Diurnal and Semi-diurnal Variations in the Large-Scale Atmospheric
Circulation over the Tropical Oceans in a Global Storm-Resolving Model
Abstract
Studies have long reported the existence of pronounced diurnal and
semi-diurnal variations in near-surface winds and divergence over the
tropical oceans. Diurnal cycles of convective precipitation and
cloudiness in the tropics are also well recognized from in-situ and
satellite observations. However, the linkages between diurnal variations
in tropospheric circulation, cloudiness and precipitation over the
tropical oceans remain to be fully documented and understood. Recently,
global storm-resolving models, which do not require convective
parameterizations, have created an unprecedented opportunity to
investigate the full three-dimensional structure of the diurnal cycle
over the tropical oceans. In this study, we used one such model – the
Model for Prediction Across Scales (MPAS) – for two main purposes:
first, to evaluate the model’s representation of semi-diurnal and
diurnal variations in near-surface winds, precipitation, and cloudiness
over the tropical oceans; and second, to extend the analyses to provide
a full three-dimensional picture of the daily variations in tropospheric
circulation and their linkage with the hydrological cycle. A 40-day MPAS
simulation (the same as used for DYAMOND-1 global storm-resolving models
inter-comparison project) was utilized in this study to examine the
large-scale geographical patterns and vertical structures of mean daily
variations of zonal and meridional wind components, wind divergence,
vertical velocity, cloudiness, water vapor mixing ratio and
precipitation. The model shows generally good agreement with the
previously reported observational results for near-surface winds and
divergence. In particular, MPAS exhibits a pronounced large-scale
diurnal cycle in the local Hadley Circulation over the Tropical Pacific
Ocean, with lower tropospheric divergence (convergence) relative to the
daily mean, maximizing around 1000 (2200) LT. The amplitude of the
diurnal variation in near-surface wind divergence at the equator is
approximately 0.8×10-6 s-1, or
approximately 44% of the daily mean. The vertical structure of this
diurnal circulation, along with its signature in vertical velocity and
its association with water vapor, cloudiness and precipitation, will be
presented.