More Snow

The wet 2019 short rains continue, according to both satellite imagery and reports from the Kilimanjaro region. Simon Mtuy wrote today that the mountain has been in clouds for the past three days, with heavy rain last night.

Above is a glimpse of the mountain two hours ago, from TPC sugar plantations (above Moshi).

Active convection continues over the anomalously-warm, western Indian Ocean. Tropical Cyclone 06A is forecast to move southwest to Somalia on a track toward Lake Victoria, with landfall on 6 December bringing heavy rainfall and high winds; not what eastern Africa needs this year.

In addition to cyclone 06A, another cyclone is forming on the other side of the Equator. Rarely do cyclones form on both sides (circulating in opposite directions). Read more about this situation, and the vigorous convection expected, here. [credit Severe Weather Europe]

Regional October precipitation

The upper photo provides another perspective on Kilimanjaro snow, complementing those of the previous post on this blog. It was sent recently by Simon of SENE, taken from Moshi in the first few days of November. On the satellite image above from 5 November, snowcover is reduced within the large summit caldera relative to that of 26 October, yet the snowline on the southwest flank appears even lower. Today's Sentinel-2 image (10 November, not shown) reveals a fresh dusting over the entire mountain, with higher amounts just west of Reusch Crater.

Simon wrote of anomalous October rainfall in the area, with a frequency more like that of the long-rain season. This appears to have also been the case for a larger region of East Africa, especially Kenya, southern Somalia and southern Ethiopia - and has led to flooding to the north and east of Kilimanjaro. The situation is shown clearly on a European Commission map for 5 November, from the Emergency Response Coordination Centre (ERCC). [Kilimanjaro can be seen, in shaded relief, where the northern border of Tanzania jogs a bit; it is southwest of the flooded area in Kenya (red dots).]

Why has the region been so wet during October? One good possibility is related to sea surface temperatures in the western Indian Ocean: anomalously warm water! Warmer than normal SSTs in the west, with cool SSTs in the east, sets up a positive Indian Ocean Dipole (IOD) event, associated with increased convection and precipitation over East Africa. During September the IOD strengthened markedly, becoming one of the most-positive events in many decades. Further information can be found here. Once precipitation data from the mountain are available, we will have a better understanding on how the 2019 IOD event is impacting the Kilimanjaro region.

Early short-rain snow

Snow conditions on Kibo have changed considerably over the past 10 days, as shown in the timelapse above. Very little seasonal snow was present on 16 October, nicely revealing the current distribution of glacier ice. Five days later the entire summit was blanketed by snow. A second-hand report from our friend Simon Mtuy indicates that the snowline on the 18th was below Kibo hut. (Simon's wonderful company is SENE).

Between the 21st and 26th, ablation of new snow appears to have dominated over additional accumulation. However, note extensive snow below the Western Breach on the 26 October image; this may have resulted from localized convection, typical on that side of the mountain. Simon was on the mountain last week, so it will be interesting to hear his observations. Low on the mountain (i.e., below 1800 m) he reports nearly non-stop rain since the beginning of October - an early beginning to the short-rains season!

Summit Snow

High elevations on Kibo received an early October dusting of snow, as shown in the Sentinel-2 image above, acquired Sunday. Until AWS data are recovered, we don't know whether this snow resulted from one event, or multiple; five days prior the summit was obscured by clouds, and it was snow-free ten days earlier.

This image reveals interesting information about ice, snow, and clouds. The brightest areas which are labeled are the remaining ice bodies. Increasing fragmentation of what was once the Southern Icefield is readily apparent. Within a few years the Heim and Decken Glacier will likely be gone, followed shortly thereafter by the Furtwängler.

Almost all other bright areas - of various sizes and shapes - are new snow (e.g., southeast of the Reusch Crater). In this scene, note how snowcover is distributed rather symmetrically on the mountain, which is typically not the case for individual snowfall events.

One large bright area to the southwest of Reusch Crater shows relatively-thick convective clouds rising above the Western Breach. Elsewhere, thin clouds appear darker and more variable in brightness, forming a annular pattern around Kibo. These clouds are low in elevation, as evidenced by the visible shadows. This annular pattern is quite common on Kibo, with clouds thickening during the day due to convection. Sometimes, the crater remains cloud-free yet encircled by clouds, if convection dominates over advection (which transports moisture laterally).

Early October snowcover usually persists for only days to weeks, with the short rains not getting underway for at least another month. Nonetheless, such events considerably influence mass balance, as snowcover greatly impacts radiative energy exchanges due, for example, to the higher reflectivity (albedo) of surfaces.

NIF shrinkage: 2015-2019

The right-hand image above depicts Kilimanjaro's Northern Icefield in mid-September (2 weeks ago). Although resolution is not ideal, minimal snowcover allows comparison with the same glacier four years earlier (July 2015). Note that the two images are not perfectly registered, so the following observations are qualitative.

Both the north and south remants of the icefield have decreased in area, especially relatively-thin portions at lower elevations. These include the northwest part of the north remnant, and the western margin of the southern part. A marginal meltwater lake is visible at the southern margin of the north portion on both images, and has been present for many years. At the eastern edge, we have observed shrinkage of several isolated blocks of ice over the years; these were present in 2017, but have now disappeared. On the south portion, thin areas and holes in the left-hand image are now ice free, including one location (southeast margin) where we have evidence that geothermal heat initiated hole formation.

The rate of glacier thinning was reduced during 2018, due to above-normal snow accumulation and the attendant increase in albedo. To illustrate, note the brighter, high-elevation portions of the glacier in the 2015 image; this is snow cover over old glacier ice.

We are hoping to visit the summit glaciers early in 2020 to measure ablation stakes, conduct GPS surveys, and photographically document changes to the glaciers since our last visit.

Regional wet-season failure

A new post on the NASA Earth Observatory website reveals the regional extent of precipitation deficit partway through the 2019 long rains. The NASA soil moisture anomaly map for April (above, from MODIS) depicts a large anomaly extending into northern Tanzania. [The Kenya-Tanzania border jogs around the mountain just below the 'KENYA' label on the image above.]

The seasonal snowcover situation on Kilimanjaro is discussed below; the extent did not increase during May.

Included in the EO article are some helpful references detailing the human impact of this developing East Africa drought.

Dry-season forecast: above-average ablation

The long rains (Masika) of 2019 are concluding with virtually no snow accumulation on Kilimanjaro glaciers, in stark contrast to last year's long-rain season - demonstrating the extreme interannual variability of precipitation at the summit.

The Sentinel-2 image above from 2 days ago (14 May) reveals a largely snow-free crater. Small areas of last year's snow persist (e.g., east of the Northern Icefield, adjacent to the Furtwängler Glacier). Elsewhere, only a dusting of snow can be seen on Kibo's south side - which not coincidentally spans the elevation range and azimuth of remnant glaciers there! (Very preliminary analysis suggests that the responsible snowfall event was somewhat more extensive, yet we know that at this SSW sector of the mountain, convection enhances snowfall and clouds reduce ablation.)

During the long rains last year - extending from 27 February until this date (16 May) - net accumulation of snow on the Northern Icefield was over one meter (as discussed here). Contrast this with 2019 long rain accumulation, shown in the figure below (blue line); prior to the minor event last week the AWS recorded a net lowering (ablation) of over 30 cm. Additional long rain snowfall may still occur this year, however, the long rains rarely extend into June at the summit.

Absent a major event bringing sufficient snow to reduce solar radiation penetration (e.g., 30-50 cm), the forthcoming extended dry season will probably begin with a snow-free crater. As a result, ablation of both horizontal and vertical glacier surfaces is likely to be dramatic in the months ahead.

(The timelapse image below provides a perspective on summit snowcover since early August of 2018. Within the crater, note the persistence of long-rain accumulation through the dry season, and the ephemeral nature of spatially-extensive-but-thin accumulation during the period February to April 2019.)

South-side glacier changes

The images above depict how Kibo's south-side glaciers have changed in area and thickness over the past 13 years. Thanks to Catrin Stadelmann and her advisor Thomas Mölg for sharing them (Friedrich-Alexander-University (FAU) in Erlangen-Nürnberg, Germany).

Note that both snow and ice are visible in these images, particularly the lower one (Jan. 2019). The largest glacier shown is the Kersten, which Dr. Mölg has published extensively on. Shortly after 2006, a gap began widening between the upper ice and the steeper slope portion of the Kersten. Similarly, a gap also formed between the Kersten and the Decken Glacier to the east (right-hand side of the Kersten). The Decken has gradually narrowed, and very little ice remains today. On the Kersten's left (western) side, remains of the Heim Glacier are visible in 2006; the lower-most extent of this glacier was still present by 2019, surrounded by considerable transient snowcover in the image above.

2018 long rains review

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 argument!

19 years on the Northern Icefield

This week marks 19 years since AWS measurements began on Kilimanjaro's Northern Icefield (NIF). With enthusiastic help from our Tanzanian crew, Mathias Vuille and I installed a tower into the ice and connected the electronics. Remarkably, the same datalogger continues measurement and control functions, and the same solar panels continue to provide power. Most sensors have been swapped out for recalibration or replacement, yet the original barometric pressure sensor continues reliable measurements every hour.

Ice ablation since 2000 has substantially reduce the areal extent of all glaciers on the mountain. However, this portion of the NIF has "only" thinned by ~5 meters, because the low surface gradient retards meltwater runoff - which then refreezes in place as superimposed ice. Other portions of the NIF, and other glaciers, have thinned more dramatically. For example, ice no longer remains at February 2000 drill sites on the Furtwängler and Decken Glaciers, which were 9.5 and ~20 m thick at the time (respectively).

Asymmetric snowfall [updated]

The pair of Sentinel-2 images above demonstrate an interesting asymmetry in snowfall, visible despite partial cloud cover. These images, acquired 5 days apart on 14 and 19 January, are closely registered and show the Northern Icefield AWS location (click image to enlarge).

The southern slopes and south side of the crater appear not to have gained any accumulation through the 5-day interval. Although slopes to the west are difficult to resolve through the clouds, new snow on the north and northeast flanks is readily apparent above ~4,800 m elevation. Snow also accumulated just south of the Northern Icefield, on the crater's west side.

AWS measurements are still being processed for this interval, which will reveal snowfall timing. In the meantime, the website "earth" allows weather conditions during this period to be visualized. The 17th appears a likely time for this snowfall pattern to have developed. Winds were light and humidity was high at 500 hPa, while a bit lower in the free atmosphere (700 hPa) winds were from just east of north, and humidity was high.

This post will be updated as AWS data and the next S-2 image become available.

[UPDATE 01/25, 2/4:  AWS data from the Northern Icefield (via telemetry) reveal the difficulty of documenting subtle climate features on a large mountain, using measurements at one location. In this case, only 3.5 cm of snow accumulation was recorded over the 5-day interval between images (above). Despite use of 2 sensors, 3 m apart, the timing of minor snowfall events cannot be precisely established from the 4-hourly satellite data, possibly due in part to wind redistribution of snowfall. Once hourly snow measurements and other data are recovered from on-site storage (e.g., solar radiation, wind speed), we may be able to better resolve snowfall timing.

A best guess from the AWS measurements on snowfall timing between these images would be the 18th. Supporting this is the lingering presence of accumulating snow at ~4,800 m, which is unlikely to persist more than a day or two - especially at this time of year. However, collaborator observations suggests that widespread snowfall on the northern slopes occurred a bit earlier. For 14-17 Jan., they report heavy rain at ~3,500 m on the Shira Plateau and to the northwest of Kibo.

Additional satellite images are available from the 9th and 24th of January. The image from yesterday (shown below) shows little change in snowcover at high elevations, consistent with AWS data, yet ablation of snow from the northern slopes.

The image from 9 January (not shown) reveals nearly-uniform cloud cover over the mountain. According to AWS data, snowfall was just getting underway at the time, and resulted in a whopping 24 cm of new snow at the summit by the 14th. This is a relatively large snowfall event for the Northern Icefield, of similar magnitude as the late-October event we experienced (see prior post, below). These two are the largest events since the 2018 long rains. Measurements at the AWS suggest most of this snowfall occurred on the 10th or 11th, 3-4 days prior to the image from the 14th (above) - plenty of time for ablation of most new snow from the slopes.]