Relating the Evolution of Storm Intensity to the Low-Wave Number Depiction of Storm Structure from Satellite-observed Precipitation and Wind of the 2017 Hurricanes Irma, Jose and Maria: Are There Telltale Signs of Forthcoming Rapid Intensification?

The tropical cyclone activity in the Atlantic basin during the 2017 hurricane season broke records for numbers and impacts, invigorating the interest in accurate forecasting of cyclogenesis and hurricane rapid intensity changes.

Tropical cyclones (TCs) are the product of complex multi-scale processes and interactions. The role of the environment has long been recognized. However, research over the past decade has indicated that the hurricane inner core processes might play a crucial role in determining the storm's intensity and size. Yet, understanding of these processes is still lacking, bringing to the forefront the need to investigate the important role of the convective organization inside the hurricane vortex.

A number of recent studies have linked hurricane Rapid Intensification (RI) to intermittent occurrence of deep, strong convective bursts within the inner core (e.g. Hendricks et al., 2004; Montgomery et al., 2006; Rogers, 2010) occupying as little as 5–10% of the area of the hurricane eyewall. An alternative hypothesis is that RI follows abundant and well organized but weaker convection in the inner-core region (Gopalakrishnan et al., 2011). A continuous (at least 90%) azimuthally symmetric eye wall (i.e., a ring) of shallow warm precipitation then indicates the imminent onset of RI (Kieper and Jiang, 2012).

A very important aspect of the RI process is regarding the location of the convective activity with respect to the vortex structure as depicted by the Radius of Maximum Wind (RMW). Rogers et al., 2013 found that for intensifying hurricanes the peak in the distribution of deep convective clouds (CBs) was preferentially located inside the RMW, whereas for steady-state hurricanes the CBs were primarily located outside the RMW. Such a difference in the radial distribution of CBs was deemed important based on balance arguments (e.g. Shapiro and Willoughby,1982; Vigh and Schubert, 2009).

Motivated by this, we examine the relationship between the structures of the 2D precipitation and that of the near-surface wind, using satellite observations of the 2017 hurricanes Irma, Jose and Maria. We relate the co-evolution of these two fields, as determined from near-simultaneous satellite observations, to the hurricane intensity changes, and look for potential predictive capabilities.

Our studies utilized observations and on-line analysis tools provided by the JPL Tropical Cyclone Information System (TCIS – https://tropicalcyclone.jpl.nasa.gov) and, in particular, by the interactive North Atlantic Hurricane Watch (NAHW- https://nahw.jpl.nasa.gov).

To investigate the 2D structure of the precipitation, we used multi-channel passive microwave observations from a number of different instruments (GMI, AMSR2 and the SSMIS’s, all available at TCIS and NAHW). The rain is inferred from the Rain Index (Hristova-Veleva et al., 2013) that combines the emission and scattering signals from the multi-channel information (19 GHz –to 89 GHz) to present a cohesive depiction of the rain and the graupel above. Previous comparisons to NexRad observations show that the Rain Index looks a lot like radar reflectivity and has a resolution on the order of 15-20km.

The wind estimates came primarily from scatterometer observations by ASCAT. However, the limited swath of the scatterometer observations and the twice-daily only re-visits severely limit the ability to find near-coincident observations of wind and precipitation. To alleviate this problem, our study also investigates the use of the CYGNSS observations for the purpose of determining the RMW.

We analyzed the 2D distribution of wind and rain using Wave Number Analysis (WNA). We focus on the low-wave numbers as they best describe the macroscale organization of convection. WNA can be used to evaluate the degree of storm symmetry, related to its intensity. It can also be used to evaluate the radial distribution of precipitation / wind, as the wave number decomposition is performed in several 20km- annuli, at distances that are progressively further away from the storm center.

In our previous research (e.g. analysis of the evolution of hurricane Joaquin – Hristova-Veleva et al., 2016), we found that RI follows after observing significant precipitation inside the RMW. In contrast, occurrence of significant precipitation outside the RMW appears to precede weakening of the storm intensity.

Here we apply the same analysis approach to the satellite observations of the 2017 hurricanes Irma, Jose and Maria (e.g. Fig. 1). In addition to studying the wind/precipitation relationship, we also investigate the evolution of the precipitation itself, as revealed by the WNA of the much more frequent passive microwave observations.

The low-wave number analysis of the precipitation structure clearly depicts the radial distribution of precipitation intensity and the storm asymmetry and its radial variability. This concise and quantitative depiction of the 2D storm structure easily reveals the evolution of symmetry inside the hurricane core, the development of pronounced rain bands, the development of a secondary eyewall and the eyewall replacement cycle. All these parameters can then be related to the evolution of the storm intensity, with the goal to develop RI predictive capability based on the passive microwave observations alone.

The research described in this paper was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA).