The estimation of convective rainfall by area integrals: 2. The Height‐Area Rainfall Threshold (HART) method

Estimates of instantaneous area average rain rate (〈R〉 in millimeters per hour) are obtained with 5–10% accuracy over a large domain simply by measuring (1) the fraction of the area, F(τ), covered by rain intensity greater than a selected threshold τ and (2) the average precipitating cloud top heights. In order to achieve this high accuracy the domain has to be large enough (about 104 km2) to include a representative sample of rain cells in different stages of their life cycle, and the measurement of the threshold rain intensity (τ) has to be unbiased. This Height-Area Rainfall Threshold (HART) method was tested with radar data of convective rains from Global Atmospheric Research Program (GARP) Tropical Atlantic Experiment (GATE) phase 3, South Africa, west Texas, and a small sample from Darwin, Australia. The results from all locations were found to be consistent and physically plausible. The cloud base temperature seems to play a major role in the determination of the constants of the HART method. The 〈R〉-F(τ) relations were found to be rather insensitive to variations in the Z-R relationships. However, any bias in the measurement of τ causes a similar bias in the derived 〈R〉. HART makes possible the accurate estimation of instantaneous rainfall from space when measuring the area and height of the convective rain systems with radar. Passive microwave observations may also be used over the ocean at wavelengths which allow setting a specified rain rate threshold, and when accompanied by IR measurements of storm top temperatures. Over tropical ocean domains of the order of 104 km2 or greater, the autocorrelation time of about 6 hours permits the area average instantaneous rate to represent the rate for a few hours.