Extreme rainfall in the tropics is frequently linked with coherent synoptic-scale potential vorticity (PV) disturbances. Here, an objective technique is used to identify coherent synoptic-scale cyclonic PV maxima with a focus on those that occur during summer over the African and Australian tropics. These two regions are chosen for comparison because of their geographical and climatological similarities. In particular, in both regions oceans lie equatorward and extensive deserts lie poleward, a juxtaposition that produces a reversal in the mean north-south temperature gradient and, through thermal wind, a low level easterly jet.In general, in the lower troposphere there are more coherent PV maxima in the tropics in the summer hemisphere than the winter hemisphere. These coherent PV maxima generally move with the background flow in the lower troposphere. The largest meridional flux of coherent PV maxima lies along eastern Australia with about half of the coherent PV maxima generated through the filamentaton and eventual isolation of midlatitude PV. In contrast, in the north African tropics, coherent PV maxima are generated mostly in the tropics and move westward through the west African monsoon region.Composites based on the extreme rainfall days for two regions are broadly similar with large, statistically significant PV maxima to the east of the maximum positive rainfall anomalies. The vertical structures of the PV fields in the two regions reveal a cyclonic PV maximum in the mid-troposphere collocated with the maximum of diabatic heating. The composite horizontal wind structures in the Australian tropics show structures similar to mesoscale convective systems (MCSs), whereas in the African tropics, they are similar to easterly waves.

Extreme rainfall in the tropics is frequently linked with coherent synoptic-scale potential vorticity (PV) disturbances. Here, an objective technique is used to identify coherent synoptic-scale cyclonic PV maxima with a focus on those that occur during summer over the African and Australian tropics. These two regions are chosen for comparison because of their geographical and climatological similarities. In particular, in both regions oceans lie equatorward and extensive deserts lie pole-ward, a juxtaposition that produces a reversal in the mean north-south temperature gradient and, through thermal wind, a low level easterly jet.In general, in the lower troposphere there are more coherent PV maxima in the tropics in the summer hemisphere than the winter hemisphere. These coherent PV maxima generally move with the background flow in the lower troposphere. The largest meridional flux of coherent PV maxima lies along eastern Australia with about half of the coherent PV maxima generated through the filamentaton and eventual isolation of midlatitude PV. In contrast, in the north African tropics, coherent PV maxima are generated mostly in the tropics and move westward through the west African monsoon region.Composites based on the extreme rainfall days for two regions are broadly similar with large, statistically significant PV maxima to the east of the maximum positive rainfall anomalies. The vertical structures of the PV fields in the two regions reveal a cyclonic PV maximum in the mid-troposphere collocated with the maximum of diabatic heating. The composite horizontal wind structures in the Australian tropics show structures similar to mesoscale convective systems (MCSs), whereas in the African tropics, they are similar to easterly waves.

Within a meteorological capacity building project in Vietnam, lightning location data and manual (human-observed) thunderstorm day observations were analyzed for the period 2015–2019. The lightning location dataset, based on the global lightning detection system Vaisala GLD360, consists of a total of 315,522,761 lightning strokes. The results indicate that, on average, 6.9 million lightning flashes per year occur in the land areas of Vietnam; this equals a lightning flash density of 20 flashes km−2 yr−1. The largest average annual flash density values occur in three regions in North, Central and South Vietnam. The majority of lightning occurs in the monsoon season (April–September), peaking in May, while in October–March, the lightning activity is very modest. During individual intense thunderstorm days, the flash density may exceed 12 flashes km−2 day−1. Thunderstorms in Central Vietnam are generally more intense, i.e., more lightning is expected on average per one thunderstorm day in Central Vietnam than in other regions. This study is a continuation of several years of meteorological capacity building in Vietnam, and the results suggest that large socio-economic benefits can be received by understanding the local thunderstorm climatology in high detail, especially in a country such as Vietnam, where lightning causes substantial socio-economic losses annually.

This study investigates the impacts of different physical parameterization schemes in the Weather Research and Forecasting model with the ARW dynamical core (WRF-ARW model) on the forecasts of heavy rainfall over the northern part of Vietnam (Bac Bo area). Various physical model configurations generated from different typical cumulus, shortwave radiation, and boundary layer and from simple to complex cloud microphysics schemes are examined and verified for the cases of extreme heavy rainfall during 2012–2016. It is found that the most skilled forecasts come from the Kain–Fritsch (KF) scheme. However, relating to the different causes of the heavy rainfall events, the forecast cycles using the Betts–Miller–Janjic (BMJ) scheme show better skills for tropical cyclones or slowly moving surface low-pressure system situations compared to KF scheme experiments. Most of the sensitivities to KF scheme experiments are related to boundary layer schemes. Both configurations using KF or BMJ schemes show that more complex cloud microphysics schemes can also improve the heavy rain forecast with the WRF-ARW model for the Bac Bo area of Vietnam.

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