Rainfields Quantitative Precipitation Nowcasts Probability Products Bureau of
Rainfields Quantitative Precipitation Nowcasts Probability Products Bureau of Meteorology Strategic Radar Enhancement Project
Motivation Whatvalue is theby probability exceeding a warning Add providingof certainty threshold? Gain more lead time than just using QPE & QPN Frequency products Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation 1. Decompose to scales 2. Calculate skill of each scale Large Scale - high skill Smaller scales - lower skill Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation 3. Apply noise rate at each scale (high skill = small noise rate over time) (low skill = large noise rate over time) Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation 3. Apply noise rate at each scale (high skill = small noise rate over time) (low skill = large noise rate over time) Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation 3. Apply noise rate at each scale (high skill = small noise rate over time) (low skill = large noise rate over time) Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation 4. Perturb Motion Vectors 5. Advect forward in time applying noise rates at different scales Perturbed Original Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation 1. 2. 3. 4. 5. Decompose to scales Calculate skill of each scale Apply noise rate at each scale Perturb Motion Vectors Advect forward in time applying noise rates at different scales Repeat this process 40 times, ie 40 ensemble members…. WHY? -- Derive Probabilistic Nowcasts Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation Example Accumulation Ensemble Member # 1 Threshold is exceeded in green area Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation Example Accumulation Ensemble Member # 2 – slightly different position and intensity Threshold is exceeded in green area Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation Example Accumulation Ensemble Member # 3 – very different position and intensity Threshold is exceeded in green area Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation Example Accumulation Ensemble Member # 4 – slightly different position and intensity Threshold is exceeded in green area Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation Overlay all Ensemble Member Accumulations 2 Members exceed threshold & overlap in this area 1 Member in this area exceeds threshold Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation Overlay all Ensemble Members 2 members / Total Members =2/4 =0. 5 50% Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation Overlay all Ensemble Members 1 member / Total Members =1/4 =0. 25 25% Bureau of Meteorology Strategic Radar Enhancement Project
QPN – Nowcast Creation Overlay all Ensemble Members 25% 50% 25% Bureau of Meteorology Strategic Radar Enhancement Project
Use ensembles to create ARI Probabilities Previous 1 hour (QPE) 1 2 3 4 5 6 7 8. 33 34 35 36 37 38 39 40 Next 1 hour (QPN) 60 min accumulation 40 ensemble members Accumulation xx xx . . Probability of exceeding 1/X year Average Recurrence Interval = Determine for ARI = 1, 2, 5, 10, 50, 100 years xx xx xx No. of ensemble members > threshold Total no. of ensemble members
Repeat for all QPE-QPF combinations Previous 1 hour (QPE) Next 1 hour (QPN) Probabilities xx xx xx And for 30 minute accumulations Previous 1 hour (QPE) Next 1 hour (QPN) Probabilities xx xx xx
Benefits… Frequency close to a warning threshold in 2 areas…? − May show one area more likely to occur − Could provide some more lead-time than just the Frequency products
Suggested Thresholds for Warning Start Getting Nervous Warning! Frequency 10 % Frequency 20 % Rapid rate of change per scan… warn earlier Review all thresholds in time with verification statistics Is it going to continue to worse? Bureau of Meteorology Strategic Radar Enhancement Project
QPN Max Probability Benefits – Detail of QPE – Better warning area – QPN – Not Smooth – no ensemble mean – Distinguish likely cells – Timely every scan! Limitations – No Dynamics – No development – Decays Convection faster – Extremes + strong gradients Bureau of Meteorology Strategic Radar Enhancement Project
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