Comparison of Surface Mountain Climate with Equivalent Free-Air Parameters Extracted from
NCEP/NCAR Reanalysis, Mount Kilimanjaro, Tanzania
Nick Pepin1 Doug Hardy2 William Duane3 Mark Losleben4
1.Department of Geography, University of Portsmouth, U.K., nicholas.pepin@port.ac.uk
2.Glacial Systems Center, University of Massachusetts, Amherst, MA, U.S.A., dhardy@geo.umass.edu
3.Department of Geography, University Brunei Darussalam, Brunei, bduane@fass.ubd.edu.bn
4. National Phenology Network Office, University of Arizona, losleben@email.arizona.edu
Surface Temperature and Humidity Data
Six hourly air temperatures, relative humidities, and u and v wind
components were interpolated from the NCEP/NCAR reanalysis
version 1 available at CDC (ftp.cdc.noaa.gov) (Kistler et al. 2001)
to exactly the same locations as the sensors in the field. Since
surface data are not in the reanalysis model, this represents free
air conditions. The reanalysis is derived four times daily,
corresponding to 0300, 0900, 1500 and 2100 LST (or East
African Time). Graphs below show the
time series of
temperature, humidity and u (zonal) and v (meridional) wind
components interpolated to site 1 (1890 m) (left hand panel) and
site 10 (5803 m) (right hand panel) respectively, using daily data
at 1500 EAT for 2004 -2007.
Surface data consists of hourly observations recorded from September 2004 until October 2007 at ten
locations on the south-western slope of Kilimanjaro (Table 1). Ten Hobo loggers were programmed to
record air temperature and relative humidity every hour, installed in home made radiation shields on
tripods, 1.5 metres above ground level. Shields were orientated horizontally, with the long-axis
orientated north-south, to minimize solar heating.
Despite some missing data, due to vandalism
and/or logger malfunctions, we retrieved some data
at all locations, and continuous records (nearly
12,000 hourly observations) have been obtained at
sites 4, 8 and 10 for the 3 year period (see Duane et
al. 2008 for a discussion of the surface data).
2005
2006
Sub-alpine heathland
6
4050m
Alpine with limited vegetation
7
4570m
Alpine with limited vegetation
8
4970m
Bare rock
9
5470m
Bare rock
10
5800m
Ice field
30
The climate sensors range in elevation from 1890 m to 5803 m above
sea-level and are representative of a range of ecological
environments. The lower slopes (sensors 1-3) are covered by
rainforest, with relatively high precipitation and humidity. The trees give
way to giant heathers around the treeline (~3500 m) as represented by
site 4. The heathland becomes drier and more sparsely vegetated as
elevation increases, becoming a semi-desert landscape by site 7
(~4570 m). Above 5000 m there are areas of permanent snow and ice
cover and the summit site (5803m) in the crater of the volcano is
installed on the northern ice field.
5
-10
-5
0
-15
5
Difference = Surface Temperature Minus Free-Air Temperature: deg C
12
12
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3
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Mean Monthly Surface/Free Air Temperature Difference: Site 8: 1500 EAT
0
3
4
5
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7
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9
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12
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-4
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10
5
-2
0
2
Temperature Difference: deg C
-4
1
1
.8
.6
.4
20
0
5
10
Upper Air Flow Strength: m/s
15
20
-5
The graphs above show annual variation (January-December) of mean surface/free-air temperature differences
and thus indicate the extent to which the mountain acts as a heat source or heat sink in comparison with the free
atmosphere. Because of the tropical latitude, there is a strong diurnal contrast, with mostly positive differences
(surface warmer than the free atmosphere) during the day (1500 EAT) and negative differences (surface colder)
at night (0300 EAT). However daytime heating is concentrated above site 3. Up to this elevation the forest
absorbs most of the solar energy, preventing the surface from heating. Negative differences remain during the
day, especially in the southern winter (July/August) when low cloud is common. In contrast above treeline,
afternoon heating is dramatic, particularly strong at mid-elevations (sites 4 and 5). This is expected to be where
montane thermal circulation (upslope flow) is strongest. The seasonal contrast in heating, with largest
temperature excesses during the south-eastern monsoon in July/August (at all sites from 4 to 9) is marked. At
first glance this suggests that the thermal circulation maybe more developed during this season, but
consideration of moisture transport (see below) paints a different picture. Heating is very small at site 10, over
the ice field, because of the increased latent heat flux at this location.
Surface Vapour Excess/Deficit vs Free Air Flow Direction: Kilimanjaro Summit
Surface/Free-Air Vapour Pressure Difference: mb/10
6
0
5
Surface/Free-Air Temperature Difference: deg C
4
15
Surface Heating/Cooling vs Free Air Flow Direction: Kilimanjaro Summit
4
6
2
0
Temperature Difference: deg C
4
3
10
Upper Air Flow Strength: m/s
Mean Monthly Surface/Free Air Temperature Difference: Site 10: 1500 EAT
-2
2
5
12
0
-4
-6
-6
1
1
.2
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1
0
10
6
4
2
0
12
Mean Monthly Surface/Free Air Temperature Difference: Site 7: 1500 EAT
4
0
11
-2
6
-2
0
2
Temperature Difference: deg C
12
-2
2
Mean Monthly Surface/Free-Air Temperature Differences: Site 9: 1500 EAT
-2
Temperature Difference: deg C
-2
-4
Temperature Difference: deg C
11
-4
1
Mean Monthly Surface/Free Air Temperature Difference: Site 10: 0300 EAT
0
Mean Monthly Surface/Free-Air Temperature Differences: Site 9: 0300 EAT
2
10
1
11
9
.8
10
8
.6
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7
.4
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.2
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0
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4
-1 -.8 -.6 -.4 -.2
5
3
2
Temperature Difference: deg C
4
2
-4
4
2
Mean Monthly Surface/Free Air Temperature Difference: Site 6: 1500 EAT
-6
-6
-6
3
1
N
E
S
Free-Air Flow Direction
W
N
N
E
1
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10 11 12
1
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9
10 11 12
1
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6
7
8
Mean Surface/Free-Air Vapour Pressure Difference: 1500 EAT: Site 2 Surface/Free-Air Vapour Pressure Difference: 1500 EAT: Site 3 Mean Surface/Free-Air Vapour Pressure Difference: 1500 EAT: Site 4
0
.2
.4
.6
.8
Vapour Pressure Difference: mb/10
-.1
0
.1
.2
.3
Vapour Pressure Difference: mb/10
-.1
0
.1
.2
.3
Vapour Pressure Difference: mb/10
.3
.2
.1
0
-.1
Vapour Pressure Difference: mb/10
9
1
10 11 12
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10 11 12
1
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7
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9
10 11 12
2
3
4
.3
.2
.1
0
-.1
0
.1
.2
.3
Vapour Pressure Difference: mb/10
Surface/Free-Air Vapour Pressure Difference: 0300 EAT: Site 9Mean Surface/Free-Air Vapour Pressure Difference: 0300 EAT: Site 10
1
5
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8
9
10 11 12
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9
10 11 12
1
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5
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7
8
0
.2
.4
.6
.8
Vapour Pressure Difference: mb/10
.2
.1
0
-.1
5
9
1
10 11 12
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3
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10 11 12
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10 11 12
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10 11 12
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10 11 12
Surface/Free-Air Vapour Pressure Difference: 1500 EAT: Site 9Mean Surface/Free-Air Vapour Pressure Difference: 1500 EAT: Site 10
.8
8
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10 11 12
Surface/Free-Air Vapour Pressure Difference: 1500 EAT: Site 5 Mean Surface/Free-Air Vapour Pressure Difference: 1500 EAT: SiteMean
6
Surface/Free-Air Vapour Pressure Difference: 1500 EAT: Site 8
.4
6
.3
Vapour Pressure Difference: mb/10
.2
.1
0
-.1
5
3
.2
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2
0
3
Vapour Pressure Difference: mb/10
2
.3
Vapour Pressure Difference: mb/10
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.2
.1
0
-.1
1
-.1
Vapour Pressure Difference: mb/10
Vapour Pressure Difference: mb/10
N
Monthly Surface/Free-Air Vapour Pressure Differences: 1500 EAT
Monthly Surface/Free-Air Vapour Pressure Differences: 0300 EAT
Surface/Free-Air Vapour Pressure Difference: 0300 EAT: Site 5 Mean Surface/Free-Air Vapour Pressure Difference: 0300 EAT: SiteMean
6
Surface/Free-Air Vapour Pressure Difference: 0300 EAT: Site 8
Such variations in the temperature contrast show that the mountain creates its own thermal circulation pattern
(Troll & Wien 1949). This has implications for vapour pressure excesses reported at the surface (graphs to the
right). At all elevations there is on average greater vapour pressure at the mountain surface than in the free
atmosphere, with surface vegetation providing an importance source through evapotranspiration. Thus mean
vapour pressure excesses decrease with elevation. The northern ice field (site 10) has very little influence on
atmospheric moisture, the largest excesses being only 1 mb. There is also a large diurnal increase in vapour
pressure excess at all sites, illustrating the importance of the thermal circulation in transporting moisture to the
upper mountain. This is particularly effective in the wet seasons but strangely less so in July/August despite the
largest mountain temperature excesses being reported then. In summary much of the atmospheric moisture at
the highest elevations is likely to have originated from the lower forest zone and not the free atmosphere.
W
Surface/Free-Air Vapour Pressure Contrasts
Mean Surface/Free-Air Vapour Pressure Difference: 0300 EAT: Site 2 Surface/Free-Air Vapour Pressure Difference: 0300 EAT: Site 3 Mean Surface/Free-Air Vapour Pressure Difference: 0300 EAT: Site 4
At night all sites show a cooling below the free atmosphere but again this effect is subdued at the lowest sites,
protected from radiative loss by the forest canopy, and perhaps surprisingly at sites 8 and 9, although sky view
factors are reduced somewhat here on the side of the steep volcanic cone.
S
Free-Air Flow Direction
.8
11
-2
0
Temperature Difference: deg C
-4
-2
Temperature Difference: deg C
-4
-2
Temperature Difference: deg C
-4
-2
Temperature Difference: deg C
-2
-4
Temperature Difference: deg C
-6
2
100
80
0
20
40
60
20
40
Free Air Humidity:
5803 m:60Station 10: 80
%
Free Air Humidity: 1890 m: Station 1: %
10
10
2007
2004
2005
2006
time
2007
2004
2005
2006
2007
time
1
2
3
4
5
6
7
8
9
10 11 12
1
2
3
4
The summit ice fields of Kilimanjaro are undergoing rapid shrinkage (Cullen et
al. 2006) and if current rates of decrease continue (a big assumption) the
summit snows may disappear by 2020 (Thompson et al. 2002). There is
debate as to the relative contributions of large scale global forcing and
local/regional drying (which may or may not be related to deforestation on the
lower slopes) (Hemp 2005, Kaser et al. 2004, Molg & Hardy 2004, Molg et al.
2003). Our comparison of surface climate with free atmospheric properties
shows that the mountain creates its own atmosphere, with enhanced moisture
availability and thermal heating by day giving way to cooling at night. This
circulation transports moisture from the lower slopes to the vicinity of the
glacier, ultimately controlling ice accumulation in the form of precipitation.
The main findings of our study are listed below:
a) Below 2500 m, despite the existence of tropical rainforest, at least some of the variability in surface and free-air temperature
anomalies is correlated, and a common response to synoptic forcing. Above treeline however, strong localised patterns of
surface heating and cooling decouple Kilimanjaro mountain climate from that of the free atmosphere, almost every day and
irrespective of synoptic scale forcing.
b) There is a strong thermally-induced circulation on Kilimanjaro, importing moisture upslope during daylight hours, well above
that available in the free atmosphere. Thus daytime temperature differences of over 5 deg C (compared with the free
atmosphere) and vapor pressure excesses in excess of 5 mb are reported. Although these excesses decline with elevation,
they are still present at the summit of the mountain.
c) Fluctuations in upper air flow strength and direction (at 500 mb) appear to have limited influence on the strength of surface
heating and upslope moisture advection, which is dominated by the diurnal radiative cycle rather than inter-diurnal synoptic
controls.
Taken together these findings suggest that local changes in land-use such as deforestation (Hemp 2005) could have a strong
influence on the mountain climate and summit ice of Kilimanjaro, making it somewhat decoupled from larger scale advective
changes. In addition, local observations are crucial since interpolation from free atmospheric models is problematic.
.6
10
.4
9
.2
8
0
7
.8
6
.6
5
.4
4
Vapour Pressure Difference: mb/10
3
6
0
0
0
0
2
Mean Monthly Surface/Free-Air Temperature Differences: Site 5: 1500 EAT
Vapour Pressure Difference: mb/10
1
1
4
6
4
12
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11
.6
10
.4
9
.2
8
0
7
.8
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.6
5
.4
4
.2
3
0
2
Mean Monthly Surface/Free Air Temperature Difference: Site 8: 0300 EAT
.8
1
.6
12
.4
11
.2
10
-1 -.8 -.6 -.4 -.2
9
Surface/Free-Air Vapour Pressure Difference: mb/10
8
0
7
-5
6
-4
5
6
4
4
3
Mean Monthly Surface/Free Air Temperature Difference: Site 7: 0300 EAT
2
2
-4
-6
1
0
12
-2
11
The relationships between
surface/free-air temperature
and moisture contrasts at the
top of the mountain (site 10)
and the free-air (synoptic
scale) forcing are weak.
Although large excess surface
heating and vapour pressure
excesses cannot occur with
high gradient wind speeds
(see top two graphs), low
speeds do not necessarily
mean
such
differences
develop. Perhaps surprisingly,
upper air flow direction
(bottom two graphs) has
relatively little influence on the
extent to which the mountain
decouples itself from the freeatmosphere.
Surface Vapour Excess/Deficit vs Free Air Advection: Kilimanjaro Summit
-4
10
Surface/Free-Air Temperature Difference: deg C
9
Temperature Difference: deg C
8
6
7
4
6
2
5
0
4
-2
3
Mean Monthly Surface/Free Air Temperature Difference: Site 4: 1500 EAT
-4
2
Mean Monthly Surface/Free Air Temperature Difference: Site 6: 0300 EAT
Temperature Difference: deg C
1
Mean Monthly Surface/Free-Air Temperature Differences: Site 3: 1500 EAT
-2
0
2
Temperature Difference: deg C
2
0
Temperature Difference: deg C
-2
-4
-4
4
Temperature Difference: deg C
6
Mean Monthly Surface/Free-Air Temperature Differences: Site 2: 1500 EAT
Temperature Difference: deg C
0
0
Mean Monthly Surface/Free Air Temperature Difference: Site 1: 1500 EAT
-2
12
Surface Heating/Cooling vs Free Air Advection: Kilimanjaro Summit
Mean Monthly Surface/Free-Air Temperature Differences: 1500 EAT
-4
11
2006
time
Conclusions and Implications for Ice Sheet Mass Balance
Influence of Upper Level Flow Regime
Mean Monthly Surface/Free Air Temperature Difference: Site 4: 0300 EAT
-6
-6
10
2005
Figure 1. Distribution of Hobo Logger Stations, SW slope of Kilimanjaro
.2
-5
0
Surface Temperature: deg C
0
-10
-2
Temperature Difference: deg C
-2
-4
-2
-4
Temperature Difference: deg C
Temperature Difference: deg C
9
2004
Relative humidities and moisture availability decrease with
elevation, particularly in winter, when extremely dry air is
common at high elevations. Changes in mean zonal and
meridional wind are complex. On average free-air flow has an
easterly component, but this becomes stronger at low elevations
in summer, and high elevations in winter. Meridional flow on the
other hand has a strong seasonal cycle at low elevations,
indicative of the northerly (winter) and southerly (summer)
monsoons. This becomes much less distinct at high elevations.
Vapour Pressure Difference: mb/10
-15
Mean Monthly Surface/Free-Air Temperature Differences: Site 3: 0300 EAT
0
0
8
2007
Temperatures show the most obvious contrast, and are
universally warmer in summer, although the annual cycle is
subdued at higher elevations, meaning steeper lapse rates during
this season.
Vapour Pressure Difference: mb/10
Free-Air Temperature: deg C
10
Mean Monthly Surface/Free-Air Temperature Differences: Site 2: 0300 EAT
-6
7
2006
There are clear differences in the free air behaviour between the
top and the bottom of the mountain. In particular seasonal
signatures change with elevation. In the following discussion,
because Kilimanjaro is in the southern hemisphere, summer
refers to January/February and winter July/August.
0
20
10
6
2005
time
Vapour Pressure Difference: mb/10
10
Surface Temperature: deg C
Free-Air vs Surface Temperature: Site 10: 5803 m: r=0.052
-5
0
5
Free-Air Temperature: deg C
2004
5
3630m
0
5
2007
-5
Transitional zone between rainforest and sub-alpine heathland
2006
-10
3170m
2005
Meridional wind: southerly is positive
5
4
Meridional wind: southerly is positive
0
Sparse montane rainforest
2004
time
-5
2760m
2007
-10
3
30
10
0
Mean Monthly Surface/Free-Air Temperature Differences: 0300 EAT
5
2006
Free Air Meridional Wind - m/s
Zonal wind: westerly is positive
-10
Free Air Meridional-5
Wind: 1890 m: Station 1: m/s
0
5
Dense montane rainforest
0
-10
Difference = Surface Temperature Minus Free-Air Temperature: deg C
4
2005
time
-15
2340m
Free Air Zonal Wind: 5803 m: Station 10: m/s
-10
-5
0
5
2
-15
Dense montane rainforest
Free Air Zonal Wind: 1890 m: Station 1: m/s
1890m
Surface/Free-Air Temperature Contrasts
3
2004
10
Zonal wind: westerly is positive
-10
30
The graphs above show the correlations between simultaneous
surface and free-air temperatures at sites 1, 4, 7 and 10. As
elevation increases decoupling intensifies, with the mountain
regime being increasingly independent of that in the freeatmosphere. The major change takes place above treeline where
local heating/cooling is induced more by the strong equatorial sun
and less by advection.
2
2007
20
Free-Air Temperature: deg C
30
15
20
25
Free-Air Temperature: deg C
10
25
0
5
Surface Temperature: deg C
Mean Monthly Surface/Free-Air Temperature Differences: Site 5: 0300 EAT
2006
Free Air Meridional Wind: 5803 m: Station 10: m/s
Site description
1
Free-Air vs Surface Temperature: Site 4: 3172 m: r=0.182
Free-Air vs Surface Temperature: Site 7: 4568 m: r=0.038
1
2005
time
10
Elevation
Table 1: Elevation and Site Characteristics of the 10 stations
15
20
Surface Temperature: deg C
Mean Monthly Surface/Free Air Temperature Difference: Site 1: 0300 EAT
2004
Free Air Zonal Wind - m/s
Surface versus Free-Air Coupling
-5
0
2007
time
Some consequences for the mass balance of the summit
ice fields are discussed.
10
100
30
20
2004
Logger No.
Free-Air vs Surface Temperature: Site 1: 1890 m: r=0.753
Free Air Relative Humidity - %
-10
-10
0
10
20
Free Air Temperature: 5803 m: Station 10: deg C
30
Free Air Temperature - deg C
Free Air Temperature: 1890 m: Station 1: deg C
This study examines the case study of Kilimanjaro, in East
Africa, the largest free-standing mountain in the world.
Surface temperatures and vapour pressures, as measured
by loggers installed on the south-west slope of the mountain
(Duane et al. 2008) are compared with free-air climate
measured at the same elevation. Unsurprisingly the
mountain modifies the climate considerably, creating its own
distinctive atmosphere, and responding strongly to the
intense radiation input at this equatorial location. This
means that the surface regime becomes decoupled from
the free atmosphere, especially above treeline.
10
Predicting and understanding future changes in climate in
mountainous regions requires an understanding of how the
mountain surface may become decoupled from the free
atmosphere, along with surface feedback effects (Bradley et
al. 2006). Most GCM’s predict future changes in climate
based on simulation of the physics of the free atmospheric
response to changes in greenhouse gases. Unfortunately
surface complexities in mountain regions, and poor grid
resolution make it difficult to downscale such predictions in
mountain areas, and temperature trends in mountain areas
are different to those in the free atmosphere (Pepin &
Seidel 2005).
NCEP/NCAR Reanalysis (Free-Air)
0
Introduction
5
6
7
8
9
10 11 12
1
2
3
4
5
6
7
8
9
10 11 12
References
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