Summer Air Temperature Lapse Rates in a Glaciated Alpine Catchment, Northwestern British Columbia, Canada
Dr. Sarah Boon (boon@unbc.ca)
Geography Program, University of Northern British Columbia, Prince George, BC, CANADA
Preliminary Findings cont’d
Methods
Introduction
a.
Air temperature lapse rates are critical for determining glacier contributions to melt in alpine catchments, and the potential impacts of climate change on these contributions, but are not often studied. Due to minimal field measurements of alpine air temperature lapse rates, they are often assumed constant at ‐6°C km‐1 (moist adiabatic lapse rate; MALR). Local lapse rates often differ from this value due to local topographic variability and the effects of glaciated terrain. b.
c.
Figure 3. Meteorological stations: (a) proglacial (585 m); (b) nunatak (1180 m); (c) ridgetop (1887 m).
It is thus important to assess temporal variability in lapse rates, their relation to the MALR, and the role of the glacier boundary layer in affecting lapse rate measurements. For this study, field meteorological measurements are used to calculate local air temperature lapse rates and determine their relationship to regional synoptic conditions.
From 13 Jul – 9 Sep, 2006, three Onset® meteorological stations were deployed within the Andrei Glacier catchment (Fig. 3a‐c): at 585 m asl, in the proglacial outwash plain ~2 km from the glacier terminus; at 1180 m asl on a nunatak 500 m south of the glacier; and at 1887 m asl on a bedrock ridge ~2 km north of the glacier margin (Fig. 1). Each station recorded hourly averages of air temperature, relative humidity, barometric pressure and rainfall. Hourly and daily air temperature lapse rates were calculated from field meteorological data. Distributed lags analyses of anomaly time series of daily temperature lapse rates and daily pressure identified relationships between surface and synoptic conditions. These relationships were further explored using 500 hpa daily synoptic maps from the Meteorological Service of Canada. Study site
Figure 7. Frequency histogram of daily air
temperature lapse rates between the proglacial
and ridgetop stations.
Figure 6. Progression of daily air temperatures
for each station.
Linking surface and upper‐air conditions
Comparison of 500 hpa synoptic configurations between days with a shallow (21 July: ‐3.2°C km‐1) and a steep (27 July: ‐8.1°C km‐1) proglacial‐ridgetop lapse rate reveal significant differences (Fig. 8). On 21 July a ridge of high pressure dominates, with strong pressure gradients and ~105 km hr‐1 WSW winds; on 27 July a low pressure cell covers the region, with weaker pressure gradients and ~65 km hr‐1 NW‐W winds.
Preliminary Findings
Andrei Glacier (56° 55ʹ N, 130° 55ʹ W) is an outlet glacier of the northeast side of the Andrei
Icefield, in the Boundary Ranges of northwestern British Columbia (Fig. 1). The glacier has a centreline length of ~13 km and is ~240 m thick at the terminus, with 50% of its surface area concentrated at 1200‐1400 m asl. BRITISH COLUMBIA
Dease Lk
Stewart
Air temperature
The proglacial station has the greatest range and standard deviation of daily air temperature (Table 1; Fig. 4), as it experiences cold air drainage during the afternoon/overnight period, and perhaps also katabatic winds3,4. Ridgetop air temperatures have the lowest range and standard deviation, as they likely respond largely to free‐air conditions. Daily air temperature anomalies are significantly correlated between the nunatak and ridgetop (r2 = 0.86; p < 0.05), but not the proglacial. Calculation of a 5‐day temperature anomaly trend increases correspondence between stations (Fig. 5). However, during periods of high positive anomalies on the ridgetop, the proglacial often has much lower anomalies (e.g., 19‐
25 July, 5‐6 August, 18‐22 August). This could be due to a thickening of the glacier boundary layer4, effectively decoupling the proglacial station from free‐air conditions.
Vancouver
Figure 1. Location and site map of Andrei Glacier; meteorological stations are marked.
The Boundary Ranges experience heavy precipitation: summers are cool and moist, winters are cool with heavy, wet snowfall. Orographic subsidence across the higher Alaska Coastal Range results in an eastward drying trend. The nearest long‐term Environment Canada climate stations are located in Stewart (55° 56ʹ N, 129° 59ʹ W; 7 m asl) and Dease Lake (58°
25ʹ N, 130° 0ʹ W; 807 m asl). The coastal climate is dominated by high annual precipitation
(1.84 m WE at Stewart) that decreases significantly inland (0.43 m WE at Dease Lake). Annual average temperature is moderated in the coastal setting, with 6°C at Stewart relative to ‐0.8°C at Dease Lake. B
Figure 8. 500 hpa synoptics on (a) 21 July and (b) 27 July, 2006. Red dots indicate Andrei Glacier location.
Next steps
Further research will involve calculation of atmospheric flow indices at the 500 mb level5,6
(zonal and meridional wind velocity, total flow strength, flow direction, and the vertical component of relative vorticity), and comparison of these values with measured surface variables. This robust, quantitative method is often used to examine relationships between surface and synoptic conditions, and can also assist in assessing the potential for lapse rate prediction using only synoptic information.
Previous research has revealed significant glacier retreat and downwasting since 19561, the year for which airphotos of the region are first available (Fig. 2). As the Iskut River is under consideration for power generation2, understanding regional glacier‐climate response and runoff variability is critical.
Figure 2. Ice thickness loss (in m) on
the Andrei Glacier from 1965-2000.
Outlines delineate ice extent in 1965
(yellow), 1988 (green), and 2000
(black).
A
Meteorological and lapse rate data will also be used to calculate melt timing and amount on Andrei Glacier using a degree‐day model, in combination with a renewed glacier mass balance program.
Acknowledgements
Figure 4. Daily air temperature at each station.
Air temperature lapse rates
Figure 5. 5-day temperature anomaly trend.
Ridge
Nun
Progl
Daily air temperature lapse rates range from Tmin
-1.24
3.89
5.56
‐9.7 to 0.8°C km‐1 and are highly variable (Fig. 6), Tmax
7.07
11.46
12.51
with a range of ±2 standard deviations. Lapse Tavg
2.35
7.54
10.14
rates are steeper between the nunatak‐ridgetop
σT
2.04
1.57
1.49
than the proglacial‐nunatak, although both are the same on 6 and 26 August. Daily proglacial‐
Range
8.31
7.57
6.95
ridgetop lapse rates are between ‐6 to ‐8°C km‐1
PDDs
115
362
487
on 52% of days, with a secondary peak (32%) July Tavg
3.26
8.49
10.96
between ‐4 to ‐6°C km‐1 (Fig. 7). Proglacial‐
August Tavg
1.76
6.91
9.61
ridgetop lapse rates have a 2‐day lagged response to changing surface pressure (r = 0.41, p < 0.05). Table 1. Summary statistics: daily air temperature data.
Further information: http://web.unbc.ca/geography/faculty/sarah
Research funded by a UNBC Faculty Seed Grant, and in‐kind support from both the Geological Survey of Canada’s National Glaciology Programme and the University of Victoria Tree‐Ring Laboratory. B. Tanasichuk and D. Lewis provided field assistance.
References
1. Wheate, R. and N. Alexander. 2006. Unpublished data; 2. Glassman, M. 2003. Forrest Kerr Hydroelectric
Project Assessment Report. BC Env. Assess. Off. 56 pp.; 3. Yoshino, M. 1984. Thermal belt and cold air drainage on
the mountain slope and cold air lake in the basin at quiet, clear night. GeoJournal 8(3):235-250; 4. Van Den Broeke, M.R. 1997. Structure and diurnal variation of the atmospheric boundary layer over a mid‐latitude glacier in summer. Bound‐Lay. Meteorol. 83: 183‐205; 5. Shea, J.M. and S.J. Marshall. In press. Atmospheric flow indices, regional climate and glacier mass balance in the Canadian Rocky Mountains. Int. J. Climatol., DOI 10.1002/joc.1398; 6. Marshall, S.J., et al. In press. Near‐surface‐temperature lapse rates on the Prince of Wales Icefield, Ellesmere Island, Canada: implications for regional downscaling of temperature. Int. J. Climatol., DOI 10.1002/joc.1396.