An analysis of the diurnal cycle of precipitation over Dakar using local rain-gauge data and a general circulation model

The representation of the diurnal cycle of local deep convection in two versions of the Laboratoire de Météorologie Dynamique-Zoom (LMDZ) General Circulation Model is evaluated using rainfall observations of a rain-gauge network in Senegal. An interpretation of the observed rainfall diurnal modes is attempted by partitioning rainfall as a function of rain-rate intensities and the origin, age and size of associated cloud systems. Our analysis shows a complex multipeak diurnal cycle and a large spatial variability over the rain-gauge domain of typically 100 km. Our results are consistent with the picture of a diurnal cycle of high convective rain rates associated with young and small cloud systems generated in the vicinity of the rain gauges, peaking in late afternoon and superimposed with precipitation associated with long propagative mesoscale convective systems or squall lines with no preferential time over the rain-gauge network. It is shown that these local observations of convection and rain can be used to evaluate the representation of the diurnal cycle of precipitation in a general circulation model with a typical horizontal resolution of 100 km. Two versions of the LMDZ model, including different parametrizations of boundary-layer turbulence, convection and clouds, are compared with observations. In the new parametrization, considering the role of boundary-layer thermals in deep convection preconditioning and the role of cold pools in its sustainment allows us realistically to shift the maximum of precipitation and cloud cover to late afternoon.