Challenges in Early Warning of the Persistent and Widespread Winter Fog over the Indo- Gangetic Plains: A Satellite Perspective - Reducing Disaster: Early Warning Systems for Climate Change - page 52

(MODIS) data. Six years of MODIS data for winter season 2000-2006 (December-
January, or DJ) are used to generate the fog/low-cloud climatology in order to gain
insight about the persistence and widespread nature of fog over IGP. In addition,
surface observations of fog occurrences and meteorological parameters such as
relative humidity (RH) were analyzed to understand the dynamics associated with
fog formation.

3.2 Spatial Distribution of Fog/Low-Cloud Occurrences

Following the methodology of fog/low-cloud detection (Gautam et al. 2007 ), the
spatial distribution of the frequency of fog/low-cloud occurrences was generated
from Level-2 MODIS swath data for the winter months (December-January) for the
period 2000-2006. Terra/MODIS data for aerosol and cloud properties (MOD04
and MOD06, respectively) over the IGP region are used in this chapter. The col-
umn mean aerosol optical depth (AOD) is produced at a 10 km spatial resolution,
while 5 km cloud properties such as cloud effective particle radius (CEPR) and
10 km for cloud top pressure (CTP) and cloud fraction (CF) datasets. Here, data
from Terra/MODIS (10:30 AM local time) were used to analyze fog/low-cloud
distribution over the IGP. In order to distinguish fog/low-cloud, high-level and precipi-
tating clouds were fi ltered out based on different criteria on CF, CTP, and CEPR
(Gautam et al. 2007 ).
Figure 3.2a shows the spatial distribution of the composite mean fog/low-cloud
occurrences from 2000 to 2006 during winter season. The average number of foggy
days over the 6-year winter period is larger in the central IGP compared to the east-
ern and western regions. The clear demarcation along the foothills of the Himalayas
is evident indicating the persistence of fog in the low topography/valley of the
Ganges basin. Majority of the foggy days occur over the IGP in India; however, a

Fig. 3.2 Top panel ( a ) shows composite mean fog/low-cloud occurrences for the 6-year winter
season derived from MODIS cloud properties. Bottom panel ( b ) shows mean fog occurrences over
the IGP (tallest bar indicates 20 fog occurrence days), during winter 2000-2006, mapped by mete-
orological surface observations overlaid onto surface topography