The Relative Importance of Stratiform and Convective Rainfall in Rapidly Intensifying Tropical Cyclones

Abstract

Using 16-yr Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) observations, rainfall properties in the inner-core region of tropical cyclones (TCs) and the relative importance of stratiform and convective precipitation are examined with respect to the evolution of rapid intensification (RI) events. The onset of RI follows a significant increase in the occurrence and azimuthal coverage of stratiform rainfall in all shear-relative quadrants, especially upshear left. The importance of the increased stratiform occurrence in RI storms is further confirmed by the comparison of two groups of slowly intensifying (SI) storms with one group that underwent RI and the other that did not. Statistically, SI storms that do not undergo RI during their life cycle have a much lower percent occurrence of stratiform rain within the inner core. The relatively greater areal coverage of stratiform rain in RI cases appears to be related to the moistening/humidification of the inner core, particularly in the upshear quadrants. In contrast to rainfall frequency, rainfall intensity and total volumetric rain do not increase much until several hours after RI onset, which is more likely a response or positive feedback rather than the trigger of RI. Despite a low frequency of occurrence, the overall contribution to total volumetric rain by convective precipitation is comparable to that of stratiform rain, owing to its intense rain rate.

Microphysical Characteristics of an Asymmetric Eyewall in Major Hurricane Harvey (2017)

“…While there are some previous microphysical observations in TC eyewalls, the number of samples in major hurricanes are very limited due to the rarity of the phenomena and difficulty in sampling. The low-level convergence and upward motion mostly occur in the downshear direction, where abundant hydrometeors are generated and then advected downwind to the left of the shear vector, where the reflectivity maximum is frequently observed (Black et al, 2002;DeHart et al, 2014;Foerster et al, 2014;Hence & Houze, 2011;Reasor et al, 2013;Tao et al, 2017). Of particular interest to the current study is the influence of VWS on Harvey's eyewall structure prior to landfall, which introduced a strong asymmetry in the observed polarimetric variables.…”

Rapid Intensification of Tropical Cyclones Observed by AMSU Satellites

“…Jiang and Ramirez (2013) found the following necessary conditions for RI in the inner core: total raining area >3,000 km 2 , total volumetric rain >5,000 mm · hr · km 2 , maximum near-surface radar reflectivity >40 dBZ, maximum 20-dBZ (40-dBZ) echo height >8 (4) km, minimum 85-GHz polarization-corrected brightness temperature <235 K, and minimum 10.8-mm brightness temperature <220 K. Using airborne Doppler radar data, Rogers et al (2013) found that intensifying TCs have a larger number of convective bursts located inside the radius of maximum wind. Tao et al (2017) showed that the onset of RI follows a significant increase in the occurrence and azimuthal coverage of stratiform rainfall in all shear-relative quadrants, especially upshear left, which are related to the moistening/humidification of the inner core, particularly in the upshear quadrants. Tao et al (2017) showed that the onset of RI follows a significant increase in the occurrence and azimuthal coverage of stratiform rainfall in all shear-relative quadrants, especially upshear left, which are related to the moistening/humidification of the inner core, particularly in the upshear quadrants.…”

“…Convective structure within the inner core of TCs also plays an important role in RI (Alvey et al, 2015;Harnos & Nesbitt, 2011;Fischer et al, 2018;Harnos & Nesbitt, 2016;Jiang, 2012;Jiang & Ramirez, 2013;Kieper & Jiang, 2012;Rogers et al, 2013;Rogers et al, 2015;Tao & Jiang, 2015;Tao et al, 2017;Zagrodnik & Jiang, 2014). Using long-term Tropical Rainfall Measurement Mission satellite data, Harnos and Nesbitt (2011) found that TCs undergoing RI have 85-GHz signatures depicting a moderately intense convective ring surrounding the storm center associated with low wind shear.…”

Evaluation and Predictability of Observation-based Surface Wind Asymmetric Structure in Tropical Cyclones

“…Using the two dimensional composites of V for wavenumber-0 and -1, ratios of wavenumber-1 amplitude to the total low-wavenumber amplitude (V 1 /V 0+1 ) are calculated and mean radial profiles of this ratio are computed for each shear relative quadrant (USL, DSL, DSR, USR). Following the description in Alvey et al (2015) and Tao et al (2017), a low wavenumber asymmetry index (AIDX) is then determined in the following manner:…”

“…Figure 3.4. In panels (a-d), TC intensity dependent radial profiles of wavenumber-1 asymmetry ratio (V 1 /V 0+1 ) are provided in the form of the four shear-relative quadrant absolute differences that define an asymmetry index (Alvey et al 2015;Tao et al 2017). 'DS' and 'US' refer to down-shear and up-shear and the additional 'L' or 'R' indicates right or left, respectively.…”