Acevedo, Otávio C.

Degrazia, Gervásio A.

Goulart, Antonio G.

Rizza, Umberto

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

                          VII Workshop Brasileiro de Micrometeorologia                                  223Large-Eddy Simulation of the planetary boundary layerunder baroclinic conditions during sunset turbulenceUmberto Rizza1, Gervasio A. Degrazia2, Otávio C. Acevedo2,Antonio G. Goulart31Istituto di Scienze dell’Atmosfera e del Clima, CNR, Lecce, Italy2Universidade Federal de Santa Maria, Brazil;3Universidade Federal do pampa, Alegrete, RS, Brazil.e-mail: u.rizza@isac.cnr.it1. IntroductionThe investigation of the evening transition and the stablystratified atmosphere with LES is in general very challenging, and therehave been few studies related to this topic (Nieuwstadt and Brost, 1986;Sorbjan, 1997; Goulart et al. 2010). The principal aim of this work is toinvestigate the effects of the baroclinic setup in the LES study of a sunsetPBL. The baroclinicity is related to temperature gradients through thewell-known “thermal wind” equations and consequently it would requirethe adaptation of model equations in order to incorporate the thermaladvection, with adjustment for the effects of buoyancy (Sorbjan, 2004).2. The case studyThe LES domain is localised around the city of Candiota (-31.40, -53.70) that corresponds to the point of coordinate (x, y)=(35,28) of the outer WRF grid. The region around Candiota is the SouthAmerican lowlands or Pampa, in which the dominant vegetation typesare grassy prairie. We assume homogeneous surface fluxes within theLES domain that has a horizontal extension of 5x5x3 km. Concerningthe geostrophic forcing two different setups are employed. The first (BCL)utilises a baroclinic geostrophic wind profile and the second (BTP) themore common barotropic geostrophic profile.3. DiscussionThe temporal evolution around the transition may be analysedby considering the PBL vertically averaged TKE, calculated following224                                             Ciência e Natura Ed. Suplementar, UFSMNieuwstadt and Brost (1986) as:[ ] ò ò ò +=h L LdzdxdySFSLESTKEhLTKE0 0 02)(1                (1)where LE and SFS are respectively the resolved and subfilter componentsof the kinetic energy.The results are scaled byW*(16Z) = W*0= 1.5ms-1,h(16Z) = h0 = 1000mand ( ′w ′θ )  (16Z) = 0.1 ms-1K,so that t*=11 min, while min180=ft , where t* = h/w*  and  is the timerequired for the sensible heat flux to switch signs. This is equivalent tothe intermediate case simulated by Sorbjan (1997), who found, inagreement with Nieuwstadt and Brost (1986), that TKE decays as:[ ]20**20*)(, --µ÷÷øöççèæ= tttttFwTKE ft                                 (2)In the present study, it is found that eq. 2 is a good representationof the TKE decay for both the LES-BTP and LES-BCL cases. This is inspite of the fact that the mid afternoon TKE magnitudes are appreciablydifferent in both cases, being larger when it is baroclinic. However, oncethe transition starts, both curves collapse to a -2 power law, as describedby equation (11). Goulart et al. (2010) proposed an analytical expressionfor the TKE decay considering the wind shear production term, which isin close agreement with the LES-BCL results during the afternoon (Fi-gure 1), and also decaying as a -2 power of the dimensionless time duringthe transition. These results show that the wind shear obviously affectsthe afternoon TKE magnitude, but does not interfere with the decayingrate, which seems to be universal.The principal result obtained by the present study is that theuse of geostrophic wind shear profiles improves the description of thedecay of the TKE at sunset.                          VII Workshop Brasileiro de Micrometeorologia                                  225Figure 1. TKE decay for LES-BCL (thick dashed line) and LES-BTP (thin dashed line).The continuous line represents the analytical model of Goulart et al (2010), the -2 powerlaw is also shown for comparison.ReferencesGoulart A, et al. 2010. On the Time Evolution of the TurbulentKinetic Energy Spectrum for Decaying Turbulence in the ConvectiveBoundary Layer. Boundary-Layer Meteor. 138: 61-75.Nieuwstadt FTM, Brost RA. 1986. The decay of convective turbulence.J.Atmos. Sci. 43: 532-546.Sorbjan Z. 1997. Decay of convective turbulence revisited. Boundary-Layer Meteor. 82: 501–515.Sorbjan Z. 2004. Large-Eddy Simulations of the Baroclinic MixedLayer. Boundary-Layer Meteor. 112: 57-80.

Large-eddy Simulation of the Planetary Boundary Layer under Baroclinic Conditions During Sunset Turbulence

https://periodicos.ufsm.br/cienciaenatura/article/download/9425/5576

Large-eddy Simulation of the Planetary Boundary Layer under Baroclinic Conditions During Sunset Turbulence

Abstract

Similar works

Full text

Available Versions

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