Deployment of new GPS dropsondes in 1997 hurricanes are stratified into two broad regions of the storm: 1) the outer wind region beyond the eyewall, consisting of stratiform rain regions between rainbands, as well as clear regions in the storm's periphery and 2) the eyewall 'inner core' region consisting of intense convection and winds well in excess of hurricane force. The outer-region group exhibit well mixed atmospheric boundary layers (ABLs) on the order of .5-1.0 km deep as defined by constant potential temperature (theta) and specific humidity (q) layers. However, a number of these layers fail to obey the traditional logarithmic decrease of wind with height. Instead, many profiles show the wind to be invariant with height in the lower portion of the ABL. This suggests either 1) a decoupling of the ABL from the ocean or 2) a positive momentum flux to the ABL from the ocean swell, which is very prominent in this region.
Most prominent among the features revealed by the eyewall-region group is the existence of double wind maxima within and just above the ABL, which is superimposed on a generally logarithmic decay of wind with height. In this region, the ABL is more dificult to define. The soundings indicate in general that the constant q layer is shallower than the constant theta layer. The lowest level wind maximum, which appears at a range of altitudes between 40 and 200m, is generally near the top of the constant q layer. The dominant low-level wind maximum, which appears at altitudes between 300 and 600m, is just above the constant theta layer. Correlations between updraft velocity maxima and minima measured by the sondes and the ABL horizontal wind maxima vary widely in phase, being positively correlated in some regions, negatively in others. Horizontal rolls tilted into the vertical and stretched by convective updrafts are hypothesized to account for these structures. Documentation of such structures in hurricane eyewalls is provided from NEXRAD and DOW Doppler radars and visual eyewall photos from aircraft. Thin constant q layers near the sea surface in the high-wind eyewall region are hyothesized to be a consequence of spray evaporation from the sea.