Abstract:
The heaviest rainfall ever recorded on Tongatapu occurred on 8 February
2008. 289.2 mm of rainfall fell in less than 12 hours at Fua'amotu. This
event was localised, highly convective, intense and poorly predicted by Global
Forecasting System (GFS) model.
Diagnosing the large-scale environment indicated that the main cloud
cluster that was responsible for the Tongan event was developed in the
southwest and propagated towards Tongatapu, and was associated with a
quasi-stationary front (with associated prefrontal trough) that maintained
the diagonal portion of the South Paci c Convergence Zone (SPCZ). Warm,
moist unstable air conveyed by a low level maximum isotach and feeding the prefrontal trough created the Mesoscale Convective Complex System (MCC)
that produced the heavy rainfall. An upper level trough with associated
subtropical jet, upper level divergence and weak mid tropospheric vorticity
advection all enhanced vertical motion. The midlevel convergence zone fed moist and dry-cool air from the westerly quarter into the rear of the MCC enforcing both the downdraft and precipitation. Near-surface advection of
warm, moist unstable air by the low level maximum isotach as well as the
advection of cold air from the south indicated that the event was synoptically
forced as indicated by the Q vectors. The Madden-Julian oscillation might
have also played role in the event.
The Weather Research Forecasting model (WRF) successfully reproduced
key features of the event. Rainfall amount, location and distribution were all
comparable with the Tropical Rainfall Measurement Mission (TRMM-3B42)
product. The Betts-Miller-Janjic scheme (BMJ) scheme at 10 km performed
better than the Kain-Fritsch scheme and the Grell-Devenyi ensemble scheme, and the Eta Ferrier scheme at 3 km resolution performed better than the
WRF 6-class scheme (WSM6), Thompson et al: scheme, Purdue Lin scheme
and the Goddard Cumulus Ensemble Model scheme (GCE). The 3 km domain
performed better than the 10 km domain indicating that the model is
sensitive to the resolution. Furthermore all the 3 km resolution simulations
reproduced the Tongan event.
