Transport, moisture, and rain in a simple monsoonlike flow

The Lagrangian transport of particles and its role in the redistribution of moisture and rainfall in a monsoonlike flow is investigated using a simple, dynamically consistent model of the heat-induced tropical circulation. The results indicate that the transport, for finite times, is filamentary in monsoonlike flows. Over the monsoon domain in the Northern Hemisphere, with increasing height in the troposphere, the moist filaments become thinner and the rainfall becomes more chaotic and scattered. Thick moist plumes found in the middle troposphere become progressively filamentary as height increases. The lower tropospheric plumes may also be filamentary if strong anticyclones are present at the surface. Larger rainfall patterns produced by ascending motion around the monsoon cyclone are well organized, but scattered rain occurring from higher levels is inherently chaotic. Scattered rainfall occurring from higher tropospheric levels in the Indian monsoon domain are likely to be less predictable. Moisture buildup in the mid- and lower-tropospheric levels or India from the north and northeastern borders during the summer monsoon can be important. Large-scale, three-dimensional laminar advection of moisture around the Asian monsoon cyclone in the lower troposphere can be a significant source of moisture for the subtropical lower troposphere. The filamentary nature of the transport has been evident from the calculations of finite-time Lyapunov exponents and correlation dimensions of dispersing clouds. The authors have noticed that in the Northern Hemisphere subtropics there is a region of strong chaotic mixing at the lower levels during the monsoon period. The relative dispersion process is found to be highly intermittent, with decaying exponential regimes interrupted by sudden shrinking of clouds, possibly in the chaotic tangle of dynamics around hyperbolic points associated with the forced part of the flow or the smaller-scale transient part of the flow. Considering the topology of the forced part and transient Rossby parts, the fact that transport is filamentary, on average, may likely be due to a predominance of chaos around saddle-foci connections existing in the forced part of the flow.