Micrometeorological conditions and surface mass and energy fluxes on Lewis Glacier, Mt Kenya, in relation to other tropical glaciers

Abstract

Abstract. The Lewis Glacier on Mt Kenya is one of the beststudied tropical glaciers, but full understanding of the interaction of the glacier mass balance and its climatic drivers has been hampered by a lack of long-term meteorological data. Here we present 2.5 yr of meteorological data collected from the glacier surface from October 2009 to February 2012. The location of measurements is in the upper portion of Lewis Glacier, but this location experiences negative annual mass balance, and the conditions are comparable to those experienced in the lower ablation zones of South American glaciers in the inner tropics. In the context of other glaciated mountains of equatorial East Africa, the summit zone of Mt Kenya shows strong diurnal cycles of convective cloud development as opposed to the Rwenzoris, where cloud cover persists throughout the diurnal cycle, and Kilimanjaro, where clear skies prevail. Surface energy fluxes were calculated for the meteorological station site using a physical mass-and energy-balance model driven by measured meteorological data and additional input parameters that were determined by Monte Carlo optimization. Sublimation rate was lower than those reported on other tropical glaciers, and melt rate was high throughout the year, with the glacier surface reaching the melting point on an almost daily basis. Surface mass balance is influenced by both solid precipitation and air temperature, with radiation providing the greatest net source of energy to the surface. Cloud cover typically reduces the net radiation balance compared to clear-sky conditions, and thus the frequent formation of convective clouds over the summit of Mt Kenya and the associated higher rate of snow accumulation are important in limiting the rate of mass loss from the glacier surface. The analyses shown here form the basis for future glacier-wide mass and energy balance modeling to determine the climate proxy offered by the glaciers of Mt Kenya.

Climatic controls and climate proxy potential of Lewis Glacier, Mt. Kenya

“…Kenya much more frequently than over the summit of Kilimanjaro, even though both summits are influenced by the same air masses. The point modelling undertaken by Nicholson et al (2013) suggests that, unlike on Kilimanjaro, the glacier mass balance variability is not dominated by a single variable or season. Building on that work, this paper aims to (i) extend the point surface energy and mass balance from to glacier wide-values for LG, (ii) evaluate the climate sensitivity of the glacier-wide surface mass and energy balance, (iii) explore climate conditions under which the late 19th century maximum extent of LG might have been sustained and (iv) discuss the potential for using shrinkage of LG to quantify climate change for a time period not covered by instrumental records. )…”

“…Although there are additional controls on the peculiar plateau ice fields Winkler et al, 2010), the ongoing retreat of slope glaciers on Kilimanjaro since the late 19th century has therefore been driven by the development of a drier regional atmosphere during the 20th and 21st century (Mölg et al, 2009b). This drying is related to changing ocean conditions which shift the Walker cell circulation over the Indian Ocean, thereby suppressing the convection along the east African continental margin (Chou et al, 2009;Lintner and Neelin, 2007;Mölg et al, 2006;Nicholson, 1996;Tierney et al, 2013;Webster et al, 1999), inhibiting the deep convection required to bring cloud cover and precipitation to the glaciated mountain summit (Mölg et al, 2009a). Both precipitation amount and frequency are reduced, and this combination reduces both the mass additions to the glaciers and the impact of frequent snowfall on surface albedo (Mölg et al, 2009b).…”

“…Although pioneering for their time, 2.5 years of in situ meteorological data are now available to allow a more rigorous assessment of the climate sensitivity of the glaciers here and explore if, due to their lower elevation, they offer a different climatic proxy than that offered by the glaciers of Kilimanjaro. Nicholson et al (2013) investigated the recent micrometeorological conditions and energy fluxes on LG at the point scale, in the context of other tropical glaciers. Conditions at the summit of Mt.…”

“…Thus, 773 days of complete data collected from the glacier surface, in three separate periods, are available for analysis. All instruments on the AWS (see Table 1 in Nicholson et al, 2013) are naturally ventilated, measured every 1 min, and half-hourly averages are stored on a Campbell Scientific CR3000 data logger. Nicholson et al (2013) demonstrated that the shielded, unventilated air temperatures and relative humidity measured at this site do not appear to suffer from distortions from solar heating of the sensors.…”

“…All instruments on the AWS (see Table 1 in Nicholson et al, 2013) are naturally ventilated, measured every 1 min, and half-hourly averages are stored on a Campbell Scientific CR3000 data logger. Nicholson et al (2013) demonstrated that the shielded, unventilated air temperatures and relative humidity measured at this site do not appear to suffer from distortions from solar heating of the sensors. On 24 February 2012, the AWS was moved off the ice and mounted on solid rock ("Radio Ridge") next to Austrian Hut at an elevation of 4809 m. The system set-up was upgraded with a data transmission via satellite, but the number of sensors was reduced: sensors to measure air pressure and surface height were omitted, the four-component radiometer was replaced by a single pyranometer (EKO MS-602) and the data logger was changed to a Campbell Scientific CR1000.…”