Case‐studies of developing east Pacific easterly waves

This paper contains a study of developing east Pacific easterly waves observed during the summer of 1991 with aircraft in situ and radar data, and geosynchronous infrared satellite imagery. The circulation and divergence profiles, surface momentum and entropy fluxes, as well as 700 mb and 975 mb saturated and actual equivalent potential temperatures, were measured repeatedly for three disturbances that evolved into east Pacific hurricanes. As the disturbances develop, the level of maximum updraught mass flux decreases, which leads to increased convergence at low levels. This process appears to be coupled to an increase in the mid‐level relative humidity, which in turn suppresses the development of convective downdraughts. Comparison of the respective tendencies of convergence and surface friction to spin up and spin down a developing disturbance suggests that friction is unimportant in the early stages of development. However, as the surface circulation develops, the two effects become comparable in magnitude if surface friction is deposited through a fairly deep layer of the troposphere. The observations appear to rule out boundary‐layer Ekman balance in these developing systems, though more‐accurate observations of the eddy fluxes of absolute vorticity into such systems are needed to answer this question with certainty.