Snowfall Events on Mt Kilimanjaro

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INTRODUCTION This study was funded by grants from the National Science Foundation (ATM-0317693) and NOAA Office of Global Programs (NA03OAR4310046) to the University of Massachusetts, Amherst. Additional support for R. Y. Chan (chan@geo.umass.edu) was provided by the Significant Opportunities in Atmospheric Research and Science (SOARS®) program of the University Corporation for Atmospheric Research, with funding from the National Science Foundation, the U.S. Department of Energy, the National Oceanic and Atmospheric Administration, and the Goddard Space Flight Center, NASA. METHODS RESULTS CONCLUSIONS FUTURE WORK REFERENCES The close resemblance of the annual precipitation cycle shown by the Kilimanjaro snowfall data, GPCP, NCEP/NCAR reanalysis, and OLR datasets suggested that Kilimanjaro snowfall is most likely tied to large-scale circulation features Global reanalysis products provide useful tools for analyzing snowfall events Seasonality in precipitation: Long Rains : low wind conditions and west to east propagation of convective activity from the continental interior toward East African coastal region Short Rains : east to west moisture transport by strong easterlies Largest snowfall events occur under conditions of Low wind speed High specific humidity Repeat study using GPCP data Compare station data Verify representation of Kilimanjaro data Interannual variability Indian Ocean Zonal Mode El Niño-Southern Oscillation Black, E., Slingo, J., and Sperber, K. R., 2003. An observational study of the relationship between excessively strong short rains in coastal East Africa and Indian Ocean SST. Mon. Wea. Rev., 131, 74-94. Camberlin, P., and Philippon, N., 2002: The East African March-May rainy season: Associated atmospheric dynamics and predictability over the 1968-97 period. J. Climate, 15, 1002-1019. Clark, C. O., Webster, P. J., and Cole, J. E., 2003. Interdecadal variability of the relationship between the Indian Ocean Zonal Mode and East African coastal rainfall anomalies. J. Climate, 16, 548-554. Kaser, G., Hardy, D. R., Molg, T., Bradley, R. S., and Hyera, T. M., 2004: Modern glacier retreat on Kilimanjaro as evidence of climate change: Observations and facts. Int. J. Climatol., 24, 329-339. Mutai, C. C., and Ward, M. N., 2000: East African rainfall and the tropical circulation/convection on intraseasonal to interannual time scales. J. Climate, 13, 3915-3939. Thompson, L. G., Mosley-Thompson, E., Davis, M. E., Henderson, K. A., Brecher, H. H., Zagorodnov, V. S., Mashiotta, T. A., Lin, P. N., Mikhalenko, V. N., Hardy, D. R., and Beer, J., 2002: Kilimanjaro ice core records: Evidence of Holocene climate change in tropical Africa. Science, 298, 589-593. Vuille, M., M. Werner, R. S. Bradley, R. Y. Chan, and F. Keimig, 2005. Stable isotopes in East African precipitation record Indian Ocean zonal mode. Geophys. Res. Lett., 32, L21705, doi:10.1029/2005GL023876. Selecting Significant Snowfall Events Persistence criterion: a three-day period with at least two days of snowfall Synoptic- vs. local- scale events Day with the highest magnitude was selected as the center date, and the neighboring values (±2 days) were removed Superposed Epoch Analysis Spatial composites for all, or subgroups (e.g. seasons, long rains, short rains, top 10) of significant snowfall events The absolute and anomalous fields of interest (e.g. wind, omega, precipitation, moisture convergence) were averaged for ± 2 days of key date Data Comparison Relationships between individual variables were analyzed by using scatter plots 3.05º S, 37.5ºE, 5,895 m above sea level Highest snow-capped mountain in Africa Ice cap shrinking rapidly during the 20th century Expected to disappear between 2015 and 2020 Kilimanjaro East African Rainfall Strong seasonality controlled by ITCZ migration Two rainy seasons Mar-May (Long Rains) Oct-Dec (Short Rains) Figure 2. An aerial photo of the summit of Kilimanjaro with a linear plot showing the decrease of total area of ice on Kilimanjaro. Figure 1. Map showing the topography of East Africa. The red dash line indicates the equator and the red arrow showing the location of Kilimanjaro. Figure 3. Monthly mean rainfall for Equatorial East Africa (Clark et al., 2003) Significant Snowfall Events A total of 70 events were identified as significant snowfall events. These events represented ~73% of the total snowfall from 2000-2005. The top 10 snowfall events ( greater than 10 cm of snow accumulation) explained ~30% of the total snowfall recorded on Kilimanjaro. The seasonal distribution of the 70 significant snowfall events included: 32 events during long rains, 18 events during short rains, and 20 events during the intermediate dry seasons. Long Rains GPCP data (Figure 6) show a west to east propagating wave-like feature across the African continent associated with precipitation toward Kilimanjaro. At the same time, a precipitation band from the Indian Ocean moves eastward and merges with the continental moisture along the East African coastline. Figure 7 shows a cyclonic vortex associated with strong rising motion at 500 hPa over the western Indian Ocean and northern tip of Madagascar. While the absolute winds show an easterly component, the anomalous westerlies observed just south of the mountain indicate a weakening of the trade winds, which allows moisture influx from the continental interior toward Kilimanjaro. During large snowfall events, the zonal and meridional winds and the calculated total wind speed (Figure 8, left) are rather close to zero, indicating an association between low wind speed at 850 hPa and large snowfall events. The horseshoe structure exhibited in Figure 8 (right) indicates an association of strong winds with low specific humidity, and weak winds with high specific humidity. The structure of GPCP precipitation patterns (Figure 9) suggests that the short rains were dominated by a general continent-wide east to west moisture transport. This reveals the Indian Ocean as the main moisture source. In addition, precipitation appears to be concentrated along the coastal region as shown by the total and anomaly of GPCP data. The pressure-longitude cross-section in Figure 10 exhibits strong easterlies throughout the vertical profile with two primary regions of rising motion over the Indian Ocean and the continent. This is consistent with the notion of moisture transport from the Indian Ocean to East Africa. Data comparison Figure 5 compares Kilimanjaro snowfall data to global analysis products, GPCP and NCEP/NCAR reanalysis data. All three datasets produce similar annual cycles, although the intensity of precipitation differs significantly. The OLR data provides an additional means of validating the Kilimanjaro snowfall data and shows similar annual cycle. Figure 5. Precipitation time series of a) Kilimanjaro (in black), b) GPCP (in red), c) NCEP (in blue), and d) OLR (in green with inverted y-axis) for 2000-2005. Global analysis data were extracted over the grid point closest to Kilimanjaro (3ºS, 37ºE). Figure 4. Histogram of significant snowfall events as a function of snowfall totals on Kilimanjaro from 2000-2005. The bracket in red indicated the top 10 snowfall events in terms of magnitude. Figure 10. Composite of pressure-longitude cross-section for “short rains” season with totals (left) and anomalies (right) on day -1 and day (0) of snowfall event. The color represents zonal (u) wind with blue indicating easterlies and red indicating westerlies. Vertical motion (omega) is shown in contours, indicating regions of rising motion (dashed) and subsidence (solid). Red triangle represents the longitude and altitude of the summit of Mt. Kilimanjaro. Figure 9. Similar to Figure 6, but for “short rains” season. Short Rains Figure 8. Scatter plots of Kilimanjaro snowfall vs. horizontal wind speed (left) and specific humidity vs. zonal (u) wind (right), all at 850 hPa. The black markers show all 2000-2005 “long rains” days, and red markers indicate significant “long rains” snowfall events. Figure 7. Superposed epoch analysis for all “long rains” snowfall events 2000-2005 with total field (left) and anomalies (right) on day -1 and day (0) of snowfall event. Arrows are wind speeds (m/s) and direction at 850 hPa. The color is a measure of vertical motion (omega) at 500 hPa with cool colors representing upward motion and warm colors indicating subsidence. Red triangle indicates location of Mt. Kilimanjaro. Figure 6. GPCP precipitation composites of all “long rains” snowfall events 2000-2005 on day -1 and day (0) of snowfall event. The total field is shown on the left, and the color is a measure of precipitation intensity. The anomalies are shown on the right, and the color indicates positive (red) or negative (blue) anomaly. Triangle represents location of Mt. Kilimanjaro.