Idealized Simulations of the Tropical Diurnal Cycle under Various BSISO States

Michael Natoli

The diurnal cycle of precipitation in the tropics is examined under varying states of the Boreal Summer Intraseasonal Oscillation (BSISO) using Cloud Model 1 (CM1) in order to test the ability of the model to represent observations despite, and explore the mechanisms leading to the observed modulation. The model is run in 2-dimensions with an idealized representation of a coastal boundary, with a sea surface covering the west half of the domain and land covering the east half. Each experiment used a different input sounding from the ERA5 reanalysis. The control experiment used an input sounding derived from the boreal summer (May-October) mean sounding inside a small box covering Luzon Island in the northern Philippines. To approximate the conditions of the large scale BSISO forcing, eight successive experiments were conducted using a composite averaged sounding based on each phase of the Lee et al. (2013) BSISO index. Additional experiments are included that test the model’s performance when only the zonal wind is taken from the BSISO composite profile, with all other variables from the boreal summer mean, as well as the opposite, where all variables except the zonal wind come from the BSISO profile. Results approximate satellite observations of precipitation reasonably well, with strong diurnal cycles simulated in the inactive and transition to active phases of the BSISO (2-4), with weak diurnal cycles in the late-active state (phases 6-8). Furthermore, the simulations with only the wind from the BSISO profile are much closer to the full BSISO than those with the mean wind cases, indicating the importance of the large scale BSISO wind in driving the observed changes in the amplitude of the diurnal cycle. Results from these simulations support the idea that the strong onshore westerly flow in the low levels of the atmosphere in the convectively active phases of the BSISO help to reduce the land-sea contrast, and thus weaken the diurnal cycle.