q 2002 American Meteorological Society Daily Hurricane Variability Inferred from GOES Infrared Imagery

Using the National Environmental Satellite, Data, and Information Service–Cooperative Institute for Research in the Atmosphere (NESDIS–CIRA) tropical cyclone infrared (IR) imagery archive, combined with best track storm center fix information, a coherent depiction of the temporal and azimuthally averaged spatial structure of hurricane cloudiness is demonstrated. The diurnal oscillation of areal extent of the hurricane cirrus canopy, as documented in a number of previous studies, is clearly identified but often found to vanish near the convective region of the hurricane eyewall. While a significant diurnal oscillation is generally absent near the storm center, a powerful and highly significant semidiurnal oscillation is sometimes revealed in that region. This result intimates that convection near the center of tropical storms and hurricanes may not be diurnally forced, but might, at times, be semidiurnally forced. A highly significant semidiurnal oscillation is also often found in the near environment beyond the edge of the hurricane cirrus canopy. The phase of the semidiurnal oscillations in both the central convective region and the region beyond the canopy remains relatively fixed during the lifetime of each storm and is not found to vary much between individual storms. This fixed phase near the central convective region insinuates a mechanistic link between hurricane central convection and the semidiurnal atmospheric thermal tide S2. Two hypotheses are constructed. The first is offered to explain the diurnal oscillation of the canopy in the absence of a diurnal oscillation of the convective regions. The hypothesized mechanism is based on the radial variation of nighttime net radiational cooling and subsidence. The second hypothesis is offered to explain the semidiurnal oscillation near the central convective region, and is based on the possible presence of a semidiurnal oscillation of local lapse rates associated with S2. 1