Characteristics Of Convective-Scale Updrafts In The Outer Cores Of Sheared Tropical Cyclones

The outer-core convective-scale updraft features of sheared tropical cyclones(TCs)simulated with TCM4 are examined in this study.Under vertical wind shear of different magnitudes,updrafts in the outer core are weighted in favor of downshear formation,and the increase in the shear value leads to more short-lived updrafts.The tops of detected updrafts tend to cluster at either 4-6 km or 12-14 km height.This indicates that most outer-core updrafts stop growing at midlevels,and a certain number can develop up to the tropopause,likely featured by convective bursts(CBs).The height of stronger vertical velocity in CBs decreases as shear increases,whereas larger vertical velocity within nonconvective-burst cells holds between 3and 4 km.Downshear-left updrafts mostly move radially inward,and the radial movement speed in the downshear-left quadrant increases as shear increases.There are no relationships between several environmental parameters(such as the supercell composite parameter and the energy-helicity index)associated with updrafts and updraft mean mass fluxes.This indicates that these parameters originally developed based on the observations in midlatitudes cannot be casually applied to the prediction of outer-core updraft strength of TCs.Most of the convective-scale updraft characteristics in TCs simulated in lower-level and upper-level shears are similar to those simulated in deep-tropospheric shear,but there remain several differences.The number of updrafts possessed top heights up to 12-14km in the lower-level shear experiment is larger than those in the upper-level shear experiment.Maximum vertical velocity and vertical mass transport of updrafts are statistically significantly stronger in the lower-level shear experiment than those in the upper-level shear experiment.The height of stronger vertical velocity within CBs in the lower-level sheared TC is larger than those in the upper-level sheared TC.The environmental parameters associated with the updrafts in the lower-level shear experiment are larger than those in the upper-level experiment.The results above indicate that deeper and stronger outer-core updrafts tend to occur under lower-level shear.The mechanism of evolution of an outer-core updraft is further investigated.A nascent updraft close to the focused updraft leads to the increase of local lower-layer equivalent potential temperature(_?),which increase the convective instability is favorable for its development.In another scenario,downdrafts associated with ambient mature updrafts may decrease the moist entropy at low levels and the convective instability,and thus suppress the growth of the focused updraft.The vertical momentum budget shows that buoyancy pressure gradient accelerlation(BPGA),thermal buoyancy(TB)and water loading jointly determine the vertical acceleration in the updraft.The phase changes of hydrometeors determines the latent heat in the updraft.Increased latent heating(cooling)associated with hydrometeor phase changes results in elevated(decreased)TB.On the other hand,the increase(decrease)in hydrometeors makes water loading larger(lower).This indicates that the imbalance of BPGA,TB and water loading largely determines the strength change of the updraft.In addition,the tilt change of the updraft also affects its development.The vertical momentum budget indicates that water loading play an important role in determining the net vertical acceleration in updraft.Water loading effects possibly interfere with the use of the environmental parameters for predicting the likelihood of updraft strength in TCs.