The effect of latent cooling processes in tropical cyclone simulations

Abstract

ABSTRACT:In order to estimate the role of latent cooling processes for tropical cyclones, we have performed sensitivity simulations with both the axisymmetric cloud-resolving model HURMOD and the fully three-dimensional COSMO model of the German Weather Service. As an idealised initial state, a convectively unstable vortex with superimposed entropy anomalies has been assumed. While growth rate and maximum wind speeds become unnaturally large in simulations without latent cooling processes, the presence of these processes proves to be necessary to maintain convection outside the eyewall and turns out to be responsible for a delayed development of the tropical cyclone. The latter result is in accordance with the findings of Gray, based on observations. Furthermore, intensity fluctuations of the mature tropical cyclone only occur in HURMOD when latent cooling processes are simulated.An analysis of CAPE, CIN and DCAPE reveals that convection outside the eyewall explains these intensity fluctuations by mixing low-entropy air into the boundary layer, while without latent cooling processes significant convective inhibition due to drying by absence of evaporation and cyclone-scale subsidence suppresses vertical mixing. HURMOD does not exhibit a threshold amplitude but a sea-surface temperature threshold for tropical cyclogenesis, while the latter only occurs with latent cooling processes. However, a threshold depending on the initial vortex amplitude, together with a stronger response to latent cooling processes, is found in the COSMO model simulations. This seems to be caused by non-axisymmetric disturbances which may circumvent cyclogenesis for a small initial vortex amplitude.

Dynamical system properties of an axisymmetric convective tropical cyclone model

“…Rotunno and Emanuel (1987), Frisius and Hasselbeck (2009)]. Instead of using Deacon's formula, the surface exchange coefficients will be determined in close agreement with the results from observation-based studies by Black et al (2007) and …”

“…the ice phase is not considered in the calculation of the water budget, and the cloudÁmicro-physical parameterisation follows a Kessler type scheme. For possible effects of the inclusion of an ice phase in axisymmetric models, we may refer the reader to previous studies by Willoughby et al (1984) as well as Frisius and Hasselbeck (2009). and l v represent the horizontal and vertical mixing length scale, respectively. The eddy-diffusivity formulated in eq.…”

“…eq. (20) in Frisius and Hasselbeck (2009)]. In the study at hand, vertical turbulent surface fluxes are calculated as a function of the horizontal wind speed V 10m at z 010m instead of that at the lowest model level…”

“…Emanuel (1989) demonstrated with a simplified axisymmetric model that the import of low-entropy air into the boundary layer by shallow clouds or precipitation-induced downdrafts can provide reasons for the finite amplitude nature of tropical cyclogenesis. On the basis of more complex models, Frisius and Hasselbeck (2009) showed that precipitating downdrafts are chiefly responsible for the suppression of initial perturbations. Therefore, it is likely that such processes are an essential ingredient to reproduce dynamical system characteristics of a TC in numeric models that are closer to those of a real TC.…”

“…In order to be able to further specify and capture physical processes on smaller scales, such as convective and turbulent exchange, a rather complex cloud-resolving model is required. For this reason, investigation of the sensitivity of PI and the bifurcation structure in terms of the respective thermodynamic parameters will be also performed with the high-resolution axisymmetric convective model HURMOD (Frisius and Wacker, 2007;Frisius and Hasselbeck, 2009). In that sense, the study at hand ties in with the investigation of TC dynamics within a conceptual model by Scho¨nemann and Frisius (2012).…”