Role of Surface Enthalpy Fluxes in Idealized Simulations of Tropical Depression Spinup

Abstract

An idealized, three-dimensional, cloud-system-resolving model is used to investigate the influence of surface enthalpy flux variations on tropical depression (TD) spinup, an early stage of tropical cyclogenesis in which the role of surface fluxes remains incompletely understood. A range of simulations supports the hypothesis that a negative radial gradient of surface enthalpy flux outside the storm center is necessary for TD spinup but can arise from multiple mechanisms. The negative radial gradient is typically created by the wind speed dependence of surface enthalpy fluxes, consistent with some previous theories for tropical cyclone intensification. However, when surface enthalpy fluxes are prescribed to be independent of wind speed, spinup still occurs, albeit more slowly, with the negative radial gradient of surface enthalpy flux maintained by an enhanced air–sea thermodynamic disequilibrium beneath the cold core of the incipient vortex. Surface enthalpy flux variations seem more important for intensification than initial conditions. For example, a vortex forms and intensifies even from a state of rest when the center of the domain is initialized to be nearly saturated with water vapor, but this intensification is modest in amplitude and transient, lasting less than 12 h, without interactive surface enthalpy flux. Sustained spinup on time scales longer than a day does not occur when surface enthalpy fluxes are horizontally homogeneous or constant, even when fixed at the high value of 200 W m−2. In the ensemble of simulations presented here, the vortex intensification rate scales linearly with the storm-scale surface enthalpy flux anomaly relative to the undisturbed environment.

The Role of Latent Heat Flux in Tropical Cyclogenesis over the Western North Pacific: Comparison of Developing versus Non-Developing Disturbances

“…The maxima of radial mean LHF associated with the developing disturbances are located at three degrees from the center from day −1 to day +1, closer to the center than the earlier stages of their evolution (Figure 4k). The developing disturbances apparently show the larger negative radial gradients in LHF between the radius of maximum LHF and the outer region than the non-developers (Figure 4k,l), consistent with [29] which showed the importance of the negative radial gradients in enthalpy flux for TC spinup using idealized simulations. Rapidly intensifying TCs are also associated with a sharper radial gradient in LHF compared to non-rapidly intensifying TCs [33].…”

“…Emanuel [27] proposed the famous wind-induced surface heat exchange (WISHE, also known as the wind-evaporation feedback) theory for TC development. Although local evaporation only contributes a small fraction of the total condensation in a simulated TC [28], Murthy and Boos [29] showed that surface enthalpy fluxes and their negative radial gradients were necessary for TC spinup using idealized simulations. A few studies have shown the crucial role of LHF in TC intensification using observation data, e.g., [30][31][32][33][34][35][36].…”

Understanding the vertical structure of potential vorticity in tropical depressions

“…advective) PV tendencies. During real‐world TD spin‐up, the diabatic heating varies with the circulation, perhaps via wind‐induced changes in surface enthalpy fluxes (Murthy and Boos, ), but we limit our focus to understanding the diabatic and adiabatic PV tendencies caused by canonical stratiform and deep convective profiles of heating (e.g. Houze, ).…”

“…Schubert and Alworth, 1987) because of weak surface winds. Indeed, Murthy and Boos (2018) found that the frictional spin-down tendency could be neglected in an approximate scaling for the intensification rate of TDs in an idealized model. We neglect PV tendencies due to friction in the remainder of this study.…”

“…advective) PV tendencies. During real-world TD spin-up, the diabatic heating varies with the circulation, perhaps via wind-induced changes in surface enthalpy fluxes (Murthy and Boos, 2018), but we limit our focus to understanding the diabatic and adiabatic PV tendencies caused by F I G U R E 7 Vertical profiles of observed (solid) and idealized (dashed) static stability, = − ∕ . The observed vertical profile is obtained by horizontally averaging the storm-centred composite mean in a 5 • × 5 • box canonical stratiform and deep convective profiles of heating (e.g.…”