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<p>Some recent global analyses using convective proxies suggest that they are inconsistent indicators of extreme rainfall intensity. Since extreme values of convective proxies are frequently observed in western and central equatorial Africa, extreme rainfall estimates in this region are subject to misrepresentation, with little ability for correction due to a sparse rain-gauge network. High-resolution, convection-permitting model simulations for November, December, and January are used to examine the relationships among surface rainfall intensity, updraft velocity, and a commonly-used convection proxy, cloud-top brightness temperature (T<sub>b</sub>), in this region. For storms with rainfall in the 99.99th percentile, a majority (64–73%) are associated with similarly-extreme T<sub>b</sub> values. These low T<sub>b</sub> storms occur throughout the region and are associated with strong updrafts (&gt; 11&#xa0;m/s), tall icy cloud-tops, high CAPE values (&gt; 2000&#xa0;J/kg), and an afternoon maximum. In December and January, however, 19–25% of extreme rainfall events are not associated with extreme T<sub>b</sub> values and are characterized by weaker updrafts (6&#xa0;m/s), lower cloud tops (550–400&#xa0;hPa), lower environmental CAPE (&gt; 1400&#xa0;J/kg), and no well-defined diurnal preference. These high T<sub>b</sub> storms occur in the saturated coastal environment of western equatorial Africa and initiate either over the elevated coastal terrain through orographic uplift of the moist, low-level flow or over the Atlantic Ocean. Regardless of genesis location, extreme rainfall is produced without strong updrafts through enhancement of warm-rain processes. These results suggest caution in using brightness temperature as indicator of extreme rainfall intensity in coastal regions where warm-rain processes and orographic uplift control rainfall production.</p>

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Cloud-top brightness temperature inconsistent indicator of extreme surface rainfall in western and central equatorial Africa

  • 期刊论文

  • 发表日期：

    2025年4月27日

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  • Patrick C. Andrews,
  • Kerry H. Cook,
  • Edward K. Vizy

摘要

Some recent global analyses using convective proxies suggest that they are inconsistent indicators of extreme rainfall intensity. Since extreme values of convective proxies are frequently observed in western and central equatorial Africa, extreme rainfall estimates in this region are subject to misrepresentation, with little ability for correction due to a sparse rain-gauge network. High-resolution, convection-permitting model simulations for November, December, and January are used to examine the relationships among surface rainfall intensity, updraft velocity, and a commonly-used convection proxy, cloud-top brightness temperature (Tb), in this region. For storms with rainfall in the 99.99th percentile, a majority (64–73%) are associated with similarly-extreme Tb values. These low Tb storms occur throughout the region and are associated with strong updrafts (> 11 m/s), tall icy cloud-tops, high CAPE values (> 2000 J/kg), and an afternoon maximum. In December and January, however, 19–25% of extreme rainfall events are not associated with extreme Tb values and are characterized by weaker updrafts (6 m/s), lower cloud tops (550–400 hPa), lower environmental CAPE (> 1400 J/kg), and no well-defined diurnal preference. These high Tb storms occur in the saturated coastal environment of western equatorial Africa and initiate either over the elevated coastal terrain through orographic uplift of the moist, low-level flow or over the Atlantic Ocean. Regardless of genesis location, extreme rainfall is produced without strong updrafts through enhancement of warm-rain processes. These results suggest caution in using brightness temperature as indicator of extreme rainfall intensity in coastal regions where warm-rain processes and orographic uplift control rainfall production.