Comparison of Surface Mountain Climate with Equivalent Free-Air Parameters Extracted from NCEP/NCAR Reanalysis, Mount Ki

1. Surface/Free-Air Temperature Contrasts 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. 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. 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. Surface/Free-AirVapour Pressure Contrasts • 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 • Department of Geography, University of Portsmouth, U.K., nicholas.pepin@port.ac.uk • 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 Introduction 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). 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. Some consequences for the mass balance of the summit ice fields are discussed. Surface Temperature and Humidity Data 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. NCEP/NCAR Reanalysis (Free-Air) 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. 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). Surface versus Free-Air Coupling 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. 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. 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. Table 1: Elevation and Site Characteristics of the 10 stations 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. 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 free-atmosphere. The major change takes place above treeline where local heating/cooling is induced more by the strong equatorial sun and less by advection. Figure 1. Distribution of Hobo Logger Stations, SW slope of Kilimanjaro • Conclusions and Implications for Ice Sheet Mass Balance • The main findings of our study are listed below: • 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. • 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. • 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. • References • Bradley, R.S., Vuille, M., Diaz, H.F. & Vergara, W. (2006),Science312, 1755-1756. • Cullen, N.J., Mölg, T., Kaser, G., Hussein, K., Steffen, K. & Hardy, D.R., (2006). • Geophys. Res. Lett., 33, L16502, doi:10.1029/2006GL027084 • Duane, W.J., Pepin, N.C., Losleben. M.L. & Hardy, D. (2008). AAAR, In press. • Hemp, A. (2005). Global Change Biology11, 1013-1023. • Kaser, G., Hardy, D.R., Mölg, T., Bradley, R.S. & Hyera, T.M.  (2004).  Int. Jnl Clim.24, 329-339. • Kistler, R. et al. (2001). Bull. Amer. Met. Soc.82, 247-268. • Pepin, N.C. & Seidel, D.J. (2005). J. Geophys. Res.110, D03104: doi:10.1029/2004JD005047. • Mölg, T., Hardy, D.R. & Kaser, G. (2003). J. Geophys. Res.108, D23,4731: • doi:10.1029/2003JD003546. • Mölg, T. & Hardy, D. R. (2004). J. Geophys. Res.109, D16104, doi:10.1029/2003JD004338. • Thompson, L. et al. (2002), Science298, 591-593. • Troll, C. & K. Wien, (1949). Geog. Ann.31, 257-274. Influence of Upper Level Flow Regime 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 free-atmosphere. 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.