Possible predictors for the rapid intensification and evolution of hurricanes from near-coincident satellite observations of the structure of precipitation and surface winds: Hurricane Joaquin

Hurricane Joaquin, the strongest Atlantic hurricane since Igor in 2010, developed on September 27th 2015. Of particular interest to our study is the evolution of Joaquin's intensity. Early in its lifecycle the hurricane underwent a Rapid Intensification (RI) and saw a pressure drop of 57 millibars in about 39 hours, going from a strong tropical storm to a Category 4 hurricane. 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 inner-core convective organization. Recent studies have linked 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 structure of the 2D precipitation and the near-surface wind fields. We relate the evolution of these two fields, as determined from near-simultaneous satellite observations, to the hurricane intensity changes and we find potential predictive capabilities. The study utilizes observations and on-line analysis tools provided by the JPL Tropical Cyclone Information System (TCIS), developed to support hurricane research. TCIS has two components: i) the Tropical Cyclone Data Archive - a 12-year global archive of multi–satellite hurricane observations; and ii) The North Atlantic Hurricane Watch (NAHW - http://mwsci.jpl.nasa.gov/nahw) - a data portal that monitors hurricanes in the North Atlantic and East Pacific ocean basins. This portal allows users to analyze and compare observation data and model forecasts during each hurricane season (June - November) from 2012 to the present day. Data, analysis and visualizations from the TCIS can be used to study hurricane process, validate and improve models, and assist in developing new algorithms and data assimilation techniques. To investigate the 2D structure of the precipitation, we use multi-channel passive microwave observations from a number of different instruments (TMI, AMSR-E, SSMI and SSMIS, all available at TCIS). In particular, 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 (Fig. 1, top panel). 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 come from scatterometer observations made by ASCAT and RapidScat. Both instruments post their retrievals at 12.5km. However, the actual resolution is closer to 20 km. Following Vukicevic et al. (2013), we analyze the 2D distribution of the wind and rain fields using Wave Number Analysis (WNA). A WNA tool is available on-line at the NAHW portal (Fig. 1). WNA can be used to evaluate the degree of storm symmetry that is 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. We perform WNA of nearly coincident wind and rain observations. Figure 2 presents the results from observations at three different times. The top panel show the relationship in the radial distribution of the two fields. Several important observations can be made: i) The storm is more asymmetric in rain than in wind; ii) RMW is just a bit larger than the Radius of Max Rain (RMR). There is significant precipitation inside the RMW. According to previous research, this suggests that the conditions are conducive to RI (e.g Rogers et al., 2013). It is very interesting to note that this analysis was performed just before the onset of the RI - it was done using observations during 13Z-16Z on 09/30 while RI seems to have begun after 18Z. It is also important to note that this RI was not predicted by the models with forecast cycles starting at 18Z on 09/30/2015. The second and third panels on Fig. 2 show the WNA of the rain field only, still relating it to the RMW determined from the WNA of the wind at the corresponding times. Note the very intense precipitation, inside the RMW, observed on 10/01/2015 at 00Z (second panel), suggesting again the strong potential for further intensification. Indeed, this analysis was performed during the early stages of RI. All this comes in contrast to the results from the analysis at 10/03/15 13Z (third panel). The storm is becoming very asymmetric away from the center. The RMR is now outside the RMW, suggesting that the conditions at this time are not conducive to intensification. Indeed, this analysis of is at the end of the peak storm intensity. We will present more analysis of this type, from observations spanning the lifecycle of Joaquin. We will present similar analysis of a couple of other hurricanes to further investigate the potential of using near-coincident satellite observations of rain and surface wind to develop predictive capabilities for RI and to shed new light on the critical process.