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Incipient vortex size-dependent evolution of tropical disturbances Part I — Results from a numerical experiment

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Abstract

The present work is concerned with the study of intensification of tropical disturbances with a view to improve prediction and early warning. The tropical disturbances are known to come in sizes (radii) ranging from 100–400 kms. Since the vortices of different sizes give rise to different initial convergence fields and since the subsequent development of the tropical depressions is very sensitive to the initial convergence fields, we argue that the size of the incipient vortex is likely to be an important factor in determining the subsequent development of a tropical disturbance.

We have examined the above hypothesis using an axisymmetric model of tropical cyclone. The incipient vortex is introduced by prescribing an initial temperature perturbation with wind in gradient balance. The results show a fairly sharp selection of scale at about 250 km radius. This implies that out of a number of initial disturbances of varying sizes and embedded in the same large scale environment, it is the vortex with about 250 km radius size that will develop to the most severe system. The sensitivity of this selective intensification at this incipient vortex radius to initial perturbation field and the mean thermodynamic state is investigated. Finally, the importance of such a selective scale of intensification for prediction, tracking and early warning of tropical cyclones is emphasized.

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Abbreviations

B:

a suffix indicating the top of the mixed layer

C:

large scale condensation per unit time and per unit mass

CD :

drag coefficient = 2.0 × 10-3

CP :

specific heat of dry air at constant pressure

D:

suffix indicating detrainment level

Fs, Fx :

air sea exchange of sensible heat and water vapour respectively from the surface to the atmosphere

f:

Coriolis parameter

g:

acceleration due to gravity

h:

moist static energy

hM :

moist static energy of the mixed layer

hs :

moist static energy at the surface

KF, KX :

horizontal eddy coefficients of viscosity and diffusivity = 103m2/s

Kv :

vertical eddy coefficient of viscosity = 10m2/s

L:

latent heat of condensation of water vapour

l:

mixing ratio of liquid water

M:

suffix indicating mixed layer

Mc(z):

total vertical cloud mass flux at level z

P:

pressure

Po :

reference pressure = 1000 mb

q:

humidity

R:

gas constant of air

r:

suffix indicating radial direction

S:

dry static energy

TM :

temperature of the mixed layer

Ts :

surface temperature

t:

time

u:

radial wind m/s

w:

vertical velocity

X:

mixing ratio of water vapour

ZB :

height of the mixed layer top

k:

(R/CP)

Kθ :

horizontal eddy coefficient of conductivity = 103m2/s

lc :

liquid water content of the cloud

λ:

entrainment rate of the λ-type cloud = constant with height

Φ:

a suffix indicating tangential direction

ρ:

density of air

ρs :

density of air at the surface

Sc :

dry static energy in the cloud

θ:

potential temperature and also θ coordinate direction

τrs :

radial stress at the surface

τθ :

small scale vertical turbulent stress in the θ direction

τr :

small scale vertical turbulent stress in ther direction

τθs :

tangential stress at the surface

us :

radial component of the surface wind

v:

tangential wind

vs :

tangential component of the surface wind

WT(Z):

weight for initial temperature perturbation at level z

Xm :

humidity mixing ratio of the mixed layer

Xs :

surface humidity mixing ratio

Δ 21 :

δ2/δ2r+1/rδ/δr

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Authors and Affiliations

  1. CSIR Centre for Mathematical Modelling and Computer Simulation, 560 037, Bangalore, India

    P. Goswami & R. Koteswar Rao

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  1. P. Goswami
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  2. R. Koteswar Rao
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Goswami, P., Koteswar Rao, R. Incipient vortex size-dependent evolution of tropical disturbances Part I — Results from a numerical experiment. Proc. Indian Acad. Sci. (Earth Planet Sci.) 102, 439–463 (1993). https://doi.org/10.1007/BF02841732

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  • DOI: https://doi.org/10.1007/BF02841732

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