The preliminary results of the experiment “Experimento de Microfisica de Nuvens-EmfiN” (Experiment of microphysics of clouds) conducted by Universidade Estadual de Ceara-UECE at Fortaleza, a semi-arid region of N-E Brazil, are presented. The mean kinematic fluxes of sensible heat and water vapor

of the surface boundary layer are estimated by the thermodynamic energy and water vapor conservation equations; and by the Monin-Obukhov similarity theory. The results of the two methods are in good agreement. It is shown that in the absence of sophisticated fast-response turbulence instrumentation and wind data the

conservations equations methods are better option for estimation of heat and water vapor fluxes. Further they are useful to study the turbulent fluxes in inhomogeneous

condition in time like early morning and late evening boundary layer transitions

Costa, A. M. N.

Da Silva, E. M.

de Fatima Correia, M.

Patel, S. R.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2005-10002-0IL NUOVO CIMENTO Vol. 27 C, N. 6 Novembre-Dicembre 2004Turbulent fluxes in atmospheric boundary layer of a semi-aridregion of N-E Brazil(∗)(∗∗)S. R. Patel(1), E. M. Da Silva(2), M. de Fatima Correia(1) and A. M. N. Costa(1) Departamento de Cieˆncias Atmosfe´ricas, Universidade Federal de CampinaGrande-UFCG, Campina Grande-PB, Brazil(2) Departamento da F´ısica, Universidade Estadual de Ceara´-UECEFortaleza CE, Brazil(ricevuto il 12 Gennaio 2005; approvato il 7 Febbraio 2005)Summary. — The preliminary results of the experiment “Experimento de Mi-crofisica de Nuvens-EmfiN” (Experiment of microphysics of clouds) conducted byUniversidade Estadual de Ceara-UECE at Fortaleza, a semi-arid region of N-EBrazil, are presented. The mean kinematic fluxes of sensible heat and water va-por of the surface boundary layer are estimated by the thermodynamic energy andwater vapor conservation equations; and by the Monin-Obukhov similarity theory.The results of the two methods are in good agreement. It is shown that in the ab-sence of sophisticated fast-response turbulence instrumentation and wind data theconservations equations methods are better option for estimation of heat and watervapor fluxes. Further they are useful to study the turbulent fluxes in inhomogeneouscondition in time like early morning and late evening boundary layer transitions.PACS 92.60.-e – Meteorology.1. – IntroductionThe importance of land surface processes on atmospheric boundary layer developmentand larger-scale weather has been widely studied for over 30 years. Reviews by Bettset al. [1] and Pielke et al. [2] detail how exchanges of energy, moisture and momentumbetween the atmospheric boundary layer and the land surface are strongly influenced byvegetation and soil moisture. Changes in the land surface and the atmospheric bound-ary layer impact larger-scale weather through entrainment with the troposphere andconvective cloud formation [3]. During the past decade land-surface models (LSM) have(∗) The authors of this paper have agreed to not receive the proofs for correction.(∗∗) Based on the work presented at the 8th Symposium on integrated observing and assimilationsystem for atmospheric ocean and land surfaces, American Meteorological Society (2004).c© Societa` Italiana di Fisica 649650 S. R. PATEL, E. M. DA SILVA, M. DE FATIMA CORREIA and A. M. N. COSTATable I. – Details of soundings used.Day Local time Code06-04-2002 10:33 0604103306-04-2002 11:50 0604115008-04-2002 11:03 0804110308-04-2002 12:49 08041249improved continuously, especially with the help of field experiments like First ISLSCP (In-ternational Satellite Land Surface Climatology Project—FIFE [4], the Boreal Ecosystem-Atmospheric Study—BOREAS [5], the Hydrologic and Atmospheric Pilot Experiment inthe Sahel—HAPEX-Sahel [6], the Northern Hemisphere Climate Processes—NOPEX [7],Observations at Several Interacting Scales—OASIS [8], etc.Because of the increasing awareness that tropical rain forest and the continentalrain forest of the Amazon basin in particular, may have an important role in globalclimatology, there have been a number of international projects on Amazon basin inBrazil as Anglo-Brazilian collaborative study of the micrometeorology and plant phys-iology of Amazon rain forest: Amazonian Region Micrometeorological Experiment—ARME [9-11], Anglo-Brazilian Amazonian Climate Observation Study—ABRACOS [12]and Large Scale Biosphere-Atmosphere Experiment in Amazon—LBA [13]. However,evaluation is still needed for semi-arid regions [14], specially the North region of Brazil.But in this region most of the works are confined to the energy balance using Bowen ra-tio method [15-18]. So, it is important to study some characteristics of the AtmosphericBoundary Layer (ABL) of a semi-arid region of N-E Brazil to better understand theparameterization of turbulent fluxes for applications among others, in regional models.2. – Experimental site and dataIn this study the data of the balloon sounding collected at Fortaleza (3.77S and38.60W) a semi-arid region of N-E Brazil, during the period 02-04-2002 to 11-04-2002as a part of the experiment EmfiN Experimento de Microfisica de Nuvens—(experimentof microphysics of clouds) conducted by Universidade Estadual da Ceara-UECE, wereused. In total 28 balloons were launched. But in this preliminary study only two daysof the following data are analyzed (see table I).3. – Methodology and discussionIt was observed that out of 28 balloons the wind data from 08 balloons, two eachday of 04-04-2002, 05-04-2002, 06-04-2002 and 09-04-2002, were lost. So to estimate thesurface layer fluxes of heat and water vapor for these days, the thermodynamics energyand humidity conservation equations methods are applied and compared by the mostcommonly used Monin-Obukhov similarity theory (MOST) [19,20].a) Thermodynamic energy and water vapor conservations equations methodsThe thermodynamic energy equation method, for estimation of sensible heat flux atthe surface and its vertical distribution (profile) in the Planetary Boundary Layer (PBL)TURBULENT FLUXES IN ATMOSPHERIC BOUNDARY LAYER ETC. 651in the absence of temperature advection, reduces to∂T¯∂t=1ρcP∂RN∂z− ∂w′θ′∂z,(1)where ρ is the density of air, cP the specific heat of air, RN is the net radiation, T¯ is themean temperature, θ′ is the temperature fluctuation, w′ is the fluctuation of the verticalvelocity, t is time and z is height.Here the time-tendency (warming or cooling rate) is retained, because it is oftenfound to be significant even when the flow field may be considered quasi-stationary. It isa manifestation of diurnal heating and cooling cycle, which is responsible for importantstability and buoyancy effects in the PBL. From eq. (1) one may see that the rate ofwarming or cooling essentially balances the convergence or divergence of radiative andsensible heat fluxes. The radiative flux divergence is usually ignored in the daytimeunstable or convective boundary layer, especially in the absence of fog and clouds withinthe PBL. It becomes more significant in the stably stratified nocturnal boundary layer.For simplification if radiative flux divergence may be ignored the integration of eq. (1)with height yields(w′θ′)0 =h∫0∂T¯∂tdz ,(2)where (w′θ′)0 is the kinematic sensible heat flux at the surface, h is the height of thePBL. In obtaining eq. (2) it is assumed that at the top of the PBL the sensible heat fluxvanishes.Similarly, from the conservation equation for water vapor, one may have the kinematicwater vapor flux as(w′q′)0 =h∫0∂q¯∂tdz ,(3)where q¯ is the mean specific humidity and q′ is the fluctuation of the specific humidity.In obtaining eq. (3) it is also assumed that at the top of the PBL (w′q′)h = 0.b) Estimation of fluxes by MOST (profile method)The most commonly used flux profile relationships are based on MOST. MOST pre-dicts that the non-dimensional gradient of velocity temperature and humidity are uni-versal functions of atmospheric stabilityφx(zLM)=κzx∗∂X∂z,(4)where ∂X/∂z and x∗ are the gradient and scaling parameter for velocity, temperatureor humidity, z is the height above the surface, κ = 0.4 is von Karman’s constant and LMis the Monin-Obukhov (MO) length given byLM = −ρcPu3∗T0κgH.652 S. R. PATEL, E. M. DA SILVA, M. DE FATIMA CORREIA and A. M. N. COSTAThe corresponding profiles may be written in the formU¯ = (u∗/κ)[ln(z/z0)−ΨM(z/LM)],(5)(Θ¯− Θ¯0) = (T∗/κ)[ln(/z0)−ΨH(z/LM)];(6)(q¯ − q¯0) = (q∗/κ)[ln(z/z0)−ΨE(z/LM)].From eqs. (5) and (6) one may haveln z −ΨM = (κ/u∗)U¯ + ln z0 ,(7)ln z −ΨH = (κ/T∗)Θ¯− (κ/T∗)Θ¯0 + ln z0 ,(8)ln z −ΨE = (κ/q∗)q¯ − (κ/q∗) + ln z0 ,where u∗ is the friction velocity or velocity scale, T∗ = −H0/(ρcPu∗) the temperaturescale, q∗ = E/(ρu∗) the specific humidity scale, κ is von Karman’s constant, H is thesensible heat flux and E is the water vapor flux, g is the acceleration due to gravity,ΨM, ΨH and ΨE are the stability functions. These flux-profile relationships have beeninvestigated during over land experiments since mid-sixties. These experiments havegenerated a number of similar semi-empirical functions, with the most commonly usedforms known as Businger-Dyer formulae [21].So the kinematic fluxes for heat flux and water vapor may be written in the formHρcP= −u∗T∗ and Eρ= −u∗q∗ .(9)Applying the least-square regression method for ln z − ΨM vs. U¯(z), ln z − ΨH vs.Θ¯(Z); and ln z − ΨE vs. q¯ at the various heights of the observations of velocity, poten-tial temperature land specific humidity from the soundings, the values of the velocity,temperature and humidity scales are estimated from the slops of the corresponding equa-tions ((7) and (8)) and consequently the kinematic fluxes of heat and water vapor areobtained from eq. (9).Table II. – Comparison of the mean kinematic heat fluxes.HeatDate Hour Eq. (9) Eq. (2)6/April 10:33–11:50 0.030509 0.72328/April 11:03–12:49 1.514075 1.7956TURBULENT FLUXES IN ATMOSPHERIC BOUNDARY LAYER ETC. 653Table III. – Comparison of the mean kinematic fluxes of water vapor.Water vaporDate Hour Eq. (9) Eq. (3)6/April 10:33–11:50 0.91229 0.76758/April 11:03–12:49 1.338492 1.1137The kinematic fluxes are calculated from the accumulation methods (eqs. (2) and (3))and from profiles methods (eqs. (9)). The height of the PBL is estimated as a heightwhere the velocity gradient is zero [22] in corresponding sounding.The calculated values by both methods for the mean kinematic heat fluxes (K m s−1)are shown in table II and the kinematic water vapor fluxes (m s−1) in table III.It can be seen from tables II and III that there are good agreements between the re-sults of the mean kinematic fluxes of heat and water vapor estimated by thermodynamicenergy and water vapor conservations equations, and the MOST. The differences of theresults from two methods are consistent with the generally estimated uncertainties in es-timates of surface fluxes using various micrometeorological methods [22]. In recent years,most of the planetary boundary layer researches are directed towards the understandingof turbulent fluxes in inhomogeneous conditions, either in time or space. The morningand evening boundary layer transitions are good examples of the inhomogeneous condi-tion in time because of the transition between the stable nocturnal boundary layer andthe convective daytime boundary layer over land. The morning transition (MT) is alsoimportant to air quality studies because of the differing concentration of pollutants thatoccur in the nocturnal boundary layer (BL) and the overlaying residual layer. Knowingthe timing of the MT is especially important during the summertime because it often308013018023028010 15 20 25Humidity (g/kg)Height (m)(a)3080130180230280298,4 298,6 298,8 299 299,2 299,4 299,6Temperature (º)Height (m)(b )308013018023028033038043014,5 15 15,5 16 16,5 17 17,5Humidity (g/kg)Height (m)(e)3080130180230280330380430300 300,5 301 301,5Temperature (º)Height (m)(f )EcompNcomp3080130180230280-0,5 0 0,5 1Velocity (m/s)Height (m)(c)30801301802302800 50 100 150Direction (º) Height (m)(d)NcompEcomp3080130180230280330380430-4 -2 0 2 4 6 8Velocity (m/s)Height (m)(g)30801301802302803303804300 100 200 300 400Direction (º) Height (m)(h)Fig. 1. – Left (a, b, c, d): Sounding on 06 April 2002, 11:50; right: (e, f, g, h): sounding on 08April 2002, 12:49.654 S. R. PATEL, E. M. DA SILVA, M. DE FATIMA CORREIA and A. M. N. COSTA308013018023028014 15 16 17 18 19Humidity (g/kg)Height (m)(a )3080130180230280301 301,5 302 302,5 303 303,5Temperature (º)Height (m)(b )NcompEcomp3080130180230280-4 -2 0 2 4 6Velocity (m/s)Height (m)(c)30801301802302800 100 200 300 400Direction (º) Height (m)(d )Fig. 2. – Sounding: 08 April 2002, 11:03.occurs during the period of increased anthropogenic and biogenic emissions. Yet, the MTis one of the more difficult features to simulate properly in numerical models becauseof insufficient vertical resolution near the ground and the BL parameterization physics.The behavior of the atmospheric boundary layer (ABL) during the period between thefully developed convection of the afternoon and the stable conditions of the nocturnalboundary layer (NBL) is poorly understood and is of interest in several areas, includingchemical and pollutant modeling. Normally the MOST assumes the time-independentcondition, so in this case the thermodynamic energy balance and water vapor conserva-tion equations may be more useful than the MOST. This is just a preliminary result ofthe experiment; further more quantitative results will be presented in a future paper.The temporal evolution of the soundings for specific humidity, potential temperature,wind speed and direction are shown in figs. 1 and 2.So, in the absence of sophisticated fast-response turbulence instrumentation and mi-crometeorological tower measurements, the thermodynamic energy and humidity conser-vation equations are quite useful in that these are based on the fundamental conservationequations and measurements of mean temperature and humidity profiles without any re-strictive assumptions. Also, in the absence of the wind data this method is useful toestimate the surface layer heat and water vapor fluxes. Further, this method is useful tostudy the understanding of the turbulent fluxes in inhomogeneous condition in time, likeearly morning and late afternoon boundary layer transitions, which are important alsoin air quality study.∗ ∗ ∗One of us (EMDS) acknowledges the support given by the Universidade Estadualda Ceara´-UECE and Centro Te´cnico da Aerona´utica—CTA (Brazil) during the datacollection for the project EmfiN (Experimento de microfisica de nuvens).REFERENCES[1] Betts A. K., Ball J. H., Beljaars A. C. M., Miller M. J. and Viterbo P., J.Geophs. Res., 101 (1996) 7209.[2] Pielke R. A., Avissar R., Raupach M. R., Dolman A. J., Zeng X. and Denning A.S., Global Change Biol., 4 (1998) 461.[3] Garratt J. R., J. Climate, 6 (1993) 419.[4] Sellers P. J., Hall G. G., Srrar G., Strebel D. E. and Murphy R. E., J. Geophys.Res., 97 (D17) (1992) 18354.TURBULENT FLUXES IN ATMOSPHERIC BOUNDARY LAYER ETC. 655[5] Black T. A., den Hartog G., Neumann H. H., Blanken P. D., Yang P. 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Turbulent fluxes in atmospheric boundary layer of a 

semi-arid region of N-E Brazil

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Turbulent fluxes in atmospheric boundary layer of a semi-arid region of N-E Brazil

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