Evidence and impacts of dry air intrusion in Hurricane Ophelia: Observations from RAINEX

Studies dating back to Gilbert and LaSeur (1957) have recognized the intrusion of dry environmental air as a potentially important influence upon the strength, internal dynamics and precipitation production of mature tropical cyclones (TC's). More recently, the work of Dunion and Velden (2004) and others has implicated the Saharan Air Layer (SAL) as an important factor modulating tropical cyclone activity within the Atlantic basin, particularly with respect to controls on tropical cyclogenesis. These exceedingly dry (and in many cases aerosol-laden) airmasses emerging from the African continent are thought to influence TC intensity through a combination of advective and radiative influences on atmospheric stratification and stability over the open waters of the Atlantic.

An unprecedented dataset suitable for further exploration of these concepts was obtained in conjunction with RAINEX (the Hurricane Rainband and Intensity Change Experiment) during the formative and mature stages of Hurricane Ophelia. Unlike other major storms probed during the RAINEX field phase (viz. Hurricanes Katrina and Rita), Ophelia was slow to intensify, exhibited a more persistent asymmetric structure, and achieved only Category-1 strength (and then only intermittently). In conjunction with RAINEX, simultaneous observations by as many as three Doppler- and dropsonde-equipped turboprop aircraft (viz. two NOAA P-3's and an NRL P-3 carrying the NCAR ELDORA radar), as well as multiple sorties into the storm by an instrumented Aerosonde drone, serve to describe the storm's thermodynamic environment, internal flow structure and precipitation processes on eight out of nine consecutive days over the interval 06-14 September 2005. During this period, the developing cyclone storm meandered slowly northward immediately off the southeastern coast of the U.S., with its outer reaches being more continuously described by land-based radar and rawinsonde measurements.

Dry air has been conjectured to reduce TC intensity by promoting convectively-driven downdrafts and associated boundary-layer cooling (e.g., Dunion and Velden, 2004), but this process has not been illustrated in detail. Airborne Doppler radar-based explorations of tropical cloud clusters over the West Pacific warm-pool by Kingsmill and Houze (1999) have shown that, in addition to such convective-scale drafts, dry environmental air entering the stratiform regions of these deep convective systems at midlevels initially exhibited characteristics of mesoscale descent [as previously described by Smull and Houze (1987)] before connecting with convective-scale downdraft flows. These west-Pacific cloud systems, however, were near-equatorial and did not generally exhibit TC characteristics.

The focus of the present study is to exploit selected subsets of these extensive airborne dual-Doppler (with more limited quad-Doppler) and dropsonde datasets provided by RAINEX in conjunction with land-based synoptic measurements to map Ophelia's humidity environment and document incursions of dry air into the storm's developing circulation in much greater detail than has heretofore been possible. These soundings will in turn be related to mesoscale circulation and attendant precipitation features that determined Ophelia's growth and intensity, which was severely curtailed relative to other storms investigated during the RAINEX field phase. Notably, a large region of unusually dry air enveloped the southeastern U.S. in the wake of Hurricane Katrina (whose remnants passed over the east-central U.S. ca. 5-6 days earlier), and was thus available for entrainment into Ophelia. Particular attention will be given to the influence of this drying on the distribution of convective vs. stratiform cloud and precipitation and associated vorticity generation within Ophelia. Access to archived output from MM5 simulations conducted at the University of Miami will allow integration of these sequential snapshot-type observations within the context of Hurricane Ophelia's extended lifecycle, as well as perhaps provide some clues regarding the relative influence of other environmental conditions (such as environmental shear and relatively cool SST's) upon this storm's protracted development.

References:

Dunion, J.P., and C.S. Velden, 2004: The impact of the Saharan air layer on Atlantic tropical cyclone activity. Bull. Amer. Meteor. Soc., 85, 353-365.

Gilbert, S.G., and N.E. LaSeur, 1957: A study of the rainfall patterns and some related features in a dissipating hurricane. J. Meteor., 14, 18-27.

Kingsmill, D.E., and R.A. Houze, Jr., 1999: Kinematic characteristics of air flowing into and out of precipitating convection over the west Pacific warm pool: An airborne Doppler radar survey. Quart. J. Roy. Meteor. Soc., 125, 1165-1207.

Smull, B.F., and R.A. Houze, Jr., 1987: Rear inflow in squall lines with trailing stratiform precipitation. Mon. Wea. Rev., 115, 2869-2889.