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.]

Wild weather on the mountain

Fieldwork is an essential component of climate and glacier research, providing basic measurements as well as a foundation for theoretical and modeling studies. Yes, fieldwork can be tremendously fun, sometimes even yielding unexpected discoveries – yet it can also be difficult and dangerous. A successful fieldwork campaign requires alignment of numerous components and factors; some of these we can control, and others we must manage. For both categories, past experience and planning is helpful. Sometimes though, the outcome also requires good luck.

After 20 prior trips to my AWS on Kilimanjaro’s Northern Icefield, plans for October fieldwork came together within only a couple weeks. Telemetry of data revealed technical problems which could only be resolved by a visit to the station. However, budgetary constraints dictated that this trip would need to be done differently. While recognizing that a lighter and faster approach would reduce the factors we could control, and increase the required management of other factors, we decided that the potential rewards of a brief visit to the station outweighed the risks of this strategy. Supportive and generous collaborators* agreed to make the visit possible.

Our quickly-hatched plan was to acclimatize normally, and then in one day ascend the final 1000 meters, undertake 4-6 hours of work, cross the summit caldera, and descend the other side to rejoin our support team. Past experience on the mountain suggested to all involved that the concept was reasonable –given just a few hours of reasonable weather. For this trip, ‘reasonable’ weather meant conditions under which an ascent of the Western Breach was safe (i.e., cold and dry), followed by a few hours at the AWS without heavy snowfall, wind less than ~20 km/hr, and air temperature above -5 °C or so; any sun would be a bonus. Once finished at the station, we were confident about descending in almost any conditions.

Reasonable weather prevailed through our first two days on the mountain, followed by conditions more typical of the wet seasons (e.g., April-May). Warmth, rain, and wind appeared in the forecasts, and on day 3 became our reality on the mountain, conditions increasingly at odds with both our work needs and those required for the safety of all involved. Ascending in wind-driven rain to Arrow Glacier camp below the Western Breach, there was little ambiguity about what we were likely to encounter the following day – which proved accurate, as illustrated below and revealed by telemetry from the AWS. That afternoon a consensus emerged: continuing with our plan would unacceptably compromise safety, and that work at the AWS would almost certainly be impossible.

Disappointing? Absolutely. Station problems remain unresolved, and the 18-year nearly-continuous record may be compromised. In addition, not measuring and documenting the summit glaciers will prevent assessing the response of anomalous accumulation during the 2018 long rains. Furthermore, any compromise on fieldwork goals is disappointing in light of the carbon cost of traveling nearly 30,000 km. However, our decision to retreat was correct, for in contrast to several other groups on the mountain, we all returned safely.

Field scientists must fully exploit observational and quantitative opportunities during every moment in the field, and learn from every experience. This trip provided new insight into the development of weather systems on Kilimanjaro. Valuable photographs and observations of the slope glaciers were obtained, and new understandings were gained through interactions with others on the mountain. More difficult to accept was something we already knew, the false economy of an ambitious undertaking in too-little time. Future fieldwork must allow adequate time to accommodate difficult weather conditions, despite the higher financial cost of extended fieldwork time on Kilimanjaro summit glaciers (e.g., extra Park fees, staff salary premiums). Finally, the experience highlights the value of high-elevation climate and glacier data, which should never be taken for granted.

*Special thanks to Nicolas J. Cullen at University of Otago (New Zealand), and Thomas Mölg at Friedrich-Alexander-University (FAU) in Germany, for their encouragement and vital support!

Fig. 1. Timelapse of clouds over Kibo, 24 Oct. at 13:30 (~1 sec interval). Wind speeds began increasing on the 23rd, and remained high for 3-4 days. Airflow throughout our time on the mountain was from just south of east, as illustrated in Fig. 3 below.

Fig. 2. Kibo on 24 Oct. at 13:30 from near Karanga Camp. Although the mountain is quite snowy for mid-October, snowcover decreased during the days prior, due to rain and a high freezing level (note lack of fresh snow east of the Rebmann Glacier, on right-hand side of image).

Fig. 3. Airflow and relative humidity at 500 hPa over east Africa and Kilimanjaro (green circle), 24 Oct. at 14:00. Cyan color indicates RH above ~95%; 49 km/hr equates to ~30 MPH, not a particularly high windspeed for a mountain summit, but difficult to work in when humidity is high (see riming in Fig. 4). Source: earth.nullschool.net (c) 2018 Cameron Beccario.

Fig. 4. Summit scenes early on 25 Oct., when apparently only 2 Norwegians and their guide reached the top. Photos courtesy of Dismas Kishingo, via Emanuel Mutta of SENE.

Fig. 5. Landsat 8 scene from 28 Oct. at 10:43 local time. Fresh snow on Kibo and Mawenzi accumulated over the prior ~4 days, when AWS data show dropping air temperature and 20+ cm of accumulation.

Fig. 6. The proverbial calm after the storm. Kibo as seen from Moshi, 28 October at 08:00 (just prior to the Landsat image above).

Crater remains snowy

Ablation of 2018 snow continues, as evidenced by the 1 October image above. Nonetheless, extensive snowcover remains within Kibo crater as the extended dry season concludes (Sentinel-2 bands 4,3,2). The mountain's south side also remains snowy, making it difficult to easily distinguish between glaciers and snow on the image.

Trails up to the crater and along the rim to Uhuru Peak appear to be free of snow. However, where snowcover remains, penitentes are likely getting steadily larger.

On the Northern Icefield, telemetry of AWS measurements reveals a surface height increase of ~30 cm for the one-year period Oct. 2017 to Oct. 2018. This accumulation was concentrated in three intervals: the first half of January, the first week of March, and a week in mid-April. In contrast, ablation was especially pronounced through the entire month of February this year. 

Whether 2018 accumulation endures will depend upon October and November weather, which typically varies considerably from year to year. Since mid-May, when peak accumulation occurred, the rate of surface lowering due to ablation has been rather constant at ~12 cm/month.

Seasonal snowcover change

 

Two views of Kibo from the plains below reveal that snowcover has largely ablated from the mountain's slopes. From the northeast (upper image) dry slopes appear capped by a fringe of snow on the crater rim, while the Northern Icefield is visible in the second image.

Both images above were taken during the 2018 Kilimanjaro Stage Run, a wonderful way to experience the mountain and the diverse cultures residing on the flanks. Adventurous runners of all abilities should look into this fun event!

Below is another view of snow on the mountain, acquired one week later. Nearly complete snowcover remains within the summit crater, although it appears that the trail to Uhuru Peak is now free of snow. Lower on the mountain, trails and camps are visible in this European Space Agency image.

Despite the extent of snow within the crater, telemetry of measurements from the Northern Icefield indicate that the glacier surface lowered by 15-20 cm, likely due to sublimation, melting, and compression of long-rain snow. This snow benefits glacier mass balance by adding mass and reducing energy exchange (e.g., reflection of solar radiation) - briefly reducing the glacier recession rate.

Kibo summit snow

 

In mid-July, Kibo remains almost entirely snow covered at high elevations. The image above depicts this snowcover along the crater rim on 10 July, looking toward Uhuru Peak with the upper Deckens Glacier on the left. Snowdepth varies considerably in mountainous terrain, due to both snowfall and ablation processes, yet nearly one meter of snow remains on the Northern Icefield (0.98 m). This represents net accumulation since the beginning of March. Relative to the glacier surface in early October - when we visited for fieldwork - the net increase in surface height is 0.72 m.

Additional detail on Kibo snow is provided by the images below. The first is a Sentinel-2 image from 13 July, with uniform snowcover in the crater and extending down all slopes. Note some thinning and emergence of bare spots in the past few weeks (see earlier posts). Below the satellite image is one from just below Stella Point, showing the depth of accumulation on 10 July. The final image also looks toward Uhuru Peak (with 40+ people), across the Furtwängler Glacier, and towards the Northern Icefield; penitentes in the foreground typically develop in deeper snow at this time of year, due to sublimation. They will likely keep growing for the next couple months.

Many thanks to our friend Timba in Moshi for providing these images!

Kibo remains snowy, as illustrated by the Sentinel-2 image above from 3 July. Over the past month, the snowline has been only slowly moving up the mountain. Accumulation at high elevation and within the crater has been ablating slightly; compare the image above with those in earlier postings.

Extensive snowcover on the glaciers and surrounding slopes is keeping the albedo high, minimizing mass loss... at least for the moment.

Kilimanjaro is not alone in being unseasonally snowy in recent months. For example, in the Karakoram Mountains (Pakistan) climbing teams on mountains such as K2 are finding dangerous avalanche conditions due to heavy snowfall, during the core climbing season. More details can be found here.

Quelccaya Ice Cap and the Cordillera Vilcanota in Peru are also unusually snowy for July, the result of La Niña accumulation during the wet season (esp. DJF) and atypical dry-season snowfall in the past couple months.

In Northeast Greenland, the winter of 2018 brought twice as much snow as the long-term average, and snowcover into early July remains so extensive that Sanderlings and other shorebirds may not even attempt nesting this year. The late snow is having large consequences for the ecosystem.

Finally, snow on portions of the Greenland Ice Sheet is resulting in the "least surface ice loss in decades". As Jason Box notes via Twitter (@climate_ice), these persistent extremes in patterns of atmospheric circulation are an expected signature of climate change.

Dry season begins

One of the most-reliable aspects of Kibo summit climate is when the extended dry season begins; typically between late May and early June. Despite considerable snow accumulation through 2018 long rains (see posts below), the dry season initiation this year appears to be right "on schedule".

Above is a view of Kibo from Moshi just after 7 am on 28 May (Simon Mtuy credit), after a long period in which the mountain was shrouded in clouds. Snowcover has changed little since March.

The timelapse below includes images every 5 days for the past month, from ESA Sentinel-2 L1C data. Note the decreasing cloud cover thickness and extent, and thinning of snowcover on the mountain flanks. Telemetry of measurements from the summit reveals little change in snowdepth on the Northern Icefield through the interval.

In the months ahead, all seasonal snow will likely sublimate and melt, exposing glacier ice to radiant and turbulent energy. Without the bright, protective snowcover, the area and thickness of the glaciers will continue to diminish.

Snowcover update

Ablation has dominated over accumulation on Kibo recently, yet the mountain remains snowy. On a Sentinel-2 image from yesterday (above), note the rising snowline and thinner snowcover on ridges and east-facing slopes - relative to images posted previously. With minimal clouds over the summit caldera, the Northern Icefield margin can now be distinguished, just to the northeast of the Reusch Crater (outer circular feature).

Below are some images contributed by Adam Quenneville, from his visit to the summit on ~26 April. The panorama looks south over the upper Kersten Glacier from near Uhuru Peak. Note how the windblown and frozen snow surface allowed climbers to walk without breaking through. The middle image shows the Northern Icefield (upper left) and the Furtwängler Glacier margin - including the tiny remaining eastern fragment. Adam's team is shown at Crater Camp in the lowest image, with the Furtwängler and Northern Icefield in the near and far background, respectively.

More March snow! [updated x3]

Numerous reports of snowfall on the mountain have been received this month, which in some cases has prevented groups from reaching the summit. While the long rains often begin during March, snowfall this month appears to be somewhat exceptional.

The previous post provides information on early March snowfall. Further details have only recently emerged, because snow on one of the solar panels prevented satellite transmissions for ~5 days during the middle of the month, and then again on 20 March. During this time, extensive cloud cover also prevented acquisition of useful satellite imagery from above.

As March comes to a close, telemetry is working well again (with thanks to Mike Rawlins at UMass Climate System Research Center for help on this). We now know that net snow accumulation for the first 3 weeks of March amounted to 63 cm on the Northern Icefield. As the ESA Sentinel-2 image above shows, snow blankets the entire summit caldera and upper slopes of the mountain (look closely, to discriminate snow from stratus fractus clouds). This is the greatest snow accumulation on the glacier in years -- with additional snowfall likely during the remaining months of the long rains (typically March through May).

For those climbing the mountain in the months ahead, fear not. Snow on the routes will quickly compact and you will have a chance to experience conditions more typical of past decades. Dust will be minimal, beautiful nieve penitentes will grow as the dry season progresses, and you will encounter much happier glaciers. It is also important to keep in mind that this accumulation is surely temporary, and will not change the reality that these glaciers are disappearing rapidly.

[UPDATE 04/02: Another Sentinal-2 image acquired 5 days later provides a clearer depiction of summit snowcover (below; centered further east than image above). Some ablation has taken place, allowing recognition of the caldera rim as well as that of the Reusch Crater and the inner Ash Pit. Snowcover remains sufficiently thick that snow and ice cannot be distinguished at this resolution. We can now see a sharp transient snowline on the west side, at approximately 4750 m - which is 1000 m below the caldera rim.]

[UPDATE 04/10: Sentinel-2 acquired a beautiful snowy image yesterday, with little cloud cover. The GIF below shows 4 registered images, including yesterday's (9 April), one from 5 days earlier, and two from late March. The red circle in the northwest corner is at ~4,700 m, while that in the southeast corner is at ~4,800 m. (Barafu Camp and adjacent trails can be seen just south of the lower red circle.) Although the transient snowline can be seen rising slightly during this period of ~2 weeks, the summit remains entirely snow covered.]

 

[UPDATE 04/11: Very clear view of Kibo from Moshi this morning, verifying the pattern and magnitude of snowcover seen in the 9 April image above. Thanks to Simon at SENE for the update!]

March snow

The first week of March brought a net snow accumulation of nearly 50 cm to the Northern Icefield, which by any measure is a snowy interval on Kilimanjaro. This precipitation follows 25-30 cm of continuous ablation during February, as illustrated in the previous post. A context for the event follows.

Figure 1 (below) shows Sentinel-2 satellite images of the exact same scene, on the last day of February and on 5 March. As detailed in another post, snowcover was primarily confined to steep north-facing slopes by the end of February. Although considerable cloud cover is present around the mountain on the 28 Feb. image, the summit caldera is mostly cloud free. Note the red squares, which are co-located on the 5 March image for orientation. High clouds partially obscure the March image, yet pervasive snowcover is visible. A sharp snowline at ~4,400 m is visible on the left-hand side of the image.

Figure 2 provides two snowy views of the mountain from the Moshi area (SENE credit). Despite low resolution of the 3 March image (upper), substantial snowfall obviously occurred since the satellite image acquired 3 days earlier. Snowcover appears to be somewhat more uniform than it was on 8 March (lower) - consistent with the timing and magnitude of snowfall recorded at the summit weather station.

At the Northern Icefield, satellite telemetry (Argos) shows ~12 cm of accumulation on 2 March, ~15 cm on the 3rd, and ~5 cm on each of the next 4 days. The precision of these daily totals will be improved when higher temporal resolution data are recovered from the automated weather station. Due to the diurnal cycle of climate on the mountain, some ablation likely also occurred on most of these days and is probably responsible for the patchier snowcover on the 8 March image.

A fascinating element of this snowfall period is provided by a depiction of regional-scale circulation (Fig. 3; Cameron Beccario credit). Here, airflow on the morning of 4 March is illustrated at the 500 hPa pressure level, equivalent to Kilimanjaro summit elevation. Airflow at this level appears to have been influenced by Tropical Cyclone Dumazile beginning on the 2nd as the storm intensified, continuing through about 7 March. The relationship between Kilimanjaro snowfall and cyclones in the southwest Indian Ocean is being investigated with collaborators Thomas Mölg and Emily Collier (Friedrich-Alexander University), along with Timba Nimrod.

On this figure, Kilimanjaro's location is shown by the green circle. Note the westerly wind, which prevailed through the snowy interval. Wind measurements at the summit (via telemetry) verify this airflow, which is atypical at the summit (only ~5% of hourly means are from 270° ±30°). Riming of the instruments appears to have occurred during the event, causing data loss particularly on the 3rd, 4th, and 6th. Nonetheless, such verification of airflow by in situ measurements is not a trivial finding - for very few continuous meteorological measurements exist from nearly 6000 m with which to compare output from numerical models.

Finally, figure 4 depicts circulation and humidity on 3 March. Here the highest humidity is shown in cyan color, suggesting a Congo basin origin for this precipitation event.

NIF surface, mid-February

Here is the Northern Icefield surface on 15 February, courtesy of Thomas Lämmle (EXTREK-africa). Our two UMass weather stations are visible on either side of the guides, with Mt. Meru in the background.

This image is particularly useful in documenting the glacier surface. Beneath the 2 ultrasonic snow sensors the surface is uniformly flat, with minor penitentes resulting from ablation of January accumulation. Therefore, further changes in height recorded at the station should be nicely representative of accumulation/ablation changes over a larger area.

Since our fieldwork in early October, net lowering of the surface has been ~15 cm. The current glacier surface at the AWS appears to be comprised of transformed seasonal snow, which is considerably brighter (i.e., higher albedo) than the immediately underlying ice.

Further ablation and lowering of the glacier surface will be determined by when the long rains begin, which typically occurs early in March.

Northern Icefield from Kenya

This is Kilimanjaro as viewed from the north this morning (Amboseli Park in Kenya, 8 AM on 25 February 2018). Part of the Northern Icefield is visible on the right-hand side of the summit. This northern portion, largely outside the crater (caldera) rim, has now separated from the southern part of the glacier which most climbers see from Uhuru Peak and within the crater.

Although the Northern Icefield is the largest glacier on the mountain, it is shrinking rapidly. The image below depicts the glacier about 20 years earlier; this is an aerial view looking south.

Thanks as always to Simon Mtuy for sending photos!

18 years!