Abstract
The tropical, oceanic mean relationship between column relative humidity and precipitation is highly non-linear. Mean precipitation remains weak until it rapidly picks up and grows at high column humidity. To investigate the origin of this relationship, a Lagrangian cloud tracking code, RAMStracks, is developed, which can follow the evolution of clouds. RAMStracks can record the morphological properties of convective clouds, the meteorological environment of clouds, and their effects. RAMStracks is applied to a large-domain radiative convective equilibrium simulation, which produces a complex population of convective clouds. RAMStracks records the lifecycle of 501 clouds through growth, splits, mergers, and decay. The mean evolution of all these clouds is examined. It is shown that the column humidity evolves non-monotonically, but that lower-level and upper-level contributions to total moisture do evolve monotonically. The precipitation efficiency of tropical storms tends to increase with cloud age. This is confirmed using a prototype testing method. The same method reveals that different tracked clouds with similar initial conditions evolve in very different ways. This makes drawing general conclusions from individual storms difficult. Finally, the causality of the precipitation-column humidity relationship is examined. A Granger Causality test, as well as regressions, suggest that moisture and precipitation are causally linked, but that the direction of causality is ambiguous. Much of this link appears to come from the lower-level moisture’s contribution to column humidity.
Highlights
In the moist tropics, clear air is often observed to give way to shallow clouds, which grow vertically, and slowly decay through a stratiform phase [1]
This is wholly expected from both meteorological intuition and past work focusing on P() [26]
This might be because low-level moisture can most be converted to surface rainfall at short timescales. u is universally the least causal moisture quantity. u and P exhibit high granger causality values but regress poorly (Figure 6c,f) onto one another. These quantities are likely linked through mutual history rather than through a direct and immediate causal sequence. This is not surprising given the results of Igel [27], where we showed that in regions of high u and low l, the variance of precipitation at a particular
Summary
Clear air is often observed to give way to shallow clouds, which grow vertically, and slowly decay through a stratiform phase [1]. This evolution describes the lifetime of some isolated clouds, and that of the population of convection at larger time and space scales during a mesoscale convective event or during the Madden Julian Oscillation. The lifetime of a tropical convective cloud is marked by a competition between processes that moisten and processes that dry, and, more broadly, processes that favor cloud growth and cloud decay.
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