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The Importance of Rainfall Characteristics in Forecasting and Verification

The Importance of Rainfall Characteristics in Forecasting and Verification. Warren Tennant. Observing Rainfall. Rain Gauge Spot measurement only Satellite Uses primarily cloud top characteristics that do not relate directly to rainfall amount/intensity RADAR

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The Importance of Rainfall Characteristics in Forecasting and Verification

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  1. The Importance of Rainfall Characteristics in Forecasting and Verification Warren Tennant

  2. Observing Rainfall • Rain Gauge • Spot measurement only • Satellite • Uses primarily cloud top characteristics that do not relate directly to rainfall amount/intensity • RADAR • Calibration of Z-R relationship difficult • Limited coverage in time (historically) and space (within 150km of RADAR)

  3. Errors in observations

  4. SYNOP errors?

  5. Rainfall Characteristics - DJF: Cross-correlation matrix

  6. Variance of sorted seasonal total rainfall for northeastern SA • Steady monotonic increase across middle three quintiles • Significant discontinuities exclusively in outer quintiles • Do terciles form an arbitrary cut in data (in this region) making forecasting more difficult?

  7. Quintile Tercile

  8. Area Rainfall Integrals • Spatial variance of rainfall is such that area integrals of spot measurements have a high degree of uncertainty • With enough stations the uncertainty decreases – but in SA we only have a handful of stations per degree grid box

  9. Rain Gauges – not enough!

  10. Summary • Existing and historical rainfall observation data does not capture the high level of spatial variability in summer rainfall • Remote sensing is an obvious path to follow but still needs work (this however doesn’t solve the problem of historical data voids) • Rainfall characteristics need to be considered much more carefully when developing forecast systems

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