The Influences of Large Scale Forcing on the Diurnal Cycle of Cloud Top Heights and Rainfall Over Darwin: Observations and Modelling
Global circulation models (GCMs) have difficulty in properly simulating convective systems due, in part, to the physics being highly parameterized. Furthermore, these simulations have difficulties in generating a realistic Madden Julian Oscillation (MJO) which can greatly impact the occurrence and intensity of convection over the tropics with ramifications for global impacts including atmospheric rivers.
The C-band dual Polarization Radar (CPOL) collected 17 seasons of full volume scans documenting the macro, microphysical and kinematic properties of precipitating systems over Darwin, Australia where the MJO and the Northern Australian Monsoon occur. From this dataset, echo top heights (ETHs) and the diurnal cycle of precipitation, key diagnostics of model performance, from the CPOL dataset are examined in order to provide an observational target for GCMs.
The CPOL observations sometimes show bimodal distributions of ETH, likely attributable to the cumulus congestus clouds and mature stages of convection. Bimodality is more commonly observed when the active phase of the MJO is over Australia due to greater mid-level moisture during the active phase of the MJO. The presence of a convectively stable layer at around 5 km altitude over Darwin inhibiting convection past this level can explain the position of the modes at around 2–4 km and 7–9 km. In general, daily rainfall accumulations are much higher during active monsoon conditions. Two distinct peaks in the diurnal cycle are present in the data. One is a stronger peak over the afternoon over mainland Australia and the Tiwi Islands that is attributable to diurnal heating and Hector that is more prevalent during the suppressed phase of the MJO. The other is a weaker peak over the oceans during the early morning hours more prevalent during an active MJO and monsoon. Finally, the diurnal cycle of rainfall over Darwin shows that the presence of the monsoon creates for an earlier onset to the afternoon peak of the diurnal cycle.
Simulations were run on the Energy Exascale Earth System Model (E3SM) at a one degree resolution. Comparisons against CPOL show that this configuration of E3SM does not properly resolve this diurnal cycle of precipitation over Darwin, highlighting a key area for future parameterization and improved convective trigger development.
Contributors: Robert Jackson, Scott Collis, Yan Feng, Valentin Louf, Alain Protat, Elizabeth Thompson, Brenda Dolan, Scott Powell, Scott Giangrande, Die Wang and Robert Warren