Z
= Geopotential height m
= Lapse rate 6.5 K km
-1
R
= Gas constant for dry air 287 Jkg
-1
·K g
= Acceleration of gravity 9.8 ms
-2
TS
= Surface Temperature K
o
p = Initial air pressure
Assumption: 1013.25 hPa
p
= Pressure at given point hPa b
Geopotential The use of pressure field as a vertical
coordinate gives some advantages to visualize both the quasi-horizontal pressure surfaces
and the structures of the atmosphere. Geopotential are used in synoptic scale to
analyze the movement of air masses in isobaric
coordinate. The
values of
geopotential are obtained as the potential of gravity per unit mass at a certain geometric
height above mean sea level.
34 ..
.......... gdz
d
= Geopotential m
2
s
-2
g
= Acceleration of gravity 9.8 ms
-2
z
= Geometric height m
3.3.4 Horizontal Wind Profile
The approaching methods to analyze and approximate wind movement from satellite
imagery are commonly based on the momentum equations. The orientation of a
pressure gradient is used to simulate wind movement and to evaluate wind field on each
pressure level. In isobaric coordinate, the pressure gradient is replaced by the gradient
of geopotential in assumption that neither frictions nor turbulence occurs.
35 ..
.......... ˆ
h v
k f
p Dt
h v
D
Dt h
v D
= Horizontal acceleration
p
= Gradient of geopotential
k f ˆ
= The Coriolis parameter
h V
= Horizontal wind components Due to the fact that the Coriolis force is
very weak near the equator, the component of Coriolis can be neglected so that the east-west
wind component u and north-south wind component v is expressed as:
The advective components were negligible since the satellite data used is in the form of
snapshot dt=1 second. As the result obtained is in the acceleration form ms
-2
and dt=1 second; the acceleration can be thought as the
speed of wind exactly at the time when the image was acquired. The magnitude of
horizontal wind can be found by using simple vector expression Stull 1995.
36 ..
..........
2 2
v u
V
. As for the wind trajectory:
=
37 ..........
arctan 360
90 o
u v
C o
o
o o
180
if
o U
180
C
= Angular rotation in full circle
2 360
o C
IV RESULTS AND DISCUSSION
4.1
Study Area
The area analyzed by MODIS sensor on board the Terra satellite for this study is
focusing around the mountain ranges located roughly in between 106
o
E and 107
o
E, 6.25
o
S and 6.85
o
S in the southern part of West Java, Indonesia. The main reason of choosing a
particular area surrounded by mountains is simply due to the noticeable differences of
surface temperature between the mountain ’s
peak and the area with lesser height as it provide some aids to analyze the wind
development. As the temperature at the surface heats the air above it by conduction,
the heated air expands and become less dense than the surrounding environment and so it
rises. Later on, the differences of air density cause a pressure gradient between one point
and another, forcing the air to flow.
4.2 Preprocessing data
The image acquired by MODIS sensor on board the Terra satellite is geometrically
distorted due to the wide-angle swath and the Earth’s curvature which lead to overlapping
data errors. The distortion is so visible that it may affect image interpretation; hence,
overlapping parts need to be removed so that x
t u
y t
v
