O jato noturno, ou jato de baixos níveis, ocorre normalmente emnoites de céu claro, i.e., sob condições estáveis. Aqui é analisada a ocorrênciado jato noturno em condições neutras por meio de um modelo teórico

Nunes, André B.

Satyamurty, Prakky

Velho, Haroldo F. de Campos

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

                          V Workshop Brasileiro de Micrometeorologia                                  355Nocturnal jet simulation under neutral conditions bytheoretical modelAndré B. Nunes1, Haroldo F. de Campos Velho2,Prakky Satyamurty11CPTEC/NPE e-mail: anunes@cptec.inpe.br2LAC/INPEResumoO jato noturno, ou jato de baixos níveis, ocorre normalmente emnoites de céu claro, i.e., sob condições estáveis. Aqui é analisada a ocorrên-cia do jato noturno em condições neutras por meio de um modelo teórico.1. The nocturnal jetThe nocturnal jet, or low level jet (LLJ), is a meteorologicalphenomenon with space scale varying from meso up to synoptic, dependingon the forcing. Usually, the formation occurs during the night and thereare many methodologies to identify it, see for example Blackadar (1957),Bonner (1968), and Brook (1985). In most cases, a standard procedure isto identify the longitudinal velocity wind peaks (supergeostrophic values)from the LLJ. During the daytime, the wind in the mixed layer tends to besubgeostrophic, due to the surface friction and vigorous mixing. At sunset,pressure gradients tend to accelerate the wind towards geostrophic value.During the process, the Coriolis force induces an inertial oscillation on thewind, producing a supergeostrophic wind at night (Blackadar, 1957). Suchprocess is more easily verified under stable conditions, when the lower halfof the Planetary Boundary Layer (PBL) close to the ground becomes stablystratified, and the upper half part of the PBL becomes more susceptible toexternal forcings (larger meteorological scale systems).2. Inertial oscillation under neutral conditionsConsidering the simplified momentum equations for the boundarylayer and the transversal wind component as zero ( 0=gV ), we have (Stull,1988):356                                             Ciência e Natura Especial, UFSMucc FfVfdtUd −=                                                 (1)( ) vcgc FfUUfdtVd −−=                                    (2)where gU  is the longitudinal geostrophic wind component, cf  is theCoriolis force, ( ) ucFfzuw =∂∂ and ( ) vcFfzvw =∂∂ , with uF  and vFgiven in units of velocity. In the stable layer case, one considers that frictionsuddenly disappears above the surface layer at sunset, remaining zerothroughout the night (Stull, 1988). For the neutral case, we consider thatfriction remains active, but varying linearly with time: it can be seen throughwu  profiles simulated by LES model (not showed here). DefiningdtdFC uFu ≡                                                            (3)where FuC is a constant, and combining (1) and (2) follows( )Fuvgc CFUUfdtUd ++−−= 222.                           (4)By means of scale analysis (based on LES data), one can –neglectthe third term RHS. The solution for the longitudinal component is:( ) ( ) Fuccgn CtfBtfAUU −++= cossin               (5)where uDFA =  and vDFB −= , D designates daytime (convectiveboundary layer, CBL) and n designates night time (neutral layer). We cannote that the LLJ equation for neutral layer is the same LLJ equation forthe stable boundary layer  (see Stull 1988, equation 12.5.3d) with additionof the FuC . Taking into account: ( ) ( )[ ]Fucc CtfBtfA −> cossin  weprobe the presence of the LLJ ( )nU  under neutral conditions. Moreover,as 0>FuC  one observes that the jet under neutral conditions tends to beless intense than the jet in stable conditions.                          V Workshop Brasileiro de Micrometeorologia                                  3573. Results and discussionIn the LLJ model (equation 5) the vertical momentum flux andthe surface wind are extracted from LES data. Here, we used the LES modeldescribed by Moeng (1984) and Sullivan et al. (1994). The LES simulationfor the neutral conditions is executed after the CBL simulation (followedby a decay and neutral phase), in order to guarantee the quasi-stationarystate.Figure 1 shows the time-integrated vertical profiles of the longi-tudinal wind component. In the Figure, the time t=0 identifies the startingpoint for the neutral phase, z is height above the ground and zi is a standardtop height (1000 m). Here, we define the occurrence of LLJ whenever thewind value is supergeostrophic, i.e., greater than 110 −= msU g . In this way,the LLJ occurrence is observed after some time period of the neutral phaseprocess, located near the top of the PBL. This means that while windincreases near the top, it decreasing slowly in the interior of the layer, i.e.,the wind profile changes from logarithmic (in CBL) to almost linear (inthe neutral layer). We also verify (LES simulation output, not shown here)that the jet tends to weaken and disappear when the convection restarts inthe morning.Figure 1. Time-integrated longitudinal wind component.358                                             Ciência e Natura Especial, UFSMFinally, despite from the simplicity of the mathematical model,the model results have a good agreement with the LES output.4. ReferencesBlackadar, A. K.: 1957, ‘Boundary layer wind maxima and theirsignificance for the growth of nocturnal inversions’. Bull. Amer. Meteor.Soc., 38, 283-290.Bonner, W. D.: 1968, ‘Climatology of the low level jet’. Mon. Wea. Rev.,96, 833-850.Moeng, C-H.: 1984, ‘A Large-Eddy Simulation model for the study ofplanetary boundary-layer turbulence’. J. Atmos. Sci., 41, 2052-2062.Stull, R. B. An Introduction to Boundary Layer Meteorology. Dordrecht:Kluwer, 1988. 666 pp.

Nocturnal Jet Simulation Under Neutral Conditions by Theoretical Model

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Nocturnal Jet Simulation Under Neutral Conditions by Theoretical Model

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