Nature does not follow calendars... but yesterday marks the typical beginning of the northern Tanzania "long rains season". Coincidentally, the long rains last year (2018) began precisely on March 1st (see earlier posts, beginning with this one). Images acquired on the day prior, both last year and this year, are shown above (Sentinel-2 L1C).
Despite partial cloud cover, both images depict limited snowcover
other than on glaciers (e.g., north-facing side of the Uhuru Peak
summit ridge, the crater's south rim). Most important to this
discussion is that the summit crater (approximately circled) is
largely free of snow. The 2019 image shows some recent snow on the
northern flanks, which was present to a lesser extent on
mid-February images (not shown).
Even limited snowcover at the end of February (images above) is in
stark contrast to the same time in 2000. Note in our prior post (link) that a snow shovel is visible;
don't be fooled by this, for there was neither snow nor firn
anywhere on the glacier or within the crater. Indeed, this was an
exceptionally dry period which continued through the long rains of
2000 (see figure below; red line); March through May snowfall that year
totaled only 26 cm, the least of any long-rain season in our
period of record.
As the long rains concluded in mid-May last year, snowcover on the
mountain was extensive - as depicted in a Sentinel image from the
29th (below). Indeed, the
daily snowfall total at the AWS for the 2018 long rains was double
the 19-year average, resulting in more than 1 m of net accumulation.
The graph below shows how anomalous this accumulation (thick blue line; 2001-2017 as thin blue lines, 2000 in red).
With even an average short rains last year (typically Nov-Dec), could crater
snowcover have persisted until these next, 2019 long rains? Quite
possibly! Instead, the 2018 short rains included just 2-3 minor
accumulation events, plus early snowfall during our late October
fieldwork (link), for a total accumulation of less than 20 cm. By
Christmas, crater snowcover was patchy. Then, despite a mid-January
event, ablation predominated; by yesterday (see above) the crater
was largely snow-free.
In contrast, the Northern Icefield surface at the AWS gained mass
over the past 12 months, increasing in height by nearly 50 cm.
Higher reflectivity and less re-radiated longwave energy from below (i.e., ice
vs. dark volcanic soil) are among the factors.
In summary, the extent of 2018 accumulation and it's persistence
demonstrates the sensitive balance of processes governing
Kilimanjaro's summit glaciers. If seasonal snowcover does ever persist
in the crater through an entire year, retention will be easier the next
year and subsequently
become even easier. This idea is explored in Kaser et al. (2010), and 2018 observations strengthen the