, 2006), the composite feedback on the ocean induced by the atmospheric response to a SST anomaly (SSTa) shown in Figure 12 is incorrect. Given the slab mixed layer ocean that we used in our coupled model, the SST interacts with the atmosphere through surface airsea (SHF) and Ekman (EKF) heat fluxes: