The variability of refractivity in the atmospheric boundary layer of a tropical island volcano measured by ground-based interferometric radar

For 24 h we measured continuously the variability of atmospheric refractivity over a volcano on the tropical island of Montserrat using a ground-based radar interferometer. We observed variations in phase that we interpret as due to changing water vapour on the propagation path between the radar and the volcano and we present them here in the context of the behaviour of the atmospheric boundary layer over the island. The water vapour behaviour was forced by diurnal processes, the passage of a synoptic-scale system and the presence of a plume of volcanic gas. The interferometer collected images of amplitude and phase every minute. From pairs of phase images, interferograms were calculated and analyzed every minute and averaged hourly, together with contemporaneous measurements of zenith delays estimated from a network of 14 GPS receivers. The standard deviation of phase at two sites on the volcano surface spanned a range of about 1–5 radians, the lowest values occurring at night on the lower slopes and the highest values during the day on the upper slopes. This was also reflected in spatial patterns of variability. Two-dimensional profiles of radar-measured delays were modelled using an atmosphere with water vapour content decreasing upwards and water vapour variability increasing upwards. Estimates of the effect of changing water vapour flux from the volcanic plume indicate that it should contribute only a few percent to this atmospheric variability. A diurnal cycle within the lower boundary layer producing a turbulence-dominated mixed layer during the day and stable layers at night is consistent with the observed refractivity

Mid-latitude westerlies as a driver of glacier variability in monsoonal High Asia

Glaciers in High Asia store large amounts of water and are affected by climate change. Efforts to determine decadal-scale glacier change are therefore increasing, predicated on the concept that glaciers outside the northwest of the mountain system are controlled by the tropical monsoon. Here we show that the mass balance of Zhadang Glacier on the southern Tibetan Plateau, 2001-2011, was driven by mid-latitude climate as well, on the basis of high-altitude measurements and combined atmospheric-glacier modelling. Results reveal that precipitation conditions in May-June largely determine the annual mass-balance, but they are shaped by both the intensity of Indian summer monsoon onset and mid-latitude dynamics. In particular, large-scale westerly waves control the tropospheric flow strength over the Tibetan Plateau remotely. This strength alone explains 73% of interannual mass-balance variability of Zhadang Glacier, and affects May-June precipitation and summer air temperatures in many parts of High Asia's zone of monsoon influence. Thus, mid-latitude climate should be considered as a possible driver of past and future glacier changes in this zone.</p