How Do Outer Spiral Rainband Affect Tropical Cyclone Structure and Intensity? Wang, Y., 2009: How Do...

How Do Outer Spiral Rainband Affect Tropical Cyclone Structure and Intensity?

Wang, Y., 2009: How Do Outer Spiral Rainbands Affect Tropical Cyclone Structure and Intensity?. J. Atmos. Sci., 66, 1250–1273.

Reference :

CONTENTS

Add your titleINTRODUCTION

Add your titleTCM4 Numerical model description

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Add your titleRESULTSFocus on the effect of the outer spiral

rainbands on both the intensity and structure

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EXPERIMENTAL DESIGN

CONCLUSIONS

Introduction

A long-standing issue on how outer spiral rainbands affect the structure and intensity of tropical cyclones

Wang(2008b) found that interaction between the eyewall and outer spiral rainbands can be lead to a size increase of storm’s eye and eyewall and formation of annular hurricane.

The particular focus is on the hydrostatic adjustment mechanism associated with diabatic heating in outer spiral rainbands and anvil clouds outside the inner core

Inner spiral rainband

Tropical Cyclone Model (TCM-4)

• Capable of simulating the inner-core structure and intensity change of TC at nearly cloud resolving resolution(Wang 2007,2008)

domain1 domain2 domain3 domain4

horizontal resolution

67.5 km 22.5 km 7.5 km 2.5 km

mesh size 201 X 181 109 X 109 127 X 127 163 X163

Vertical level

26 vertical levels

Tropical Cyclone Model (TCM-4)

An unperturbed surface pressure of 1010 hPa.

38 km(Durran and Klemp,1983)

solve sound and gravity waves problem

Fully compressible nonhydrostatic

primitive equation model

Explicit treatment of mixing-phase cloud microphysics(Wang

2001)

No cumulus parameterization is considered in any

domain

Experimental designAfter a spinup period of 48 h ,the modle TC develops a structure similar to real TCs.

outer spiral rainbands are mainly driven by diabatic heating due to phase changes in the rainbands.

The heating due to condensa-tion, depostion, and freezing while the cooling due to sublimation of ice particle ,evaporation of rain and cloud droplets ,melting of snow and guapel.

The effect on TC intensity and structure can be evaluated by artifcially modifying the heating and cooling rate due to phase change .

Experimental design

Experimental design

CTRL

C120

HC80,C80,H80

H110

Outer Spiral RainbandsInner Spiral Rainbands

RMW

Results

9h

(K/h)

Weak tangential wind

Results

Reducing the heating rate (H80) or increasing the cooling rate (C120) considerably decreased the size of eye and eyewall relative to CTRL.

In contrast, reducing the cooling rate (C80) or increasing the heating rate (H110) considerably increased the size of eye and eyewall relative to CTRL.

The heating is critical to the maintenance of outer spiral rain-bands, whereas cooling is destructive.

>120 h

小結• The stronger storms in HC80,H80, and

C120 than in CTRL imply that outer spiral rainbands weaken a storm.

Results

Rain rate

120 h

Results

Transition period

Results

The extension of the warm core in the upper layer provides a more stable vertical structure .

moist

drydry

moist

warm

Results

The correlation between surface Rain Rate and surface Pressure Drop

Results72-144 h time average

Results

L

Heating

Reduce pressure gradient force

r

z

9 h

120 h

This is eyewall

Conclusion I

• The previous views on the effect of outer spiral rainbands on TC intensity :

1. Blocking of the boundary layer inflow

2. Subsidence forced by diabatic heating

3. cooling and drying of the boundary layer inflow due to convective downdraft

• Internal atmospheric heating (cooling ) would tend to decrease(increase) surface pressure underneath the column.

• As a result above, it would reduce the horizontal pressure gradient across RMW and increase the inner-core size of the .storm

Conclusion II

• Heating/cooling outside the inner core depends strongly on the relative humidity in the near-core environment.

• Deep moist layer in the near-core environment may favor the development of large tropical cyclone ,annular hurricane, and concentric eyewall.

• A relatively dry environment may favor small, compact tropical cyclones and is unfavorable to the formation of annular hurricane or concentric eyewall.

• May and Holland(1999),PV generation in the outer spiral rainbands could contribute to the formation of a concentric eyewall.

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INTRODUCTION

TCM4

RESULTS

CONTENTS

EXPERIMENTAL DESIGH

DISCUSSION

CONCLUSIONS

How Do Outer Spiral Rainband Affect Tropical Cyclone Structure and Intensity? Wang, Y., 2009: How Do...

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