Computation of Atmospheric Water Vapour Map from MODIS Data for Cloud-free Pixels
Agnes Lim
E-mail: crslima@nus.edu.sg
Chew Wai Chang
E-mail: crsccw@nus.edu.sg
Soo Chin Liew
E-mail: crslsc@nus.edu.sg
Leong Keong Kwoh
E-mail: crsklk@nus.edu.sg
Centre for Remote Imaging, Sensing and Processing (CRISP)
National University of Singapore
Blk SOC, Lower Kent Ridge Road, Singapore 119260
Tel: (65) 8744411 Fax: (65) 7757717
Singapore
Abstract:
Water vapour is a highly variable constituent in the atmosphere. From the visible region to thermal region of the electromagnetic spectrum, it is one of the culprits hindering the measurement of actual radiance and reflectance from the surface. Water vapour maps can be generated from MODIS data by measuring the relative transmission for a suitably chosen water vapour absorption band and two “window” bands where water absorption is negligible. With the transmittance map, a binary search is performed using lookup tables generated using MODTRAN to retrieve the total precipitable water, giving a water vapour map. With this map, data can be corrected for atmospheric water vapour absorption effect.
1. Introduction
The Moderate Resolution Imaging Spectrometer (MODIS) onboard Terra satellite views the earth every 1 to 2 days. It is thus well suited for global monitoring of atmospheric properties from space. Moreover, it acquires data in 36 spectral bands and hence allowing for multidisciplinary studies of earth-atmosphere and ocean -atmosphere studies. Five near infrared (NIR) bands are useful to the remote sensing of water vapour, making it possible to derive total columnar water vapour from MODIS data at a spatial resolution of 1km. By taking the ratio of 1 absorbing infrared band to the weighted average of 2 non-absorbing bands, the two way transmission through the atmosphere can be derived and used for remote sensing of total columnar water vapour
The ability to obtained information on the distribution of precipitable water vapour will improve the understanding of the hydrological cycle, biosphere-atmosphere interaction, the energy budget and monitoring climate change due to greenhouse gases. Besides their utilization in atmospheric correction of remote sensing imagery, the water vapour distribution maps generated at different dates also enable multi-temporal monitoring of atmospheric moisture, which can be an indication of drought conditions in the region.
