Abstract:
Integrated precipitable water vapor (PWV) retrieved from Moderate Resolution Imaging Spectro-radiometer (MODIS) in near-infrared channels is examined with the PWV estimated from the Global Positioning System (GPS) data and measured surface temperature and pressure values over Hanle, located at
a high-altitude (∼4500 m) trans-Himalayan region. Since the accuracy of MODIS derived PWV over the
large elevated topography of the region is not well studied, the present work focus on the validation of
MODIS PWV using the GPS data during 2005–2012. The study reveals that MODIS PWV compares well
with the GPS PWV data with bias 0.018 cm, root mean square error (RMSE) 0.137 cm and coefficient of
determination (R2) 0.91. The two types of data products are compared on short (daily) and long temporal
(monthly-seasonal) scales in order to evaluate the seasonal dependence of PWV. The peak values of PWV
from MODIS and GPS data at the site varied from 1.7 to 2.05 cm and 1.2 to 1.4 cm as the daily and the
monthly means, respectively, which occurred during August. However, the seasonal peak of PWV occurred during summer as 0.88 and 0.97 cm for MODIS and GPS data, respectively. The present study
noticed that about 85% of the total PWV estimated from MODIS as well as GPS data lies below 1.0 cm over
the site. The seasonal study of PWV reveals that MODIS data is found to be underestimating the PWV
when compared to GPS data with bias 0.095 cm and RMSE 0.215 cm particularly during summer. This
is due to dry and high-altitude terrain of Hanle particularly during summer months. However, there are
marginal differences (bias varied from 0.007 to 0.017 cm) during the remaining seasons and seasonally R2
varies from 0.62 to 0.87. GPS PWV exhibits distinct diurnal cycle with minimum in morning as well as in
night and peak in the late afternoon (16–18 h) irrespective of the seasons. However, the diurnal cycle is
very prominent during summer than the rest of the seasons.
