THE ENERGY BALANCE OF A MELTING SNOWPACK IN THE FRENCH PYRENEES DURING WARM ANTICYCLONIC CONDITIONS

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 16, (1996) THE ENERGY BALANCE OF A MELTING SNOWPACK IN THE FRENCH PYRENEES DURING WARM ANTICYCLONIC CONDITIONS GLENN R. MCGREGOR AND ANNE F. GELLATLY School of Geography, The University of Birmingham. Edgbaston. Birmingham B15 2T?: UK Received 20 Januaty 1995 Accepted 20 July 1995 ABSTRACT The bulk aerodynamic approach was used to measure the energy balance over an isothermal melting snowpack at 2600 m a.s.1. during warm anticyclonic conditions in the French Pyrenees. Net radiation contributed the majority of energy for melt (67 per cent), whereas sensible heat (33 per cent) played a secondary but, nevertheless, important role, because air temperatures were anomalously high during the measurement period. Latent heat as a heat source for melt or as a heat sink for evaporation was insignificant. Study results are discussed in the context of possible synoptic-scale forced energy balance controls on ablation climates in the Pyrenees. KEY WORDS: French Pyrenees; energy balance; bulk aerodynamic approach; snowmelt; synoptic climatology. INTRODUCTION The cascade of atmospheric processes from the synoptic to local scale is important for determining the nature and magnitude of alpine boundary layer climatic processes and the hydrological response of alpine terrain dominated by snow and ice. However, little is understood about the way in which alpine surfaces respond to the thermodynamic and radiative transfer processes in the lower troposphere. To increase our understanding of the ways in which large-scale atmospheric processes possibly force surface climates at the catchment level in alpine environments, descriptions of heat and moisture fluxes over alpine snow and ice surfaces are required for a wide range of synoptic conditions. Such descriptions are of utility not only for 'understanding atmospheric thermodynamics and physics, but also for parameterization or explicit specification of boundary layer processes in meso- to macroscale climate models, modelling glacier and snow-melt runoff, alpine water resource planning, validation of general circulation model results, and climate change studies. The objective of this paper is to evaluate the relationship between snowpack ablation and synoptic-scale climatological processes for a 4-day period characterized by exceptionally warm anticyclonic conditions in the French Pyrenees. This will be done by describing the vertical fluxes of energy and mass over a melting snowpack and discussing these in relation to the prevailing synoptic conditions. This specific objective forms part of a broader aim, which is to model the hydrological response of glacierized basins in the French Pyrenees to potential climate change through evaluating linkages between synoptic-scale circulation patterns and moisture fluxes and surface hydroclimatological processes. This aim is justified as only when a full understanding of the nature of the linkages between synoptic-scale circulation patterns and hydroclimatological processes in glacierized basins is developed, can suitable transfer functions be constructed and then applied to general circulation model output for the assessment of the impact of climate change on snow and ice resources in alpine environments. The Pyrenees was chosen as the study area because it is considered an ideal area for assessing the linkages between snow and ice ablation and accumulation processes and synoptic climatology in the context of climate change studies. This is because the glaciers and snowpacks in this area are extremely sensitive to climate, as manifest by the disappearance of a number of glaciers over the last century (Gellatly et al., 1995). The study of climatically sensitive snow and ice environments, such as the Pyrenees, is important because shifts in the CCC /96/ by the Royal Meteorological Society

2 480 G. R. MCGREGOR AND A. F. GELLATLY climatology of synoptic and surface hydroclimatological processes in such marginal snow and ice environments may represent an early signal of increased climatic variability or anthropogenically forced climate change. STUDY AREA Measurements for the evaluation of the surface energy balance were carried out over the snow covered Taillon Glacier in the Cirque du Gavarnie at 43"6'N, 0" 1O'W. The snout of the glacier lies at 2526 m above sea-level and 144 km from the Atlantic Ocean (Figure 1). The glacier has a north-east aspect and covers an area of 20 h. It is the third largest of 50 remaining glaciers in the Pyrenees massif and currently has a negative mass balance (Gellatly et al., 1995). The nearest permanent climate station is located at the Pic du Midi at 2862 m above sea-level (43"4'N, 0"9'E), 10 km to the north of the Taillon Glacier. Bucher and Dessens (1991) have presented a detailed analysis of an 89-year temperature record for this location and have shown that the mean annual temperature has increased by 0.8"C, due mainly to daily minimum temperature increases of 2.1"C and daily maximum decreases of 04 C. The mean temperatures ( ) for the warmest and coldest months (August and January) are 7.0"C and -7.5"C respectively. Mean annual precipitation is 925 mm. July and December are the driest (53 mm) and wettest months (106 mm) respectively (Bucher and Dessens, 1991). McGregor et al. (1995) have shown for the winter accumulation season marked negative departures of precipitation since the mid- 1950s. This trend of below average precipitation in conjunction with increasing regional temperatures most likely accounts for the decreasing extent and number of glaciers in the Pyrenees massif. METHODS The energy and mass exchanges over a melting snow surface are described utilizing the energy balance concept (Oke, 1987). The energy balance at the snow surface during melt, which may be written as Qm = Q* + Qh + Qe + Qp + Qg(W mp2) where Qm is the energy available for melt, Q* is the net radiation, Qh is the sensible heat flux, Qe is the latent heat flux, Qp is the precipitation heat flux, and Qg is the subsurface heat flux, was solved for the 4-day period 29 June 1994 to 2 July 1994 (day 1 to day 4). On an instantaneous time basis, terms in the energy balance equation are expressed as energy flux densities (W mp2). On a daily basis flux densities are reported as MJ m-'. Following Ishikawa et al. (1992) positive fluxes are those towards the surface. The methods used for measuring or estimating the components of the energy balance equation are those outlined by Hay and Fitzharris (1988) and Ishikawa et al. (1992). Net radiation (Q*) and two of its components, incoming and reflected shortwave radiation, were measured directly with a net pyrradiometer and pyranometers respectively. The longwave components of the net radiation balance were derived following Ishikawa et al. (1992) and Saunders and Bailey (1994). For the determination of the turbulent heat fluxes (Qh and Qe) the bulk aerodynamic approach was used. During periods of snow melt, snow surface temperatures are at or near O"C, with surface humidities near saturation. This situation makes the bulk aerodynamic approach appropriate for determining the turbulent fluxes of sensible and latent heat. The bulk aerodynamic approach has been applied widely to the problem of assessing turbulent fluxes over melting snowpacks (Hicks and Martin, 1972; Prowse and Owens, 1982; Grainger and Male, 1978; Hay and Fitzharris, 1988; Marks and Dozier, 1992; Kang et al., 1992). Associated with the determination of turbulent fluxes using the bulk aerodynamic approach are a number of assumptions. These include the validity of the bulk aerodynamic equations for neutral conditions only, and the similarity of bulk exchange coefficients for water vapour, heat, and momentum. If surface stability conditions are not neutral, the exchange coefficients must be corrected. To evaluate stability conditions the dimensionless bulk Richardson number (Rb) was used as described by Oke (1987). Based on values of Rb the generalized bulk exchange coefficient was corrected (Thom, 1975; Ishikawa et al., 1992).

3 ENERGY BALANCE OF A MELTING SNOWPACK 48 1 Vallee de Pouey Aspc '. -GI Sud-Est Taillon L. de Roland \ kilometre I I i Key a Glacier a Bedrock Terrace,-. Moraine A-UL Ice Limit Bedrock slopes km u 3352 / Figure 1. a) General location of study area. (b) The Glacier du Taillon and Cirque de Gavarnie

4 482 G. R. MCGREGOR AND A. F. GELLATLY For the study period, temperature inversions characterized the surface layer, with conditions of stability and weak horizontal motion predominant. The bulk exchange coefficient averaged during the study period, with a standard error of 0-002, and is similar to values reported by Kuhn (1979), Gray and Male (1981), Hay and Fitzhanis (1988), and Ishikawa and Kodama (1994). The roughness length parameter required for the calculation of the bulk exchange coefficient was evaluated from wind profile measurements during periods of neutrality. This value was found to be m, with a standard error of Zero precipitation was recorded for the study period, thus negating the necessity for consideration of precipitation heat flux (Qp) in the solution of the energy balance. Subsurface heat flux (Qg) was also zero, as confirmed by heat flux plates and thermocouples placed at 0.01 m, 0.1 m, and 0.15 m in the snowpack. Snow-melt heat flux (em) was found by measurement and estimation. A snow stake ablation network was used to measure Q,,. Twenty ablation stakes in a regular grid were set up covering an area of 400 m2. Stakes were monitored at 2-h intervals. Ablation measurements were used to calculate daily ablation totals. Based on measures of daily ablation and snow density, Qm was calculated as follows: where Mi is the mean snow ablation (m day-'), pi is the snow density (340 kg m-3), and Lf is the latent heat of fusion (0.333 MJ kg-'). In addition to measuring Q,, with the ablation stake network, Qm was estimated using the energy balance equation. Meteorological sensors for measurement of air temperature, humidity, wind speed, atmospheric pressure, and the measured components of Q* were scanned at 10-s intervals using a Campbell Scientific 21XL logger, with meteorological variables averaged and stored for 15-minute intervals. Energy balance calculations were based on these 15-minute averages. Specific humidity (g kg - ') was estimated from humidity measurements (McIlveen, 1992). Based on instrument error terms, errors of 22 per cent for Qh and Qe were estimated. This is close to that suggested by Moore (1983) for turbulent flux measurements estimated by the bulk aerodynamic approach. Violation of the assumptions associated with the bulk aerodynamic approach are also likely to generate errors (Hay and Fitzhanis, 1988). Because quantitative estimates of assumption-related error magnitudes are not available (Moore, 1983; Hay and Fitzharris, 1988), subjective assessments of the extent to which measurement conditions depart from those assumed in the application of the bulk aerodynamic approach must be made. For the study period advection is considered to be unimportant owing to fetch lengths in excess of 350 m and an associated constant flux layer depth of 3.5 m (Gray and Male, 1981). Because measurements over the melting snowpack were made at 1.O m and 2.0 m, they were well within the estimated constant flux layer, thus avoiding advective fluxes. Average wind speeds were also low during.the study period thus reducing the likelihood of advection. Because stable conditions prevailed it is assumed that errors due to inequality of exchange coefficients are minimal. General weather during study period RESULTS The synoptic situation for 29 June to 2 July 1994 was dominated by a ridge of high pressure at 700 hpa stretching from north Africa. This brought a flow of warm south-westerly air into the Pyrenees region, with 700 hpa temperature reaching 11 C. Strong subsidence dominated the Pyrenees, with related stable conditions and freezing levels of 4000 m. Temperatures during the study period remained well above freezing, with a study period average of 13.9"C. Minimum temperatures were high, considering the absence of nocturnal cloud cover, with an average of 12.9"C. The 15-minute maximum and minimum temperatures recorded were 18.6"C and 9.8"C respectively. Relative humidities averaged 46 per cent but demonstrated a decrease during the study period equivalent to 0.24 g kg-' day-' specific humidity. Wind speeds during the study period averaged 2.5 m s-', with 15-minute averages ranging between 0.83 and 4.46 m s-'. Maximum wind speeds, reaching 10.1 m s- ' on a daily basis, all occurred during the falling limb of the diurnal temperature cycle, suggesting that these pulses of air were of a katabatic origin.

5 ENERGY BALANCE OF A MELTING SNOWPACK F OoO E c v) n X LL 5 F r-.~ Day 1 Day 2 Day 3 Da;4- Figure 2. Radiation balance components for the 4-day study period. Q* is net radiation, K, and KO are incoming and reflected shortwave radiation respectively, and L* is net longwave radiation Table 1. Radiation balance components (MJ m-2). K and L are shortwave and longwave radiation respectively. Subscripts i and o refer to incoming and outgoing respectively. Q* is net radiation and a is albedo Day Ki KO a 4 LO Q* Mean Energy balance components Radiation. Apart from some transient cumulus and cirrus, cloudless skies predominated. Sunshine duration was 69 per cent of that possible owing to high solar altitudes in comparison to the local horizon. Solar radiation inputs were high, with a daily average of 28.7 MJ m- (Table I). Instantaneous inputs reached a 15-minute maximum of 1058 W m- at 1400 hours on day 3 (Figure 2). This is equivalent to 85 per cent of the possible incident shortwave (1 124 W m- ) for the study location and time of year. Maximum temperatures lagged maximum shortwave peaks by approximately 3.5 h. Because the snow surface was clean, apart from some mild discoloration due to dust, albedos were high, averaging Reflected shortwave was therefore high, averaging MJ m- day- (Table I). Outgoing longwave radiation at 267 W m- exceeded incoming longwave for the duration of the study period, producing a negative net longwave balance (Figure 2). Net radiation followed the diurnal cycle of incoming shortwave closely, and reached an instantaneous maximum of 398 W m- on day 3 (Figure 2). Maximum net radiation losses up to -47 W m-* occurred around 0700 hours, coincident with minimum temperatures. Although shortwave inputs were high, net radiation totals (Table I) remained low owing to the high snow surface albedo.

6 484 G. R. MCGREGOR AND A. F. GELLATLY Table 11. Energy balance components (MJ m-*). Q*, Qh, and Qe are net radiation, sensible heat, and latent heat respectively. Q,(est.) and Qm (meas.) are snow-melt heat as estimated by the energy balance method and measured by the ablation stake network respectively Q* Qe Mean Table 111. Percentage contribution of energy balance components to melt heat Mean Turbulentjluxes. Most noticeable was the unimportance of Qe as either a heat sink or heat source through evaporation or condensation respectively. Daily contributions of Qe were negligible, accounting for only a few thousandths of a megajoule. This represents infinitesimal heat gain due to condensation (Table 11). The only important turbulent heat input to the snow surface was Qh (Table 11). However, in percentage terms, Qh contributed from 25 to 42 per cent of the heat to the surface for melt (Table 111). On a diurnal basis Qh was the only nocturnal heat source, because Q* was negative. Pulses of nocturnal Qh often were associated with bursts of katabatic drainage. During the day Q* exceeded Q h (Figure 3). Although air temperatures were high, wind speeds were generally low, thus limiting the turbulent transfer of heat to the surface. Snow-melt heat. On all days snow ablation reached a maximum between 1400 and 1600 hours, when Q* was approaching a maximum. Daily energy consumption for melt, as calculated from the ablation stake network, is compared with that estimated using the energy balance approach in Table 111. Little day-to-day variability in the energy consumed in snow melt exists because meteorological conditions were stable throughout the study period, with similar daily energy inputs to the snow surface. Energy consumed in melt reached a maximum on day 3, coincident with the period of maximum energy inputs. Average total ablation measured over the ablation network during the study period was 535 mm, whereas that estimated was 518 mm. The estimated total falls within two standard errors of that measured. DISCUSSION AND CONCLUSIONS This study was conducted under somewhat ideal conditions of an extremely stable, almost cloudless atmosphere, slack upper level pressure gradients, and within several days of the summer solstice, when maximum insolation amounts may be expected. For this reason ablation totals were high and measured ablation parallels closely that estimated by the energy balance approach, with a root-mean-square error of 0.26 MJ m-2. Although the agreement of calculated and measured Qm is of interest, it is perhaps the partitioning of the available energy for melt that is of greater interest, because this reflects the nature of the synoptic-scale atmospheric processes at the time and sheds some light on atmosphere-surface climate interaction.

7 ENERGY BALANCE OF A MELTING SNOWPACK 485 Figure 3. Energy balance components for the 4-day study period. Q* is net radiation, Qh and Qe are sensible and latent heat respectively It is perhaps of no surprise, given the meteorological conditions, that Q* dominated, contributing over the study period 67 per cent of the energy inputs to the surface. Despite high temperatures, low wind speeds constrained the sensible heat flux to an average of 33 per cent of the energy available (Table 111). Although Qh was exceeded by Q*, it is somewhat higher than expected given the altitude at which the study was conducted. This is because of largescale upper level advection of warm air into the study area from the African continent. Consequently, air temperatures during the study period were anomalously high, with positive anomalies of 6"-8"C (NOAA, 1994). Because of the large-scale advection, sensible heat in the air mass overlying the study area was not altitude limited, thus giving rise to this exceptionally warm event. This points to the importance of air-mass enthalpy characteristics and anomalously warm events produced by an anticyclonic synoptic regime for enhancing the available energy for snow melt or for satisfying snowpack cold deficits in alpine environments, where Qh is otherwise altitude limited. In relation to the above Grainger and Male (1978) have noted for the Canadian Prairies the importance of the energy content of air masses at 850 hpa for determining the magnitude of sensible heat supply to a melting snowpack. Similarly, Yarnal (1984) has shown, for periods of enhanced summer glacier ablation in western Canada, the importance of anticyclonic synoptic regimes of subsidence and associated positive temperature anomalies at the 500 hpa level. The importance of anticyclonic types in providing the synoptic set-up conditions for high ablation rates is also clear in the work of Hay and Fitzharris (1 988) for New Zealand, Braze1 et al. (1 992) for Alaska, and Aizen and Aizen (1 993) for the Central Tien Shan region of China. For the Tien Shan region warm anticyclonic events produced total ablation amounts three times that of warm cyclonic events. Aizen and Aizen (1 993) also noted that cool anticyclonic events, because of their greater inputs of net and shortwave radiation, are of greater importance in terms of melt than warm cyclonic and especially cold cyclonic events. As for the alpine environments discussed above it appears that based on this study's results anticyclonic regimes and their associated air enthalpy and radiative transfer properties are possibly of great importance for determining ablation magnitudes in the Pyrenees. Clearly, given the supportive evidence from other alpine environments, the spectrum of atmospheric circulation types that exist for the Pyrenees area and the extent to which these are important for controlling ice and snowpack ablation climates needs to be established if the climatic controls on Pyrenean snow and ice resources are to be understood and modelled. For example, if with an enhanced greenhouse effect there is an increased incidence of anticyclonic type circulation systems in the Pyrenees, because of a northwards extension of the subtropical ridge from the Afiican-Mediterranean region similar to the synoptic

8 486 G. R. MCGREGOR AND A. F. GELLATLY situation analysed in this study, the Pyrenees would not only become warmer but also her. This is because of the high heat content and large inputs of net and shortwave radiation, as seen in this study, associated with African anticyclonic ridge systems, which are also very effective in steering moisture-bearing cyclonic systems to the north of the Pyrenees. The fact that a number of glaciers have disappeared over the last century and precipitation anomalies have been predominantly negative for the last 40 years in the Pyrenees suggests that there has been a shift in the synoptic climatology of this area, with the possibility that anticyclonic-related ablation processes have become climatologically more important than westerly and northerly cyclonic accumulation processes. Clearly synoptic climatology has a role to play in testing these ideas and the relationship between circulation patterns and ablation and accumulation processes for the Pyrenees. REFERENCES Aizen, V B. and Aizen, E. M Glacier runoff estimation and simulation of streamflow in the peripheral temtoly of Central Asia, Int. ASSOC. Hydd Pub., 218, Bucher, A. and Dessens, J Secular trend of surface temperature at an elevated observatory in the Pyrenees, 1 Climate, 4, Brazel, A. J., Tempe, F., Chambers, D. and Kalkstein, L. S Summer energy balance on West Gulkana Glacier, Alaska, and linkages to a temporal synoptic index, Z. Geomorph. NE SuppL-Ed., 86, Gellatly, A. F., Grove, J. M., Bucher, A,, Latham, R. and Whalley, W. B Recent historical fluctuations of the Glacier du Taillon, Pyrenees, Phys. Geogr, 15, Grainger, R. J. and Male, D. H Melting of a prairie snowpack, 1 Appl. Meteorol., 17, Gray, D. M. and Male, D. H Handbook of Snow. Pergamon Press, Toronto. Hay, J. E. and Fitzharris, B. B The synoptic climatology of ablation on a New Zealand glacier, 1 Climatol., 8, Hicks, B. B. and Martin, H. C Atmospheric turbulent fluxes over snow, Bound. Layer Meteoml., 2, Ishikawa, N. and Kodama, Y Transfer coefficients of sensible heat on a snowmelt surface, Meteorol. Amos. Phys., 53, Ishikawa, N., Owens, I. F. and Sturman, A. P Heat balance characteristics during fine periods on the lower parts of the Franz Josef Glacier, South Westland, New Zealand, Inrl. 1 Climatol., 12, Kang, E., Yang, D., Zhang, Y., Yang, X. and Shi, Y An experimental study ofthe water and heat balance in the source area of the Urumqi River in the Tien Shan mountains, Ann. Glaciol., 16, Kuhn, M On the computation of heat-transfer coefficients from energy balance gradients on a glacier, 1 Glaciol., 22, Marks, D. and Dozier, J Climate and energy balance at the snow surface in the alpine region of the Sierra Nevada. 2. Snow cover energy balance, Water Resour Res., 28, McIlveen, R Fundamentals of Weather and Climate, Chapman and Hall, London. McGregor, G. R., Gellatly, A. F., Bucher, A. and Grove, J Climate and glacier response in the Pyrenees, Z. Glerscher Glacialgeol., in press. Moore, R. D On the use of bulk aerodynamic formulae over melting snow Nord. Hydml., 14, Oke, T. R Boundary Layer Climates. Methuen, London. Prowse, T. D. and Owens, I. F Energy balance over melting snow, Craigiebum Range, New Zealand, 1 Hydml. (NZ), 21, NOAA Weekly Climate Bulkfin. Climate Analysis Centre, National Oceanic and Atmospheric Administration, Washington, DC, No Saunders, 1. R. and Bailey, W. G Radiation and energy budgets of alpine tundra environments of North America, Progr Phys. Geogr, 18, Thom, A. S Momentum, mass and heat exchange of plant communities, in Monteith, J. L. (ed.), Vegetation and Atmosphere, I, Principles. Academic Press, London, pp Yamal, B Relationships between synoptic scale atmospheric circulation and glacier mass balance in south-westem Canada during the International Hydrological Decade, , 1 Glaciol., 30,