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more than 10 years ( Haiden et al. 2018 ). A region generally characterized by low forecast skill and high uncertainty is the tropical belt. Haiden et al. (2012) note that 1-day precipitation forecasts at low latitudes have skill similar to 6-day forecasts in the extratropics. Little progress has been made also for free-tropospheric winds in the tropics ( Haiden et al. 2018 ). For variables with large forecast uncertainty, ensemble prediction is of particular importance, even for short ranges
more than 10 years ( Haiden et al. 2018 ). A region generally characterized by low forecast skill and high uncertainty is the tropical belt. Haiden et al. (2012) note that 1-day precipitation forecasts at low latitudes have skill similar to 6-day forecasts in the extratropics. Little progress has been made also for free-tropospheric winds in the tropics ( Haiden et al. 2018 ). For variables with large forecast uncertainty, ensemble prediction is of particular importance, even for short ranges
precipitation and their representation in numerical weather prediction (NWP) models over northern tropical Africa. Equatorial waves are a potential source of predictability in the tropics, as they can be considered preferred eigenmodes of the tropical atmosphere. These waves can couple with deep convection and subsequently modulate rainfall on the synoptic to subseasonal time scale throughout the tropics ( Wheeler and Kiladis 1999 ). Equatorial waves interacting with precipitation are thus called
precipitation and their representation in numerical weather prediction (NWP) models over northern tropical Africa. Equatorial waves are a potential source of predictability in the tropics, as they can be considered preferred eigenmodes of the tropical atmosphere. These waves can couple with deep convection and subsequently modulate rainfall on the synoptic to subseasonal time scale throughout the tropics ( Wheeler and Kiladis 1999 ). Equatorial waves interacting with precipitation are thus called
mm day −1 during the monsoon season. Individual types can explain up to one-third of rainfall variability on the daily to subseasonal time scales. The TD and Kelvin modes control precipitation on the daily time scales. ER waves and the MJO dominate longer time scales (see Part I and the references therein). Disturbances in the tropics are not independent from the extratropics. Using weather observations made during the Second World War, Riehl (1950) documented how tropical disturbances
mm day −1 during the monsoon season. Individual types can explain up to one-third of rainfall variability on the daily to subseasonal time scales. The TD and Kelvin modes control precipitation on the daily time scales. ER waves and the MJO dominate longer time scales (see Part I and the references therein). Disturbances in the tropics are not independent from the extratropics. Using weather observations made during the Second World War, Riehl (1950) documented how tropical disturbances
1. Introduction The bulk of precipitation in the tropics is related to moist convection, in contrast to the frontal-dominated extratropics. Because of the small-scale processes involved in the triggering and growth of convective systems, quantitative precipitation forecasts are known to have overall poorer levels of skill in tropical latitudes ( Haiden et al. 2012 ). This can be monitored in quasi–real time via the World Meteorological Organization (WMO) Lead Centre on Verification of Ensemble
1. Introduction The bulk of precipitation in the tropics is related to moist convection, in contrast to the frontal-dominated extratropics. Because of the small-scale processes involved in the triggering and growth of convective systems, quantitative precipitation forecasts are known to have overall poorer levels of skill in tropical latitudes ( Haiden et al. 2012 ). This can be monitored in quasi–real time via the World Meteorological Organization (WMO) Lead Centre on Verification of Ensemble
predictability. In general, the ensemble predictability of 24-h rainfall totals for a given station or grid point barely increases compared with climatology in many parts of the tropics because of the stochastic nature of convection. The Quang Ninh extreme event is a salient example of a case where if a strong synoptic forcing is present, the predictability of convection can suddenly appear once the synoptic forcing processes are well forecasted. 6. Summary and discussion Record-breaking precipitation
predictability. In general, the ensemble predictability of 24-h rainfall totals for a given station or grid point barely increases compared with climatology in many parts of the tropics because of the stochastic nature of convection. The Quang Ninh extreme event is a salient example of a case where if a strong synoptic forcing is present, the predictability of convection can suddenly appear once the synoptic forcing processes are well forecasted. 6. Summary and discussion Record-breaking precipitation
(color shading, in m s −1 ), and (c) filtered LWA (color, shading in m s −1 ). The color bar for both (b) and (c) is provided below (c). We also computed the analogous LWA field of Fig. 2b but from the quasigeostrophic version of (15) (i.e., Ertel PV is replaced by quasigeostrophic potential vorticity). In the extratropical upper troposphere (at 300 hPa), quasigeostrophic LWA showed a very similar picture, but there were major differences in the tropics, with a spurious longitudinally extended
(color shading, in m s −1 ), and (c) filtered LWA (color, shading in m s −1 ). The color bar for both (b) and (c) is provided below (c). We also computed the analogous LWA field of Fig. 2b but from the quasigeostrophic version of (15) (i.e., Ertel PV is replaced by quasigeostrophic potential vorticity). In the extratropical upper troposphere (at 300 hPa), quasigeostrophic LWA showed a very similar picture, but there were major differences in the tropics, with a spurious longitudinally extended
. Cook and Vizy (2015) found suspicious upward trends in the TPW over the Sahara and global tropics in the NCEP-2 and MERRA reanalyses, but not in ERA-I. The latter has been verified for the Dakar and Ouagadougou area (not shown). c. Satellite data In large parts of Africa, where the coverage of weather stations is low, SRFEs offer the only possibility to obtain area-wide observations of precipitation. We have opted to test the allegedly best rainfall product at daily time scales, the TRMM 3B42
. Cook and Vizy (2015) found suspicious upward trends in the TPW over the Sahara and global tropics in the NCEP-2 and MERRA reanalyses, but not in ERA-I. The latter has been verified for the Dakar and Ouagadougou area (not shown). c. Satellite data In large parts of Africa, where the coverage of weather stations is low, SRFEs offer the only possibility to obtain area-wide observations of precipitation. We have opted to test the allegedly best rainfall product at daily time scales, the TRMM 3B42
convection scheme . Geophys. Res. Lett. , 42 , 3056 – 3062 , https://doi.org/10.1002/2015GL063525 . 10.1002/2015GL063525 Selz , T. , and G. C. Craig , 2015b : Upscale error growth in a high-resolution simulation of a summertime weather event over Europe . Mon. Wea. Rev. , 143 , 813 – 827 , https://doi.org/10.1175/MWR-D-14-00140.1 . 10.1175/MWR-D-14-00140.1 Straus , D. M. , and D. Paolino , 2009 : Intermediate time error growth and predictability: Tropics versus mid-latitudes . Tellus
convection scheme . Geophys. Res. Lett. , 42 , 3056 – 3062 , https://doi.org/10.1002/2015GL063525 . 10.1002/2015GL063525 Selz , T. , and G. C. Craig , 2015b : Upscale error growth in a high-resolution simulation of a summertime weather event over Europe . Mon. Wea. Rev. , 143 , 813 – 827 , https://doi.org/10.1175/MWR-D-14-00140.1 . 10.1175/MWR-D-14-00140.1 Straus , D. M. , and D. Paolino , 2009 : Intermediate time error growth and predictability: Tropics versus mid-latitudes . Tellus
1. Introduction Tropical transition (TT) describes the phenomenon when a tropical cyclone (TC) emerges from an extratropical cyclone ( Davis and Bosart 2003 , 2004 ). During TT, the extratropical cyclone transforms from a cold- to a warm-core system. A cascade of events commonly precedes the TT: anticyclonic wave breaking (e.g., Thorncroft et al. 1993 ; Postel and Hitchman 1999 ) causes an upper-level precursor potential vorticity (PV) trough to penetrate into the (sub)tropics ( Galarneau et
1. Introduction Tropical transition (TT) describes the phenomenon when a tropical cyclone (TC) emerges from an extratropical cyclone ( Davis and Bosart 2003 , 2004 ). During TT, the extratropical cyclone transforms from a cold- to a warm-core system. A cascade of events commonly precedes the TT: anticyclonic wave breaking (e.g., Thorncroft et al. 1993 ; Postel and Hitchman 1999 ) causes an upper-level precursor potential vorticity (PV) trough to penetrate into the (sub)tropics ( Galarneau et
. Early during ET, ascent and associated latent heat release occurs mainly in the convection near the cyclone center (exemplified by trajectories depicted in Fig. 5a ). At the same time, the cyclonic circulation of the transitioning cyclone advects warm and moist air masses out of the tropics toward the midlatitudes. When impinging on the baroclinic zone ( Figs. 5a,b ), these air masses begin to ascend slantwise along the sloping moist isentropes and convective bursts with associated latent heat
. Early during ET, ascent and associated latent heat release occurs mainly in the convection near the cyclone center (exemplified by trajectories depicted in Fig. 5a ). At the same time, the cyclonic circulation of the transitioning cyclone advects warm and moist air masses out of the tropics toward the midlatitudes. When impinging on the baroclinic zone ( Figs. 5a,b ), these air masses begin to ascend slantwise along the sloping moist isentropes and convective bursts with associated latent heat
