The analysis of typhoon parameters by using AMSU data
Chien-Ben Chou1
Ching-Yuang Huang2
Mei-Hui Liao1
Kung-Hwa Wang1
Lee-Nan Shee1
1
Central Weather Bureau, Taipei, Taiwan
2
Institute of Atmospheric Physics, National Central University, Chung-Li, Taiwan
Abstract
The Advanced Microwave Sounding Unit (AMSU) data is useful for retrieving the parameters of
typhoons because the AMSU observations are high resolution and are not subject to the obstruction of
cloud.
The three-dimensional rotational wind component associated with a typhoon can be obtained
by solving the nonlinear balance equation using the retrieved temperature from AMSU, assuming
hydrostatic and gradient-wind balance and that the 50 hPa height at the top of a typhoon is uniform
(same as the environmental value).
equation.
The divergent wind component can be evaluated from the omega
The diabatic term in the omega equation is estimated from the rainfall rate obtained from
AMSU observations.
parameterization.
The friction-induced convergence in the boundary layer is represented by a
With these techniques, in this study we formulate a procedure to analyze the
structures of temperature and wind in typhoon based on the AMSU data.
basic structures of a typhoon are captured by the AMSU observations.
In several case studies, the
A simulation made with the
MM5 model and the retrieved wind indicates the potential of using AMSU data in numerical weather
forecast.
1. Introduction
Th e AMSU d a ta ha s b e en us ed to ana lyz e trop ical-cyclon e (T C) p arame ters du e
to the h igh reso lu tion of the d a ta and the ab ility of the AMSU observ a tion to
p enetr ate cloud. Most trop ical cyclones d ev elop ov er the trop ical o cean wh ere
tr ad itional observ a tions (sound ings) are sp ar se. Ro senkr an z e t. a l (1978) d e tected
th e temp eratur e ano malies in the cor e of typhoon u s ing micr ow ave observ a tion.
K idd er e t. a l . (1978, 1980) no ted that th e te mp eratu re anomalies are related to the
pr es sur e a t th e cen ter of a trop ic a l c yc lon e and th e as soc iate d ou ter surf a ce w ind.
Th e in tensity, or th e min imu m sur face pr essure, of a typhoon h as been estimated
using th e brigh tn ess temp er atur e and the r e tr ie ved temp erature at 250 hPa fro m th e
Microwav e
Sound ing
Un it
( MSU)
d a ta
( e.g.,
V en ld en
e t.
a l.
1991).
Th e
r e la tion sh ip b e tween the grad ien t of microw av e br igh tness temp eratur e and 500
hPa w ind has also been repo r ted b y Grod y et. a l. (1982).
K idd er e t . a l. (2000)
used A MSU d a ta to analyze th e stru ctures of a trop ical cyclone.
The tang en tial
w ind w a s calcu lated as function of rad iu s and h igh fro m th e r e tr ieved temp er ature
f ield.
Th e ax is ymme tr ic temp eratur e r e tr ieved fro m A MSU and the w ind d er iv ed
fro m a (gr ad ien t-w ind) b a lance equ a tion h ave b e en used to stud y th e r e lationsh ip
b e twe en the s tr u c tu r es o f a tro p i cal c y c lo n e and i ts env ir o n me n t a l v e r t ic a l w in d
sh ear.
Knaff et al. (2004) showed that an in cr eased w ind shear lead s to a
sh allow er balan c ed vortex.
Zhu . e t .a l (2002) bu ilt an algor ith m to r e tr ieve th e
thr ee-d ime nsion a l te mp eratu re and w ind stru ctur es of a hurr icane from th e A MSU
d a ta and incorpor ated th e r e tr ieved vor tex into a nu me rical weath er p red iction
mo d e l.
Th e y show ed pro mis ing r e su lts in th e fo re cas t tha t incorpor a ted th e
A MSU d a ta .
In th is paper w e w ill adop t and mo d if y th e pro cedu re of Zhu et al.
(2002) to analyze th e stru ctur es of typhoons and p erfor m for ecast exper ime n ts th at
in corpor ate th e r e tr iev ed typhoon- lik e vor tices in to a me so -scale nu me r ical
w eath er p red iction model.
2. Method and data
Th e procedur e of Zhu .et .a l (2002) that w e w ill adop t first retrieves th e
temp eratu re fro m th e A MSU data b y a statis tic me thod. Assu ming h yd rostatic
b a lan c e, th e h e igh t o f ea ch lev e l c an b e ob ta ine d b y a vertic a l in teg ra tion. U s ing
t h e e s t i ma t ed h e igh t a t e a ch l ev e l , t h e s tr e a mf u n c t io n a t th a t lev e l c an b e so lv ed
fro m a b a la nce equation. The rotational comp on en t of th e w ind can th en be
calcu lated fro m the streamf unction.
In th is work, we u se the fr amework of Zhu
et al. (2002) bu t adop t th e assu mp tion of K idder (2000) that the height at th e 50
hP a lev e l is un ifor m (a s su me d to b e the environ me n ta l v a lue) . W ith th e he igh t
f ield at 50 hPa g iv en, a downward in tegration w ith th e hydro s ta tic equ a tion and
th e retr iev ed te mp erature wou ld produ ce th e heigh t field s at all lev e ls.
After the
h e igh t of th e 920 hPa level is calcu la ted w ith an assu me d sur face temp eratu re,
su rface pressur e can be calcu lated through the hyd rostatic equ a tion. Th e 50 hPa
h e igh t for th e ar ea contain ing a typhoon is evaluated from th e env iron men tal d ata
(ou tside th e typhoon) in wh ich the surf ace pr essur e is prov id ed b y a nu me r ical
w ea th er f o r e c as t s ys t em. W h en the sur f a ce p r e s sur e and te mp e r a tu r e a r e spe c if i ed,
th e h ydro s tatic equ a tion c an b e in tegr a ted upw ard to d e ter min e th e h e igh t a t the 50
hPa lev e l.
In the algor ith m of Zhu e t a l. (2002), th e d iverg en t w ind is calcu lated in a
similar ma nn er as Tarbell e t a l. (1981), i. e., by so lv ing the o meg a and con tinu ity
equation s. In the o mega equ a tion, th e v er tical d istr ibu tion of laten t heat is r e la ted
to pr ecip itation r a te, wh ich can be estima ted fro m the A MSU data as d escr ib ed by
Grod y et al. (2000) . Th e sur face str ess that is need ed as th e low er bound ar y
cond ition is pr esen ted by a bu lk aerod yn a mic for mu la (Smith, 2002).
Th e AMSU d a ta used in th is work w as co llected p er iod of 2004 for th e ar ea in
th e v ic in ity of T a iwan. W e h av e s e lec ted the obs erv a tions in wh ich a trop ica l
cyclon e is fu lly covered b y on e swath. The AMSU ch annels f ro m 6 to 11 ar e use to
r e tr ieve th e te mp er a ture prof iles . Chann e ls 1 and 2 ar e u sed to es tima te th e
p arame ter s of cloud liqu id w a ter and th e pr ecip itation r a te.
3. Results
a. Analysis of the structures of typhoons
The first case to be analyzed is typhoon MEARI (2004) at 0457 UTC on
29 September when it was on track to Japan.
Figures 1 and 2 show the
retrieved temperature anomaly and wind at 850 and 250 hPa using th e
method described in Sec 2.
Notably, the circulation center of the typhoon
at 850 hPa does not coincide with that at 250 hPa (the center tilts slightly
northeastward with height), likely because cold air coming from the north
eroded the lower-level thermal structures of the typhoon. At the upper level
(250 hPa) where the influence of the low-level inflow of cold air is no t
strong, the circulation center coincides with the center of the temperature
anomaly .
The circulation at 850 hPa shown in Fig. 1 is representative of
those at other levels from 950-500 hPa (not shown). In general, the
circulation of the typhoon is not axisy mmetric. The southern branch of the
vortex is stronger than the northern one. The influence of the low-level
northerly inflow of cold air is that it destroys the axisy mmetric thermal
structure of the typhoon at the lower levels.
It might have also slowed
down he moving speed of the typhoon, given that the direction of the cold
air stream is opposite to that of the track of the typhoon.
The next case is typhoon MINDULLE (2004) at 0541 UTC on 29 June that
is developing over the tropical ocean in the vicinity of the Philippines and
slowly moving westward. Figure 3 shows that the retrieved temperature
anomaly and wind at 850 hPa for this typhoon are more axisymmetric than
the first case shown in Fig. 1.
Figure 4 shows a vertical cross section of
the tangential wind field associated with this typhoon.
The center of this
typhoon is well-defined for all levels and it does not significantly tilt with
height. The intensity (measured by the magnitude of the maximu m tangential
velocity) of the typhoon also does not change significantly through the
troposphere. These structures, derived from the AMSU data, indicate that
the tropical cyclone has developed into the mature stage at this time.
Comparing the vertical cross section in Fig. 4 to analy sis field shown in Fig.
5, we can find that the overall structures are similar between the two but
some differences remain.
The radius of the typhoon, measured by the
distance between its center and the location of the maximu m wind, is
smaller in the circulation derived from AMSU in Fig. 4.
In the analysis
field the tangential velocity decreases more rapidly with height, such that at
the upper troposphere the AMSU-derived wind is stronger than that in the
analysis.
Figure 6 shows the structures at 500 hPa of the same typhoon but for a
later time at 1812 UTC on 29 June. A closed circulation is still well-defined
for the typhoon, but its shape becomes more elliptical than circular (this is
true for most of the lower troposphere up to 400 hPa, not shown). How the
structures of the typhoon evolve during the 12-hour period from Fig. 3 to
Fig. 6 remains to be investigated.
b. A case study of forecast
The fifth-generation Pennsylvania State University-National Center for
Atmospheric Research Mesoscale Model (MM5) is used to test the results of
typhoon forecast when the 3-D temperature and circulation fields within the
typhoon are estimated from AMSU data.
Other initial fields required for
the forecast are generated from the regional model of CWB in Taiwan. The
horizontal resolution of the mesoscale model is 45Km and 15Km in two
nests grid and vertical resolution is 24 levels. We simulate typhoon
MINDULLE with initial condition at 0600 UTC on 29 June and at 1800 UTC
on 30 June, incorporating the AMSU data at 0541 UTC on 29 June and 1801
UTC on 30 June to provide the retrieved temperature and circulation
structure for the typhoon. Fig.7 shows the tracks of the typhoon from the
best track analysis and model simulations. For the simulation, that started
from the initial condition at 0600 UTC on 29 June (blue line in Fig. 7), the
initial westward and then northwestward movement of typhoon is reproduced
by the model.
After 0000 UTC on 1 July the deviation of the simulated
track from the observation (the “best track”) becomes significant. The
simulation from the initial condition at 1800 UTC on 30 June (green line in
Fig. 7) captures the landing of typhoon, although the forecast is degraded
after landing. These preliminary results are encouraging and point to the
potential of using the AMSU data for further analy ses typhoon parameters.
4. Concluding remarks
In this work the AMSU data is used to retrieve the structures of several
typhoons. A preliminary comparison of the AMSU-derived structures of a
typhoon with those in the analysis field reveals encouraging similarities.
Our results demonstrate the potential of using the AMSU data to depict the
three-dimensional temperature and velocity structures of typhoons.
The
outlook is promising since the AMSU observations provide an excellent
coverage over the tropical ocean where conventional observations are sparse.
The AMSU data may also be used to derive the values of a set of dynamical
and thermodynamical parameters for a typhoon, defined in the sense of Hart
(2003) for depicting the different stages of a tropical cyclone.
In this study we assume that the height at 50 hPa is constant and
integrate the hydrostatic equation from 50 hPa downward to obtain the
height field at each pressure level. This approach is likely simpler than
integrating the equation from surface upward, as the lower boundary
condition
is
strongly
inhomogeneous
and
not
trivial
to
determine.
Integrating from top to bottom, one needs an accurate estimate of the
atmospheric state in the upper troposphere to begin with. In addition to
improving the retrieval techniques for the upper levels using the AMSU data,
a good estimate of the atmospheric state at the upper troposphere may also
be aided by the uses of the Atmospheric Infrared Sounder (AIRS) data, since
contamination by cloud is not an issue above the cloud top.
Fig1 . The r e tr iev ed 850 h Pa temp eratur e
ano maly ( co lor imag e) and hor iz on ta l w ind
d er ived from a balan ce equation (v ecto rs) for
typhoon MAERI (2004) at 0457 U TC, 29
Sep temb er. Th e so lid lin e is the track of th e
typhoon th at mo ves f rom sou thw est to nor th east.
Th e ma gn itud e of th e ma x imum v e c tor is
0.178 E+02 m/s as ind icated at bo tto m.
Fig2 . Same as Fig. 1. bu t for th e 250 h Pa lev el.
Th e ma gn itud e of th e
ma x imum v e c tor is
0.201 E+02 m/s.
Fig3 . Same as Fig. 1 bu t for typhoon
MINDULLE (2004 ) at 0541 U T C, 29 June. Th is
typhoon is mo v ing from east to west. Shown is
its structu re at 850 hPa. Th e ma gn itud e of th e
ma x imu m vector is 0.380E+02 m/s.
Fig4 . A vertical cross section o f th e tang en tial
w ind dr iv en fro m th e A MSU d a ta for the s ame
typhoon and selected time shown in Fig.3.
Fig5 . Same as Fig . 4. bu t for analys is fro m th e
Reg ion Mod e l of Cen tr a l W e a ther Bur e au in
T a iw an.
Fig6 . Same as Fig 3. bu t for a la ter ( af ter
abou t 12 hour s) time at 1812 UTC, 29 Jun e.
Th e ma gn itud e of th e ma x imu m v ecto r is
0.237 E+02 .
Fig7 . Th e track of typhoon MINDULLE (2004). Red line represent best track.
Green line represents the track of initial condition at 1800 UTC on 30 June.
Blue line represents the track of initial condition at 0600 UTC on 29 June.
Acknowledgement
The first author thanks Professor Da-Lin Zhang of University of
Maryland for his valuable help and instruction. He also thanks Dr. N. Grody
for his help on estimating the precipitation rate.
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