The Unusual Intensity Pattern of OH(6,2) and O(1S) Airglow Observed Over the Andes Lidar Observatory

<p>Simultaneous observations of OH(6,2) and O(<sup>1</sup>S) nightglow at the Andes Lidar Observatory (ALO) from September 2011 to April 2018 have been analyzed to investigate an unusual intensity pattern showing an O(<sup>1</sup>S) nightglow intensity enhancement concurrent with an OH(6,2) nightglow intensity weakening. About 142 nights have been identified in the time period showing a remarkable biannual occurrence rate with maxima during the equinoxes. A semidiurnal (12-h) tide fitting applied to the 30-min bin size monthly averaged data shows that the largest amplitudes of the semidiurnal tide were observed for the months of April and August-October in the OH(6,2) data and April and September in the O(<sup>1</sup>S) data. It was also found that SABER’s atomic oxygen at the O(<sup>1</sup>S) peak height is 1.3-2.5 times higher during the nights that displayed the unusual intensity pattern. Simulations using the nonlinear, time-dependent, OH Chemistry Dynamics (OHCD) and Multiple Airglow Chemistry Dynamics (MACD) models have also been used to investigate the effect of a long-period wave on the OH(6,2) and O(<sup>1</sup>S) airglow intensities. The simulation results are in good agreement with the observations and replicate the unusual intensity pattern observed in the OH(6,2) and O(<sup>1</sup>S) airglow data.</p>

Impact of Secondary Eyewall Heating on Tropical Cyclone Intensity Change*

The primary goal of this study is to explore the factors that might influence the intensity change of tropical cyclones (TCs) associated with secondary eyewall replacement. Concentric eyewall structures in TCs with and without large intensity weakening are compared using the Tropical Rainfall Measuring Mission (TRMM) 2A12 and 2A25 data. It is found that the secondary eyewalls with a stratiform-type heating profile show a marked weakening, while those TCs with a convective-type heating weaken insignificantly or even intensify. This observed feature is supported by a set of sensitivity numerical experiments performed with the Weather Research and Forecasting model. With more active convection, the latent heat released in the outer eyewall and moat region can better sustain storm intensity. The prevailing stratiform precipitation results in low equivalent potential temperature air in the moat and reduces the entropy of the boundary layer inflow to the inner eyewall through persistent downdrafts, leading to a large intensity fluctuation. Comparison of observations and numerical model results reveals that the model tends to overproduce convective precipitation in the outer eyewall and the moat. It is possible that the model underestimates the storm intensity changes associated with eyewall replacement events.