Impacts of Evaporation of Rainwater on Tropical Cyclone Structure and Intensity—A Revisit

Abstract

The impact of evaporation of rainwater on tropical cyclone (TC) intensity and structure is revisited in this study. Evaporative cooling can result in strong downdrafts and produce low-equivalent potential temperature air in the inflow boundary layer, particularly in the region outside the eyewall, significantly suppressing eyewall convection and reducing the final intensity of a TC. Different from earlier findings, results from this study show that outer rainbands still form but are short lived in the absence of evaporation. Evaporation of rainwater is shown to facilitate the formation of outer rainbands indirectly by reducing the cooling due to melting of ice particles outside the inner core, not by the cold-pool dynamics, as previously believed. Only exclusion of evaporation in the eyewall region or the rapid filamentation zone has a very weak effect on the inner-core size change of a TC, whereas how evaporation in the outer core affects the inner-core size depends on how active the inner rainbands are. More (less) active inner rainbands may lead to an increase (a decrease) in the inner-core size.

Effect of the Eyewall Cold Pool on the Inner Rainband of a Tropical Cyclone

“…To maintain conservation of mass, there is strong upward motion at the outer edge of the ECP (Figure b), which drives the formation of new convective cells. This is similar to the results of some other studies (Chen et al, ; Li et al, ; Moon & Nolan, ). Moreover, the ascent is associated with positive vertical mass transport (Figure d).…”

“…Since the formation of the cold pools has been attributed in previous studies to different processes such as subgrid‐scale vertical transports (Barnes et al, ) and the evaporation of precipitation (Frisius & Hasselbeck, ; Li et al, ; Sawada & Iwasaki, , ), the ECP needs to be examined further. The model equation for the potential temperature tendency is used here to analyze the corresponding processes as follows: …”

“…This phenomenon has been observed not only in landfalling TCs (Eastin et al, ; Meng & Wang, ) but also in TCs over the open ocean (Barnes et al, ; Tang et al, , ). Cold pools in TCs over the open ocean have also been investigated using numerical simulation in many studies (Chen et al, ; Li et al, ; Li & Wang, ; Moon & Nolan, , ; Sawada & Iwasaki, , ; Yamasaki, ).…”

A Numerical Study on the Formation and Maintenance of a Long‐Lived Rainband in Typhoon Longwang (2005)

“…In view of the cold‐pool dynamics of convective systems (e.g., rainbands), there must be a large enough surface cold pool under the rainband to generate negative vorticity to balance the positive horizontal vorticity caused by low‐level ambient VWS in the front of rainbands (e.g., Figure ; RKW). In the rainband studied here, there was no surface cold pool occurring under the rainband (or its predecessor) before 0600 UTC (Figures a, b, and a), indicating that the triggering and early maintenance of the rainband was not contributed by cold‐pool dynamics, which was consistent with the conclusion of Li et al (). Interestingly, by 0600 UTC a surface cold pool started to form in the upwind sector of the rainband (Figure c), which became better organized later (Figure f2) and was likely contributed by the growth of the cold pool (Figures c and d).…”

“…The presence of IGWs in TCs has been evidenced by the horizontal wind vector rotating clockwise with height (Kumar et al, ; Kumar & Ramkumar, ), a similar fluctuation period of the horizontal winds to the inertial oscillation (Kumar et al, ), and a quadrature relationship between the vertical velocity and the surface pressure perturbation in a rainband (Yu & Tsai, ). In addition, IGWs have been found to be associated with squall lines (e.g., Abdullah, ; Yang & Houze, ), their dynamics have been regarded as being important for the maintenance of outer rainbands (e.g., Li & Wang, ; Powell, ; Tang et al, ; Yu et al, ; Yu & Tsai, ), but not necessary for the triggering of outer rainbands (Li et al, ). In addition to the properties noted above, one key manifestation of IGW is its high frequency and fast propagation speed (e.g., propagated outward by 22 m/s in Nolan & Zhang, ).…”