Simulations of the Transformation Stage of the Extratropical Transition of Tropical Cyclones

Abstract

The physical mechanisms associated with the transformation stage of the extratropical transition of a tropical cyclone are simulated with a mesoscale model using initial environmental conditions that approximate the mean circulations defined by Klein et al. The tropical cyclone structural changes simulated by the U.S. Navy Coupled Ocean-Atmosphere Model Prediction System mesoscale model during the three steps of transformation compare well with available observations. During step 1 of transformation when the tropical cyclone is just beginning to interact with the midlatitude baroclinic zone, the main environmental factor that affects the tropical cyclone structure appears to be the decreased sea surface temperature. The movement of the tropical cyclone over the lower sea surface temperatures results in reduced surface heat and moisture fluxes, which weakens the core convection and the intensity decreases. During step 2 of transformation, the low-level temperature gradient and vertical wind shear associated with the baroclinic zone begin to affect the tropical cyclone. Main structural changes include the development of cloud-free regions on the west side of the tropical cyclone, and an enhanced ''delta'' rain region to the northwest of the tropical cyclone center. Gradual erosion of the clouds and deep convection in the west through south sectors of the tropical cyclone appear to be from mechanically forced subsidence due to the convergence between the midlatitude flow and the tropical cyclone circulation. Whereas the warm core aloft is advected downstream, the mid-to low-level warm core is enhanced by subsidence into the tropical cyclone center, which implies that the low-level cyclonic circulation may continue to be maintained.Step 3 of transformation is the logical conclusion of structural changes that were occurring during steps 1 and 2. Even though the tropical cyclone circulation aloft has dissipated, a broad cyclonic circulation is maintained below 500 mb. Although the low-level warm core is reduced from step 2, it is still significantly stronger than at step 1, and a second warm anomaly is simulated in a region of strong subsidence upshear of the tropical cyclone remnants. Whereas some precipitation is associated with the remnants of the northern eyewall and some cloudiness to the north-northeast, the southern semicircle is almost completely clear of clouds and precipitation.

Simulations of the Extratropical Transition of Tropical Cyclones: Phasing between the Upper-Level Trough and Tropical Cyclones

“…Such a trough position is consistent with 200-mb divergence in the northwest quadrant. Significant precipitation in the northwest quadrant is consistent with both the location of an upper-level trough to the west of the tropical cyclone and with enhanced precipitation in the northern sector of the tropical cyclone due to its interaction with vertical shear associated with the increasing west- erly midlatitude flow (Ritchie and Elsberry 2001). A number of researchers (e.g., Klein et al (2000) have identified the development of asymmetric precipitation with a maximum in the northern sector as typical of extratropical transition in the western North Pacific region.…”

“…The core structure is approximately symmetric with cyclonic winds extending to 50 mb in the core and a maximum temperature anomaly of 12 K at 450 mb (see Fig. 5 in Ritchie and Elsberry 2001). The 300-mb winds turn weakly anticyclonic at 400-km radius.…”

Analysis of the Wind Field Evolution Associated with the Extratropical Transition of Bonnie (1998)

“…This wind field expansion can be characterized by two components: the outward movement of the radius of maximum winds (RMW; Jones et al 2003) and a flattening of the azimuthally averaged tangential wind profile concordant with an acceleration of the wind field outside the RMW, as will be shown later. Little research has been performed towards understanding this phenomenon, with most attention paid to its occurrence (e.g., the model analyses of Ritchie and Elsberry 2001) rather than to its underlying causes. Because of this limited understanding, operational forecasts of the wind field evolution associated with transitioning TCs in the North Atlantic basin are often formulated with a relatively large focus upon climatology, persistence, and structural features shown in the various observation platforms (J.…”

Regional climate model simulations of North Atlantic cyclones: frequency and intensity changes

“…This is very important because of the highly destructive potential of such cyclones (Hart & Evans 2001). Many studies were restricted to selected or idealized cases (Ritchie & Elsberry 2001, Klein et al 2002. They investigated the sensitivity of extratropical transition (ET), and reintensification to SST, SST gradient and vertical wind shear.…”