The purpose of this paper is to study the daily cycle of rainfall over Fortaleza, Ceará State. A 30 year time series of rainfall data at five minutes intervals was used. The data came from rain gauge records obtained at the meteorological station at Universidade Federal do Ceará, located at the "Campus do Pici". To obtain the daily rainfall cycle, non-linear regression analysis was used. We searched for fitting to two types of curves: polynomial and sinusoidal. The fitting to a third degree polynomial resulted in an approximately sinusoidal shape, with a determination coefficient r2 = 0.9118. However, the fitted curve showed discontinuity between the values of the ordinates at the beginning and ending of the daily cycle. We developed a new polynomial curve removing the discontinuity. The obtained curve was markedly sinusoidal, with r2= 0.8798. Then, sinusoidal functions fitting were done. We tested the simple sine and a sine-cosine association. With the combination of sine and cosine, the best fitting found is a curve with r2= 0.9113, considered as the best representation of the diurnal cycle. The curve equation has the form: P(h)=113.7422sen(2πh)+26.5895cos(2πh)+142.5764, where h is the event time.
daily cycle of precipitation; sinusoidal model of rainfall
