Snow line characteristics from the AWS driven distributed model 2010-2012

2.4 Results and Discussion

2.4.4 Snow line characteristics from the AWS driven distributed model 2010-2012

A comparison of the temporal evolution of the mean altitude of the transient snow line at Zhadang glacier is presented in Fig. 2.13. Due to the installation period of the time-lapse camera system, ca-mera snow lines are available for the ablation seasons in 2010, 2011 and 2012 (see section 2.4.2.5).

At days, when mapped snow line is missing but model snow line is visible, the camera view field was hampered by low clouds or fog all day. In case only the picture at 16 BT was unusable, the snow line of a different picture of the same day was selected. At days, when both snow lines are missing, the glacier area within the view field of the cameras (Fig. 2.1) was either totally snow covered or snow free. The latter is the case only in 2010 after the snow line exceeds 5700 m a.s.l. On average, a snow line becomes visible at the lowest parts of the glacier tongue between May and June. In 2010 surface melt starts relatively late in the beginning of June but was very strong until September (Fig. 2.8, 2.19). This is clearly visible in the snow line evolution (Fig. 2.13, 2.19). The spatial and temporal variability of the transient snow line in 2010 is visualized in Fig. 2.19 by a series of ortho-rectified camera images. The animated time series for 2010 to 2012 is available online¹. In 2010, a snow line develops not until 11 June but then mean altitude rises quickly until the glacier is completely snow free. The following months are characterized by intermediate snow fall events and strong melt resulting in rapid changes in transient snow line altitudes. In September the permanent winter snow cover builds up in the course of storm events in autumn both in 2010 and 2011 (see section 2.4.3). In 2011 the first snow line is visible on 3 June, around one week earlier than in 2010, and rises very

slowly, repeatedly interrupted by snowfall events (Fig. 2.13). The uppermost areas of the glacier re-main snow covered throughout the whole summer. The rather small ablation in 2011 is also evident in measured and modelled surface height changes (Fig. 2.8) and in modelled SEB and MB compo-nents (Fig. 2.17). In 2012 the lowest parts of the glacier tongue become snow free already 23 May but snow line altitude again increases slowly (Fig. 2.13). The observed melt patterns develop in a very inhomogeneous way. They are not clearly to map. This limits the ability of reconstruction through the MB model in 2012 (Fig. 2.18). At first sight, the slow increase in snow line altitude seems to be in conflict with the rapid surface height loss observed at AWS1 (Fig. 2.8) between 19 May and 4 June 2012. However, accumulation through solid precipitation especially in September 2011 and April 2012 (Fig. 2.17) resulted in a snow pack of approx. 1 m at AWS1 (Fig. 2.8). The snow pack was com-pletely removed by melting only by 9 June. This is in agreement with the mean snow line altitude in Fig. 2.13 compared to the measured surface height at AWS1 (5665 m a.s.l., Fig. 2.8). In the lowermost regions of the glacier tongue the winter snow cover was not permanent and therefore rather thin in 2012 (see section 2.4.5). This results in snow lines developing rather early in the melt season of this year.

Fig. 2.18: Transient snow line on 4 June 2012 (for more information see Fig. 2.15).

Spatial patterns of the transient snow lines in most regions of the glacier tongue within the view field of the cameras are clearly dominated by altitude (Fig. 2.19) and therefore mainly by air temperature.

This is reasonable as SWin is distributed homogeneously and in the open valley hardly influenced by topographic shading. The eastern edge of the glacier tongue is an exception with consistently fast in-creasing snow line altitudes (Fig. 2.15, 2.18, 2.19) probably because of enhanced melt through long-wave radiation and sensible heat release from the adjacent rocks. In 2012 large snow patches re-mained at the western glacier tongue around 5600 m a.s.l. (Fig. 2.18). This could be the effect of to-pography induced decreased solar insolation described in section 2.4.3. The middle part of the gla-cier is very flat. The glagla-cier tongue ends in a concave steeper north-facing snout. This shape may support the long duration of the snow cover in this part of the glacier. In the uppermost south eastern glacier regions and in the steep areas in the west topographic shading generally results in long lasting snow cover and slowly retreating snow lines (Fig. 2.15, 2.19).

Fig. 2.19: Spatial and temporal evolution of the transient snow line at Zhadang glacier 2010; the blue line is the mapped transient snow line from the ortho images (see section 2.4.2); the red area indicates the transient snow line generated by the MB model (see section 2.5.2.5). The figure continues on the next pages.

Fig. 2.19 continued from preceding page

It could be shown in section 2.4.2.2 and Fig. 2.9 that measured surface height change using HAR data cannot be reproduced as good as with in-situ measurements what is probably caused by uncertain-ties introduced through HAR precipitation. Therefore, at this point we do not compare observed transient snow lines with modelled snow lines by the HAR forced MB model because mean altitude and temporal evolution at such small scale highly depend on surface melt and therefore intensity and frequency of solid precipitation as α is governing SWnet (see section 2.4.3). Nevertheless, comparing monthly mean snow line altitudes or end-of-summer snow lines from the HAR forced MB model with results from remote sensing is a promising option for further evaluation.

In document Energy and mass balance modelling for glaciers on the Tibetan Plateau : extension, validation and application of a coupled snow and energy balance model (Page 89-105)