Amplified water vapour feedback at high altitudes during winter



During the last five decades, the Tibetan Plateau has experienced a warming trend of 0.4 °C/decade in winter, which is at least twice that of any other season. Some studies have suggested that this anomalous winter warming is caused, in part, by the observed increases in near-surface water vapour and its amplifying effect on the surface longwave downward radiation (LDR). This study uses observations of surface-specific humidity (q) and temperature as input to a one-dimensional radiative transfer model to assess the influence of lower atmospheric increases in water vapour on surface LDR, and the sensitivity of this process to different elevations and seasons on the Tibetan Plateau. The results from three idealized experiments are examined based on realistic atmospheric column profiles of temperature and moisture. They show that when an equal mass of water vapour is added into the atmospheric boundary layer during winter, a substantially greater increase (8×) in LDR is found at the high-elevation site relative to the low-elevation site. During summer, the LDR increases are much smaller as are the differences between the two sites. Experiments, where both q and temperature are increased, suggest that the influence of temperature changes on LDR is much greater than those caused by changes in q in all cases, except for the high-elevation-winter case when the opposite is true. These results provide further evidence for the possibility of a strong modulation of surface LDR caused by increases in atmospheric water vapour in high altitude regions (>3000 m) during the cold season. Copyright © 2012 Royal Meteorological Society