A Multiscale Numerical Study of Hurricane Andrew (1992). Part I: Explicit Simu

lation and Verificantion

Liu, Y., D.-L. Zhang, and M. K. Yau, 1997, Mon. Wea. Rev., 125, 3073-3093

　黃小玲　 2004/03/08

Introduction

• The hurricane is a violent atmospheric vortex characterized by strong multiscale interactions.

• Previous studies have shown that the tropical synoptic conditions and the sea surface temperature (SST) tend to control the general development of a hurricane(Gray 1979…….) .

• Its track and intensity can be affected by its internal dynamics and thermodynamics, the formation and distribution of clouds and precipitation, and the interaction between the hurricane and its larger-scale environment (Holland and Merrill 1984……….)

• Observations reveal many interesting phenomena and structures of mature hurricanes.

Overview of Hurricane Andrew

• Hurricane Andrew cost a total of $ 25 billion in damages.

• The storm originated from a tropical disturbance near the west coast of Africa on 1992/08/14, and deep convection began to organize into a narrow, spiral cloud band an 08/17.

http://www.nhc.noaa.gov/1992andrew.html

• Model integration is initialized at 08/21/1200 UTC (began to intensify) ~ 08/24/1200 UTC(about move out Florida)

• Its rapid deepening stage, the mature stage, the maximum intensity stage near Bahamas, and its landfall stage over Florida.

Model description and initial conditions• An improved version of the PSU-NCAR nonhydrostatic, mo

vable, triply nested grid, 3D mesoscale model (MM5).• 23 σ layers, a two-way interaction, movable, triply nested gr

id.• The Betts-Miller parameterization for shallow convection is

applied over mesh C to treat reasonably shallow convective clouds at the outer edge of the hurricane.

• The SST is held constant, and use the NCEP data(2o).• The NCEP analysis is always too dry, particularly in the low

er troposphere, as compared with the Omega dropwindsondes (ODWs) observation that were taken during Andrew’s development stage.

Model verification

The storm translates at a speed of 6-8 ms-1, the deviation in track less than 100 km at the time of landfall.

parameterized deep convection over the mesh B domain

922 hPa

919 hPa

C1(>33)

C2(>41)

C3(>48)

C4(>57)

C5(>68)

CTL compared with 49 ODWs (released at 400 hPa level).

08/23/0000 UTC

Miami WSR-57 radar at 08/24/0830 UTC

CTL at08/24/0800 UTC

Andrew moves over land:(1) the eye begins to fill,(2) the eyewall expands in size, and(3) the radar reflectivities or the rainfall rates weaken rapidly.

The strong-wind zone near the coastline to the north results from the intensified deep convection, which is in turn attributable to the rapid increase in surface friction and the enhanced low-level convergence of mass and moisture.

From Powell and Houston(1996)

Vertical structures08/23/2000 UTC 08/23/2000 UTC

08/24/0800 UTC

08/23/2000 UTC

cloud water(0.8-2.0 g kg-1)/ice (0.8-1.2 g kg-1) rain water (4-6 g kg-1) /snow (0.5-0.8 g kg-1)

graupel (2-4 g kg-1)

08/23/2000 UTC

08/23/2000 UTC

08/23/2000 UTC at the center (eye) 08/23/2000 UTC at the eyewall

08/23/2000 UTC

Summary and conclusions• The model captures successfully the track, propagation, and rapid dee

pening of the storm during the 3-day period, as verified against the best track analysis.

• The model simulates well the larger-scale environment in which Andrew is embedded.

• The model reproduces the visible cloud structures in terms of their size, shape, and intensity, as compared to the satellite and radar imagery.

• It is found that Hurricane Andrew is characterized by a shallow layer of intense cyclonic inflows in the PBL and intense outflows above 300 hPa, with much weaker and less organized radial flows in between.

• The streamlines in the central core tend to rotate cyclonically outward and converge in the eyewall with the cyclonic inflows from the far distance.