Carvalho, J. C.

Degrazia, G. A.

Filho, E. P. Marques

Mangia, C.

Miglietta, M.

Moscatello, A.

Puhales, F. S.

Rizza, Umberto

English

Universidade Federal de Santa Maria: Portal de Periódicos Eletrônicos da UFSM

                          V Workshop Brasileiro de Micrometeorologia                                  337LES modeling of a diurnal cycleUmberto Rizza1, C. Mangia1, M. Miglietta1, A. Moscatello1, J. C.Carvalho2,  G. A. Degrazia3,  F. S. Puhales3, E. P. Marques Filho41ISAC-CNR - Lecce, Italy2UFPel - Pelotas, RS3UFSM - Santa Maria, RS4UFRJ - Rio de Janiero, RJ1. IntroductionOver the last decades, the Large-Eddy Simulation (LES) of thePlanetary Boundary Layer (PBL) has evolved in a significant way. Being apowerful generator of high-resolution four-dimensional turbulence data,LES has gained a better understanding of some of the complex PBLphenomena. In this work we aim to investigate the daily cycle of heating/cooling of a clear-sky boundary layer with a well known PBL-LES, whichis widely used in literature (Moeng 1984, Sullivan et al, 1996).2. Large-Eddy Simulation modelThe LES model utilised is based on the incompressible Boussinesqform of the Navier-Stokes equations and considers a horizontallyhomogeneous boundary layer. The boundary layer variables (mean windcomponents, pressure, temperature etc) are spatially filtered to defineresolved component and sub-grid scale (SGS) component. The large-scalemotion is explicitly solved while the smallest scales (typically in the inertialrange) are parameterized in terms of the resolved large-scale velocity andtemperature fields.3. Description of simulations: WRF case study and the LES setupEmploying idealized initialization (Moeng and Sullivan 1994) wehave ingested into the LES the vertical profiles of wind and temperaturetaken from a simulation performed by the model WRF (http://www.wrf-338                                             CiŒncia e Natura Especial, UFSMmodel.org/index.php) in the southern Italy (fig.I(a)). The WRF simulationlasted three days, from 16 to 18 May 2005, and it was characterised bytypical spring weather with no clouds. Figure I(b) shows the daily cycle ofground-level (2 meters) temperature as simulated by WRF during thesecond day of simulation (17 may). This is a typical profile for a springperiod at mid-latitude in the northern hemisphere with a minimum at 6:00a.m., an average warming rate of 0.884 °K/h from 06:00 a.m. to 4:00 p.m,and an average cooling rate of  0.714°K/h from 5:00 p.m to 12 p.m.In Figures  I(c) and (d) the interpolated mean wind speed andpotential temperature  profiles at 6:00 a.m.  are shown. The temperatureprofile shows a capping inversion above 200 meters with a stable profile  inthe upper part. The computational setup for the LES model is summarisedin Table I, together with the micro-meteorological parameters taken fromWRF simulation. The extension of domain (3,3,1) km and the number ofgrid points in each direction (96,96,96) is a compromise between the needto resolve the physics (in particular during stable conditions) and thecomputational costs. The LES run started the 17 May 2005 at 06:00 a.m. incorrespondence of the minimum of the temperature profile and end after18 hours at 12:00 p.m. covering almost all the stability regimes. The warmingrate and the cooling rate were derived from the temperature profile anddiscussed above.4. Results and discussionsFigures II (a-d) and III (a-d) show the results obtained duringconvective ( from 6:00 a.m. until 4:00 p.m.) and stable conditions (4:00 pmuntil 12:00 pm) respectively. Figure II (upper left)  shows  the temporalevolution of the total (resolved+subgrid) turbulent kinetic energy averagedover all the domain. Figures II (lower left and right)  show the simulatedhorizontal mean wind speed ( 2 2 1/ 2( )M U V= + ) and temperature profileat different hours. It is evident the formation of the CBL due to the heating(figure II, lower right). This injection of TKE is also confirmed by theincreasing of the vertical velocity variance showed (figure II, upper right,CBL 1 through CBL 6).  During the formation of the CBL it is interestingto point out the evolution of the mean wind with  and the sub-geostrophicbalance (CBL 4). Starting at 4:00 pm we cooled the CBL of almost 6°K in8 hours. Due to inertial oscillation a low-level jet (LLJ) developed with amagnitude of the maxima increasing in time SBL 1 SBL6 and decreasingfrom SBL 6 to SBL 8.Results indicate that LES run realistically captures at leastqualitatively the development of the PBL and of the LLJ.                          V Workshop Brasileiro de Micrometeorologia                                  3395. ConclusionsIn this preliminary work we have simulated with LES the diurnal/nocturnal evolution of  the atmospheric boundary layer. The initialconditions were obtained interpolating the wind and temperature fields (inone point of the numerical domain) from a mesoscale model (WRF) runningfor weather forecast on the southern Italy.At this stage we present results on convective and stable PBLsfeatures. Further developments will concern the transition between unstableto stable, i.e. the formation of the neutral PBL and the residual layer,together with the comparison with the mesoscale model outputs runningon a finer grid.6. ReferencesMoeng, C.-H.: 1984, A Large-Eddy-Simulation Model for the Study ofPlanetary Boundary-Layer Turbulence,  J. Atmos. Sci. 41, 20522062.Moeng, C.-H. and Sullivan, P. P.: 1994, A Comparison of Shear andBuoyancy Driven Planetary Boundary Layer Flows, J. Atmos. Sci. 51,9991021.Sullivan, P.P., Mc Williams, J.C., Moeng , C.H.: 1994, A subgrid-scalemodel for large-eddy simulation of planetary boundary layer flows,Boundary-Layer Meteorol. 71, 247.Table IInternal parameters for LES taken from WRF simulation day2 at06am, ustar is the friction velocity, qstar the kinematic heat flux at ground,(zi)0 is the initial capping inversion,T0 the ground level temperature (2meters), (Lx,Ly,Lz) is the extension of the LES domain and (Nx,Ny,Nz) thenumber of grid points.ratsU ratsq z( i)0 T0 L( x L, y L, z) N( x N, y N, z)sm 1- sKm 1- m K° 01 3m62.0 5610.0 312 57.782 )1,3,3( )69,69,69(340                                             CiŒncia e Natura Especial, UFSMFig.I : (a) WRF numerical domain, x represent the central point of the LES domain; (b)diurnal evolution of ground level (2 meters) temperature from WRF in the point X ofthe domain; (c) initial potential temperature profile from WRF in the point X of thedomain and its interpolation used in LES; (d) initial wind profile from WRF( 2 2 1/ 2( )M U V= +  in the point X of the domain and its interpolation used in LESFig.II: upper left: evolution of the mean turbulent kinetic energy; upper right: evolutionof vertical variance; lower left: evolution of mean wind velocity; lower right: evolutionof potential temperature profiles

Les Modeling of a Diurnal Cycle

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Les Modeling of a Diurnal Cycle

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