Climate Prediction of precipitation for the Nordeste rainy season

of MAM 1999

contributed by Iracema F.A.Cavalcanti, Luciano P.Pezzi, Gilvan Sampaio, Marcos Barbosa Sanches

Centro de Previsao de Tempo e Estudos Climaticos (CPTEC)

Instituto de Pesquisas Espaciais (INPE) Brazil

Monthly to seasonal dynamical atmospheric prediction at CPTEC has been performed since January 1995. The model used for these predictions is the CPTEC version of the COLA AGCM which was derived from the NCEP model ( Kinter et al., 1988) and includes a sophisticated biosphere model (Xue et al., 1991). The horizontal resolution of the AGCM is T62 and there are 28 levels in the vertical (L28; Cavalcanti et al., 1995). The seasonal predictions at CPTEC, during the first three years, were results of an ensemble of four members, using four consecutive days as initial conditions. In 1998 there was an increase in the number of ensemble members, with the prediction based on 25 integrations.

The application of boundary conditions also changed, from the use of persisted SST anomalies, to predicted SST. In the tropical Pacific the SST is given by the NCEP coupled model forecast, and in the Atlantic the SST is predicted using a statistical model (Pezzi et al., 1998). Outside these regions presisted SST anomaly is used.

Predictions for the rainy season of Nordeste (Northeast of Brazil) in 1995 were discussed in Nobre et al. (1995), and those for 1996, in Nobre and Cavalcanti (1996). Results from simulations (with a T42 L18) version of the model of the Nordeste rainy season of 1993 and 1994 using observed monthly Sea Surface Temperature (SST) as boundary conditions and T42 L18 compared well with observed values of precipitation, and with NCEP reanalyses data (Cavalcanti et al., 1996). Precipitation anomalies in Nordeste (area averaged, 20S-120S; 45W-35W) from an 11 year simulation (1986 to 1996), using the T42L18 model reproduces much of the interannual variability in this region, (Cavalcanti et al.; 1998). The forecast of the March-May 1998 rainy season in Nordeste verified well.

The seasonal prediction made in the beginning of March 1999 for the rainy season of Nordeste (MAM) was based on the results from 25 integrations using initial conditions from the first 25 days of December, with predicted SST in the Tropical Pacific Ocean and Atlantic Ocean, and persisted SSTA elsewhere. The model climatology is the average (ensemble) of 23 integrations using climatological SSTA. Fig. 1 shows the precipitation prediction for March, April and May, respectively, as well as for the seasonal mean (MAM). Above normal precipitation is seen in the interior areas of Nordeste in March, and close to normal over north and east of the region. In April close to normal is predicted in the extreme northeast, above normal in the extreme northwest and below normal precipitation in central and southeast area of Nordeste. In May the area of negative anomalies extends over Nordeste region with a maximum over the east states.

In the seasonal average, negative anomalies are concentrated over the east states and parts of the semi-arid regions indicate close to normal precipitation. To the north and south of the dry area there are two regions of positive anomalies, one related to the ITCZ and other related to an anomalous SST in the subtropical South Atlantic which induces precipitation in the model. These two ascending regions can produce decending motion over the east part of Nordeste. The Tropical Atlantic was colder than normal in the predicted fields, although colder in the North than in the South Atlantic. Nevertheless, the ITCZ is situated at around the equator producing a large impact over the extreme northwest of Nordeste.

Dispersion maps of several regions of Brazil are shown in Fig.2. The Nordeste region (Fig.2g,h) shows a large dispersion in March and April, reducing the confidence in the results.

Satellite images show that the ITCZ in the first half of March was active and below the equator, bringing precipitation over north of Nordeste. During this period, frontal systems have been affected south of Nordeste. From these observations and from the model results, the prediction for the rainy season of Nordeste is for near normal, above normal in the extreme northwest and below normal in the extreme east, with low confidence.

References

Cavalcanti,I.F.A.; P.Nobre; M.L.Abreu; M.Quadro; L.P.Pezzi,1995. Vertical and horizontal resolution comparisons of CPTEC/COLA GCM. Proceedings of the twentieth annual climate diagnostics workshop., Seattle,Washington, Oct. 23-27, 1995. pp 73-76.

Cavalcanti, I.F.A., P.Nobre ; I.Trosnikov. Simulacao de verao e outono de 92/93 e 93/94 com o GCM CPTEC/COLA. IX Congresso Brasileiro de Meteorologia, Campos do Jordo, Nov.1996, pp. 807-811.

Cavalcanti, I.F.A.; L.P.Pezzi, P. Nobre; G.Sampaio., H.Camargo Jr, 1998. Climate prediction of precipitation in Brasil for the Northeast raining season (MAM) 1998. Experimental Long -Lead Forecast Bulletin, 7, No. 1, 24-27.

Kinter,J.L.;J.Shukla; L.Marx; E.K. Schneider, 1988. A simulation of winter and summer circulations with the NMC global spectral model. J.Atm.Sci.,45, 2486-2522.

N Nobre, P.; I.F.A.Cavalcanti, 1996. Previso Climatica Sazonal no CPTEC-A estacao chuvosa de 1995 e 1996 no Nordeste do Brasil. Congresso de Meteorologia Argentino (Congremet VII e Climet VII), Buenos Aires, 2-6 setembro, 1996, pp.351-352.

Nobre, P.; M.L.Abreu, I.F.A.Cavalcanti; M.Quadro; L.P.Pezzi, 1995. Climate ensemble forecasting at CPTEC. Proceedings of the twentieth annual climate diagnostics workshop., Seattle,Washington, Oct. 23-27, 1995. pp 417-420.

Pezzi,L.P.; C.A.Repelli; P.Nobre; I.F.A.Cavalcanti;G.Sampaio., 1998.Forecasts of Tropical Atlantic SST anomalies using a statistical Ocean Model at CPTEC/INPE-Brazil. Experimental Long -Lead Forecast Bulletin, 7, No. 1, 28-31.

Xue,Y.; P.Sellers.J.L. Kinter; J.Shukla, 1991. A simple biosphere model for global climate studies. J.Climate, 4, 345-364.