water vapor in the sub-Saharan is high and varies
significantly temporally and spatially. A strong
relationship of the surface recorded water vapor
amount (via the surface dew point temperature) to
the vertical atmospheric water content was
reported by Mohamed and Frangi (1983) for
atmospheric conditions near Niamey, Niger. A
linear comparison of surf ace dew point
temperature versus atmospheric precipitable water
indicates a linear correlation coefficient of
r^0.89.
In Ps = 0.0581*Td + 0.23 (3)
Ps - precipitable water (cm)
Td - dew point temperature (0°)
Hence, using the surface dew point temperature
from NCDC archived data to adjust the atmospheric
water vapor model (lb^N) was considered to
provide a reasonable estimate of the water vapor
atmospheric optical depth near the time of the
satellite overpass. The ozone atmospheric
optical depth derived using the 15° N tropical
model in Lowtran-6 was used as input to the Ahmad
and Fraser atmospheric radiative transfer model.
Water vapor optical depth estimates were
derived for Tambacounda and Podor. The magnitude
of the water vapor optical depth is high, ranging
for values above 0.17 during the rainy season and
below 0.08 during the dry season. Inspection of
the monthly dew point temperatures (Figures 2 and
3) indicates that only the winter months are
associated with a low atmospheric water vapor.
ALthough Tambacounda receives considerable more
rainfall than Podor, the atmospheric water vapor
recordings are comparable.
c aeros °l optical depth in the western sub-
baharan may be extremely high with optical depths
averaging over one during the spring-summer
months when there is high solar insolation
(D Almeida 1987 and Holben et al. 1989b) . The
nuu^w e u 1C aeroso1 density distribution
(McClatchey et al. 1971) was adjusted in the
Ahmad and Fraser radiative transfer model through
specification of an aerosol optical depth. Site
specific aerosol optical depths were estimated
trcm surface meteorological ranges via an
observer recorded visibility. The visibility was
obtained from archived data at NCDC for Podor and
tambacounda at a time near the satellite
overpass. The observer visibility (V 0 b s ) was
converted to a meteorological range (V) through
Sal P S V 3 ■ (V=V ? b ?* :l ' 3 (±°' 3 » (Kneizys
et, al. iy»3;. The horizontal range was converted
to an aerosol optical depth using an empirically
based transformation (r-0.96) derived by
Almeida (1986). D’Almeida analyzed west sub^
Saharan aerosols in Assekrem, Algeria and
rtoutilimit, Mauritania.
0 12 24 36 48 60
MONTH
Figure 2. Tambacounda monthly mean precipitation,
surface air and dew-point temperature, and height
of lifting condensation level.
257
Figure 3. Podor monthly mean precipitation,
surface air and dew-point temperature, and height
of lifting condensation level.