Tuesday, May 15, 2018

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.

Friday, March 30, 2018

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!]

Wednesday, March 14, 2018

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.

Monday, February 26, 2018

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.

Sunday, February 25, 2018

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!

Saturday, February 24, 2018

Tuesday, February 20, 2018

January snowcover [updated]

The 2017 'short rains' season brought very little snow accumulation to the summit this year. Although snowfall is often variable during the short rains (typically November and December), this year was especially dry and resulted in a net lowering of glacier surfaces.

Early January brought the first noteworthy snow accumulation to the glaciers since the dry season began in June 2017. The graph above,
illustrating January snowfall, is based on snow measurements obtained from the Northern Icefield AWS by satellite telemetry. Such measurements must always be viewed cautiously, as wind redistribution of snow and other factors can complicate data interpretation - yet as average measurements from 2 sensors, their reliability is improved.

Snowfall is critically important to Kilimanjaro glaciers, primarily by controlling the amount of solar radiation reflected from the surface. When a bright snowcover exists, the reflectivity (albedo) is high; aging snow becomes gradually less reflective, and thinning snow allows radiation to penetrate through to the underlying glacier ice.

On the graph above, numbered squares correspond to a selection of natural-color images from the ESA (European Space Agency) Sentinel-2 satellite, shown below. These depict snow accumulation steadily increasing during early January, culminating in the events of 10 and 11 January. With little precipitation through the balance of the month and into early February, the mountain's snowcover then thinned and became patchier.

Here are a few notes on the Sentinel-2 images, which qualitatively demonstrate that
Kilimanjaro glaciers are sensitive to the magnitude, frequency, and spatial extent of precipitation events!

Image #1, 30 December 2017:  Clouds to the west of Kibo, filling the Western Breach and obscuring the southern slope glaciers. Within the caldera and south of the crater rim, uniformly-white areas are glaciers (likely with some snowcover). No snowcover is present within the caldera, with only a light dusting on the highest northern slopes, and some accumulation on the north-facing section of the caldera rim (e.g., below Uhuru Peak).
Image #2, 9 January 2018:  Extensive clouds, yet uniform snowcover is visible within the caldera. Note the lack of snow on a small portion of Reusch Crater (dark area), where geothermal heat flux is probably responsible for melting any accumulation. The graph above shows that this image represents snowcover prior to the two largest snowfall events of January, on the 10th and 11th.

Image #3, 14 January:  Little or thin cloud cover is present over the Kibo caldera, in contrast to the clouds obscuring the mountain's western and southern flanks. Snowcover is extensive, as suggested by the graph above.

Image #4, 24 January:  Scattered clouds surround the mountain at elevations below ~5,000 m. Snowcover from early January has thinned within the caldera and on the glacier (see graph), and generally become patchier. Albedo of the glacier surfaces remains high, reflecting most of the incoming solar radiation. The Reusch Crater is now mostly snow-free, and limited snow remains within the Western Breach. Climbers going to the summit on this date were likely walking on snow most of the distance above Stella Point.

Image #5, 8 February:  A typical February day on Kibo, with scattered clouds concentrated to the south and west. Snowcover patches within the caldera have shrunk further. Note the almost complete lack of snow on eastern and south-facing slopes; even at only 3° south latitude, solar radiation receipt is greater on south-facing than north-facing slopes in early February!

Image #6, 18 February (not shown on graph):  Without measurable snowfall since image #5 was acquired (see above), the extent of snowcover continues to decrease, lingering primarily on steeper north-facing slopes. During this time the Northern Icefield surface decreased in height by another 10 cm, and with ablation of January snow the surface is likely bare glacier ice once again. On the southern slope of this image, distinguishing between snow and ice is difficult for those unfamiliar with the glaciers, yet these ice bodies were all contiguous only ~10 years ago. As shown below - and explained in the 21 November post - this is no longer the case, as the glaciers continue shrinking.