Convective Downdrafts and Boundary Layer Recovery in Hurricane Earl (2010) Before and During Rapid Intensification

Using a combination of NOAA P-3 aircraft tail Doppler radar, NOAA and NASA dropsonde, and buoy and drifter based SST data, different types of convective downdrafts and their influence on boundary-layer thermodynamics are examined in Hurricane Earl (2010) during periods right before rapid intensification (RI; defined as at least 20 knot increase in intensity over 24 hours) and during RI. Right before RI, the atmospheric boundary layer (BL) was generally warm and moist close to the radius of maximum wind speed (RMW). Convectively driven downdrafts underneath strong updrafts inside the RMW in the upshear-right quadrant and vortex-tilt induced downdrafts outside the RMW in the upshear-left quadrant were determined to be the largest hindrances for intensification before RI. Possible mechanisms for overcoming the low entropy (theta-e) air induced by these downdrafts are BL recovery through air-sea enthalpy fluxes and turbulent mixing in the inner core.

During RI, convective downdrafts of varying strengths in the upshear-left quadrant had differing effects on the low-level entropy and surface fluxes. Interestingly, the stronger downdraft in the upshear-left quadrant corresponded with a maximum in 10-m theta-e. It was hypothesized that the large amount of evaporation that can take place underneath a convective core can lead to high amounts of specific humidity in the air parcels associated with the downdraft. On the contrary, a weaker downdraft that contained a lower evaporation rate corresponded with a minimum in 10-m theta-e. This type of downdraft has more impact on the low-level entropy than the moist downdraft, requiring more surface fluxes to recover the downdraft-driven low entropy air before being lifted in the downshear-right quadrant, where convection is usually initiated. In all cases, a simple integration of the surface fluxes showed that the ocean supplied enough energy to replenish the BL entropy. This study emphasizes different types of convective downdrafts in TCs and how they influence the latent and sensible heat fluxes, which is tied to hurricane intensity change.