Verifying temperature lapse rates in the Eastern Himalayas using Landsat 7 and 8

Abstract

This paper investigates the use of Landsat-7 and Landsat-8 thermal bands to assess the accuracy of temperature lapse rate relationships used in hydrological modelling for the Eastern Himalayan region. The temperature at high altitude is an input to conceptual gridded and lumped hydrological models that many studies use to understand and predict the relative contribution of melt-water to streamflow for Himalayan catchments. Temperature observations in the Eastern Himalayas are limited to meteorological stations below 2000m above sea level (ASL), except for a few stations near Mt Everest, that are part of the Ev-K2-CNR project, and some observations from short field campaigns. Many studies extrapolate temperatures at high altitudes from local meteorological observation stations using a temperature lapse rate. The most common method is to develop a seasonal temperature lapse rate from averages of ground-stations, typically between 1000mASL and 2000mASL. The aim of the analysis was to: a) determine if accurate temperatures at higher altitudes can be calculated from Landsat imagery; b) understand if a linear lapse rate can be inferred from Landsat imagery; and c) investigate the relationship between air temperature and land surface temperature for snow. The method involved (1) identifying cloud-free areas of snow, (2) generating temperature surfaces from Landsat thermal bands using algorithms from Jimenez-Munoz et al. (2009) and Jimenez-Munoz et al. (2014), and (3) generating lapse-rate relationships from comparison with elevation surfaces (derived from Aster). The lapse rates generated were cross referenced with ground station measurements and a serendipitous field campaign. It was found that the correlation between observed temperature at Pyramid station (5035mASL) and Landsat inferred temperature was 0.82 R 2 (see Figure 1). The correlation degraded as elevation reduced (i.e. to 0.35 R 2 at 1732mASL). The statistical methods applied were not capable of distinguishing between the contribution of error from the Landsat classification and the error from extrapolation of a linear lapse rate, so it was not possible to determine if the drop in temperature with elevation was linear or non-linear. Further work is required to use this method to confirm that a linear temperature lapse rate is reliable at high elevations. As expected, there were clear differences between temperatures on the northerly and southerly facing sides of mountains. These were especially pronounced during winter.