A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

The modification of the ABL structure due to a Friagem event in Amazonia: a case studyGilberto FischCentro Técnico Aeroespacial (CTA/IAE-ACA)São José dos Campos, 12228-904, SP, Brazilgfisch@iae.cta.brAlistair D. CulfCenter of Ecology and Hydrology (CEH), Wallingford, OX10 8BB, UKRoberto Fernando da Fonseca Lyra (UFAL)Departamento de Meteorologia/Universidade de MeteorologiaMaceió, CEP 57000-000, AL, BrazilAbstract: A cold front invasion into Amazonia during the winter times is known regionallyas Friagem. These phenomena is an important feature of the climate, as intermittent eventscan modify the meteorological conditions, altering the way of life from Amazonidas as wellas the flora and fauna. During the field campaign of RBLE 2 (july 1993), a Friagem eventhas happened in the Ji-Parana area (Rebio Jaru tropical forest) and was fully measured.Using temperature data from ABRACOS Project (1992-1993), usually there are 6-7 eventsof Friagem during the year, with 2-3 cases in July. The invasion of cold air (classified asmoderate) had occurred in early evening on July 6, provoking a light rain (3.6 mm at 6 AMon July 7). The radiation integrated fluxes (solar and net radiation) showed a remarkablechange from a value of 17.1 and 11.8 MJ.m -2. day -1 for solar and net, respectively, on July 6to 7.1 and 5.2 MJ.m -2. day -1 during the event (July 7,1993). The air temperature also showeda dramastic change from a daily average of 25.3 °C to a value of 18.7 °C. The specifichumidity also reduces from 18 g.kg -1 to a value around 13 g.kg -1 . The windspeed hadincreased from 1.5 m.s -1 to 3.8 m.s -1 . All these changes are expected by the modificationfrom a cold and dry air mass. The following day (July 8, 1993), as the cold front was indissipation, the radiation fluxes return to their typical values. The behavior of the sensibleheat flux is interesting as it has a reduced during the friagem (as normally one expected) butit was very high: 3.5 after on July 9, 1993. This value is very high (correspond a sensibleflux of 200 W.m -2 which is not typical). This can suggest the hypothesis that the principalmechanism chosen by the plants were the sensible heat flux instead the usual latent heatflux. In terms of the atmospheric boundary layer (ABL), prior of the Friagem the height ofthe convective boundary layer was 1350 m, with an average virtual potential temperature of307.8 K. The ABL was very well mixed at late afternoon with a temperature discontinuityalmost null. On June 7, there was a cooling of the layer (temperature of 293.5 K) and only ashallow but still well mixed layer (height around 420 m). This value is 1/3 of the typicalvalues and the mechanical turbulence was very important. The temperature jump at the topof mixed layer was 9 K, indicating a strong subsidence acting at Ji-Parana area. The nextday the insolation has begun to heat the layer and the averaged virtual potential temperaturewas 300.5 K, with a discontinuity of 3.5 K and still shallow CBL. The cooling of theatmosphere was so intense in the lower 1-2 km that first the mixed layer has to be heatedand then it can grow its depth. Also the intensity of jump of temperature was so strong thatit may inhibited the entrainment, which act on the CBL development. The timescale