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<br />two-dimensional cloud models predicted deep convection in this <br />environment when given an adequate initial impulse and diameter <br />at cloud base. Observations indicated that deep convection only <br />developed where mesoscale ascent of 90 to 180 cm/s was present <br />0.5 to 10 km ahead of the precipitation zone. <br /> <br />Houze (1977) found that tropical squall lines tended to be com- <br />posed of line elements of thunderstorms which form along the leading <br />edge of the line ahead of existing precipitation cells. They were <br />initiated by convective saturated downdrafts (10-40 km wide) produced <br />by precipitation drag in a manner suggested by Moncrieff and Miller's <br />(1976) model and described by Brooks (1922), Byers and Braham (1949) <br />and others. Larger (300-500 km wide) mesoscale downdrafts were found <br />in the anvil region behind the squall line. These were unsaturated <br />regions of widespread subsidence produced by evaporation of anvil <br />debris described by lipser (1969, 1977). He found sinking motion of <br />10 to 20 cm/s which produced regions that were 3-40C warmer than <br />ambient pre-squall air in the 850-900 mb layer. In advance of the <br />squall line, Zipser estimated mesoscale vertical motion of 20 to <br />50 cm/s which is consistent with that of Gamache and Houze (1982), <br />who found values of 20 to 50 ~bar/s in the active region of tropical <br />squall lines from 800 to 300 mb levels. Leary and Houze (1980) also <br />estimated-that squall line anvil updrafts reached 40 cm/s. Houze <br />(1977) also estimated that about 40% of tropical squall lines' <br />total rainfall was stratiform and the remaining 60% convective <br />showers. He hypothesized that a broad region (100-300 km wide) of <br />mid-tropospheric mesoscale convergence and lifting of -10 cm/s in <br /> <br />19 <br />