Global Comparisons of Regional Life Cycle Properties and Motion of Multiday Convective Systems: Tropical and Midlatitude Land and Ocean

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Abstract Tracking of convective cloud systems (cloud-top temperature <245 K) in geosynchronous satellite images at 3-h intervals is used to determine life cycle statistics of convective systems in four regimes: tropical land and ocean and midlatitude land and ocean, including seasonal comparisons. The ISCCP tracking dataset covers the period 1984–2006. Only systems with lifetimes greater than or equal to 1 day that were moving predominantly eastward or westward are considered, with splits and merges combined into larger extended convective systems. The life cycle variables are lifetime (duration), maximum area, and minimum cloud-top temperature. These are compared to each other and to the speed of longitudinal motion. Distributions and relationships between the life cycle variables are similar to previous studies based on single-day lifetimes, but the current study is globally extensive (all longitudes at lower and middle latitudes) and multidecadal, which allows extension of such results to rarer, larger, and longer-lived convective systems than previous work. Velocity distributions were monomodal with tails skewed in the direction of the zonal mean wind, being almost purely eastward in the midlatitudes but nearly symmetric in both directions with a small westward bias in the tropics. Representative life cycles for each geographical region are formed by averaging together systems with similar lifetimes. These composite life cycles show that, except for the first and last days, the daily evolution of most system variables exhibits little variation during the average multiday convective life cycle, although the cloud area goes through one cycle of expansion and contraction in a lifetime.