International Journal of Fieldwork Studies, 2003 1 (1)

An Experimental Optical Instrument for the Determination of Snow Accumulation in Alpine Environments

Derrick J. Lampkin
Department of Geography and Regional Development, Department of Hydrology and Water Resources, University of Arizona, Building 11, University of Arizona, Tucson, Arizona, United States.

Abstract

Variability in regional climate can affect snow covered amount, and timing of water yields. Substantial changes in regional snow cover extent can have an impact on the availability of water resources in the Southwest U.S. There is a need for improved ground-based instrumentation that can monitor snow accumulation distribution across spatial scales ranging from a meter to tens of square meters in area. An experimental, prototype instrument was developed to monitor ground accumulated snow using coupled optics and voltage measuring sensors. The system was deployed on January 29, 2002 and March 18, 2002 atop Mt. Bigelow in the Santa Catalina Range, north of Tucson, Arizona. Results indicate that snow accumulation equal to or greater than 3cm occurred on the January 2002 deployment at the Ecm1 site. This was validated by the sonic depth sounder measuring ~ 2cm of snow at the Wtm2 site. Results during the March test indicate little accumulation and subsequent melt at Ecm1 site. Near surface air temperature, coupled with relative humidity, net radiation, and near surface soil temperature at Ecm1, indicate conditions were sufficient for accumulation and ablation of snow.

Key Words

Alpine Snow Cover, Instrumentation, Snow Hydrology, Optical sensor, Experimental, Alpine Vegetation

Résumé

Variabilité dans les climats regionaux peut affecter la couverture de neige, et le timing des rendements d'eau. Changements substantiaux dans la couverture regionale de neige peuvent avoir un impact sur la disponibilité de resources d'eau dan le sud-ouest des États-Unis. Un instrument est necessaire pour controler l'accumulation de neige à travers sur des superficies d'un mètre à des dizaines de mètres carrés. Un prototype experimental a été developpé pour contrôler la neige accumulé qui utilise des détecteurs voltaïque et optique couplé. Le system a été déployé le 29 janvier 2002 and le 19 mars 2002 sur le mont Bigelow dans les montagnes Santa Catalina au nord de Tucson, Arizona. Les résultats indiquent que l'accumulation était equivalente à 3cm ou plus pendant le déployement du janvier 2002 au site ECm1. Ces résultats ont été validés par des mesures en profondeur soniques de approximativement 2cm au site Wtm2. Les résultats du test de mars
indiquent peu d'accumulation et fonte subséquente au site ECm1. La température au sol, l'humidité relative, la radiation nette, la température du sol á la surface au site Ecm1 indiquent que les conditions étaient suffisantes pour de l'accumulation et de l'ablation de neige.

Mots clefs

Couverture de neige alpine, Instrumentation, Hydrologie de neige, Détecteur optique, Experimental, Végétation alpine

Rezumat

Diferentele regionale climatice pot afecta grosimea stratului de zapada, si periodicitatea topirii acesteia.Schimbarile regionale substantiale la nivelul extinderii stratului de zapada pot avea impact asupra resurselor de apa din sud-vestul Statelor Unite. Exista necesitatea imbunatatirii echipamentului de sol in vederea monitorizarii distributiei acumularilor de zapada pe suprafete incepand cu cele de un metru patrat pana la cele de zeci de metri patrati dintr-o zona. A fost pus la punct un instrument prototip, care monitorizeaza experimental acumularile de zapada la sol, utilizand senzori optici si de masurare voltaica. Sistemul a fost pus in functiune pe 29 Ianuarie 2002 si pe 18 Martie 2002 pe varfulBigelow din lantul Santa Catalina, nordul Tucson, Arizona. Rezultatele indica faptul ca acumularile de zapada din Ianuarie 2002 se afla in jurul valorii de 3 cm sau peste la locatia Ecm1. Acestea au fost validate prin masuratori sonice care au indicat 2 cm de zapada la locatia Wtm2. Rezultatele testului din Martie 2002 indica acumulari mai mici si topiri subsecvente la locatia Ecm1. Temperatura aerului la nivelul stratului de zapada impreuna cu umiditatea relativa, radiatia neta, temperatura aerului la nivelul solului la Ecm1 au indicat conditii suficiente pentru acumulare de zapada si ablatie.

Cuvinte cheie

Strat de zapada alpin, Instrumente, Hidrologia zapezii, Sensor optic, Experiment, Vegetatie alpina


Introduction

An understanding of global and regional changes in climate and the assessment of water resources requires that we monitor the temporal and spatial variability of snow cover from local to global scales (Dozier, 1989). Seasonal snow cover comprises about 30% of the Earth's land surface, while 10% is perennial cover by glaciers (Dozier, 1989). Variability in regional snowfall can be indicative of changes in the Earth's global climate.

Frozen reservoirs of water in alpine environments of the mid-latitudes are important in the management of water resources, particularly in locations such as the southwestern United States. Substantial changes in regional snow cover extent can have an impact on the availability of water resources in the Southwest. A thorough understanding of various characteristics about alpine snow cover, such as density, depth, accumulation, and temporal variation in extent, are vital to the effective management of this resource.

There are established methods for assessing snow cover characteristics. Alpine snow cover data have been collected by various field-based methods. These methods include snow gauges to measure snow water equivalence, graduated stakes and aerial markers to capture snow depth, automated data collection apparatus, manual data acquisition, and snow surveys (Goodison et. al., 1981). Current field-based techniques for determining snow accumulation, suffer from a lack of either spatial or temporal resolution. This research will improve acquisition of snow accumulation data through the use of an automated optical system, which will use a coupled laser-sensor system. The system, composed of these fundamental components, can be replicated to assess an integrated depth of accumulation across an area.

Background

Optical methods have been used to capture information about alpine snow cover. Lundberg and Johansson (1994) used two pairs of transmitter-receivers fashioned from a light-diode and photo-detector. They were designed to monitor and discriminate solid phase precipitation. Warren and Gunn (1968) used a pulsed light source detector to estimate hourly snow accumulation. Their work involved the use of a pulsed light source operating in the visible region of the EM spectrum with peak intensity at 0.45µm. A transmitter and receiver were arranged between two buildings with a distance of 71 m and elevated 20m above ground. Warner and Gunn's (1968) work extended the work of Lillesaeter (1965), which involved the use of a narrow light beam and receiver to determine an empirical relationship between beam attenuation and the extinction. Lundberg and Johansson (1994), Warren and Gunn (1968) and Lilleseater (1965) used optically instruments to determine solid phase of precipitation, but failed to record information about snow accumulation on the ground.

Method

a) Theoretical Basis and Design

The instrument uses a laser that operates in the 630-680nm range of the electromagnetic spectrum (EM). The basic component of the system is a laser/sensor array, which measures snow accumulation through ground-accumulated snow interrupting the beam, causing a subsequent drop in measured voltage, over moderate spatial and temporal resolutions. The instrument does not measure particle type or snowfall, but strictly ground-accumulated snow. Additionally, the prototype system cannot resolve accumulations derived from lateral redistribution of snow due to wind flow. Therefore, all measurements effectively represent peak accumulation between laser and sensor towers, derived from vertical input from the atmosphere, as well as canopy intercepted snow fall, and laterally redistributed accumulations. Separating measurements into their constituent input components will be a focus of future work.

Figure 1. Characteristic growth modes of columnar and planar ice crystals from Ono, (1970).

A primary concern regarding such a system is its degree of sensitivity to accumulating snow grains, which vary in size and accumulation rate. Precipitating ice crystals form as small hexagonal plates that can grow along the six prism faces (a-axis) or perpendicularly along the two basal planes (c-axis) (Fassnacht et. al., 1999). Work by Ono (1970), describes the change of ice crystal shape with temperature. Plates, stellar, and dendrites grow along the a-axis, while columns and needle-shaped crystals grow primarily along the c-axis (Fassnacht et. al., 1999). The range of values for lengths in both the a and c axis directions illustrate that the laser wave front would be sensitive to falling snow. The lower range for the c axis is ~ 0.01mm (10,000nm) at -7 degrees C and 0.08mm (80,000nm) along the a-axis (see Figure 1). At this temperature, the smallest dimension along the c-axis of precipitating snow crystals is 16 times larger than the laser wave front and 127 times larger along the a-axis. The instrument should theoretically be sensitive to the attenuation of the laser beam by accumulated particles within the 630-680nm range of the EM spectrum, as a result of snow's low transmission and absorption relative to its high reflectance within the dynamic range of the laser. Figure 2 displays the basic design of the optical snow accumulation instrument. The system is composed of an array of lasers set at user-specified heights above the ground. Each laser is set to a corresponding light-sensitive sensor, which measures changes in an amplified voltage derived from an illumined solar cell. As snow accumulates, each beam is interrupted with a corresponding decline in sensor voltage. The optical component houses a switch, and laser diode. The sensor apparatus is composed of a silicon-based solar cell, which is routed to an operational amplifier as well as two adjustable resistors (potentiometers). The potentiometers are used to set limits on the minimum and maximum amplified output voltage. The laser signal received by the solar cell is amplified and output voltage is logged by an ONSET HOBO VOLT® data logger.

Figure 2. Conceptual design of the optical system. Laser and sensor casing dimensions (31cm x 31cm x 16cm) with laser and sensor instruments contained with protective weather casings.

b) Field Measurements

A single laser-sensor system was deployed in the Santa Catalina Mountain Range just North of metropolitan Tucson, Arizona. Figure 3 illustrates a map of sites for the deployment of the system. Each site corresponds to a location of a micro-meterological tower erected and maintain by Sustainability of Semi-arid Hydrology and Riparian Areas (SAHRA). Each site contains array of instruments designed to monitor and quantify hydrometerological fluxes in alpine environments. Table 1 summarizes the instruments deployed at both the Wtm2 and Ecm1 sites. The optical sensor was deployed on January 29, 2002 and March 18, 2002 at the Ecm1 site. This site was selected because it was at the highest elevation and most accessible of all the available locations. Additionally, the Ecm1 site is a little more than a kilometer south of the Wtm2 site. Unfortunately, the Ecm1 site's resident sonic snow depth sounder experienced malfunctions due to tower proximity to commercial high frequency radio communication towers. Therefore, snow depth information from the nearest tower to the Ecm1 (Wtm2) was used to validate the optical instrument measurements.

Figure 3. Topographic map displaying locations of SAHRA micro-meteorological towers and optical system test site. Data from WTm2 tower were used to validate optical system results collected at the ECm1 Site. Map projected in UTM (zone 12). Contour intervals set at 150 metres.

The optical array was deployed on January 29, 2002 at the ECm1 site at 5:30pm. The single coupled sensor/laser system sits ~ 3cm above the snow-free surface. Therefore, the lowest resolvable measure of snow accumulation is ~ 3cm. Any accumulation less than this amount will not be detected by the system. The datalogger attached to the sensor and housed within the sensor module was set with a sampling interval every 0.5 seconds for 1 hour and 15 minutes. This high sampling rate was mistakenly set due to operator's error, when deploying the datalogger. The high sampling rate facilitated an increase in accuracy in temporal resolution, but restricted monitoring through the entire accumulation event. The laser /sensor modules were deployed in January 2002, approximately 78cm apart and a distance of 8 meters south from the micrometeorological tower, near the edge of a clearing in the canopy. The system was orientated with the sensor tower facing towards the northwest, while the laser tower was aimed at the sensor housing with an orientation towards the southeast. The data was retrieved on January 30, 2002. Deployment of the instrument on March 18, 2002 was at the Ecm1 site with the sensor tower facing due north, while the laser tower was facing south. The system was placed at the southeastern edge of the forest clearing about 10 meters south of the SAHRA instrument tower. The system was activated at 7:00pm on March 18, 2002 with a total sampling rate every 5 minutes. The site was characterized by bare ground with a small patch of ice less than 2cm thick on the southwest edge of the canopy clearing. The data was retrieved on March 19, 2002.

Table1. Instruments mounted at SAHRA micro-meteorological stations

Results and Discussion

Figure 4 displays results from the optical instrument as well as validation data from the Wtm2 snow depth sounder and the Ecm1 air temperature probe for the January 29, 2002 deployment. The air temperature at the Ecm1 site demonstrates a significant decline with values dropping below zero. This drop occurs during an increase in snow depth as measured by the sonic depth sounder at the Wtm2 site. Simultaneously, there was a significant decline in voltage as measured by the optical instrument at the Ecm1 site. A substantial decline in voltage occurred at about 5:16pm. This decline, indicative of a substantial amount of accumulated snow, is validated by an increase in sonic snow depth sounder measurements at the Wtm2 site.

Figure 4. Graph of results from 1-29-02 deployment at the ECm1
site as a function of time, averaged over 15 minute intervals: A) air temperature from ECm1 tower, B) Snow depth measured at WTm2 site, C) Voltage measured by optical system at ECm1 site.

Results from the March 18, 2002 deployment were more complex. The site during retrieval was characterized by bare ground, with no snow present between the laser and sensor towers. There was present a dusting of snow, less than 1cm, covering the patch of ice that was present pre-deployment. The soil between the instrument towers and within the canopy was damp. The sonic depth sounder at the Wtm2 site was of no use as a validation data set because the amount of accumulation, if any was less than (accumulation < 3cm) the sensor's ability to detect snow without substantial errors. Therefore, the use of other data derived from the Ecm1 met tower, such as air temperature, relative humidity, surface soil temperature, and net radiation were used to validate the optical instrument results. Figure 5 displays graphs of optical instrument voltage, as well as the previously mentioned data as a function of time during the course of the instrument deployment period. This demonstrates a decline in optical meter voltage for approximately a two-hour period (13:45-15:45) on March 20th. The decline in voltage was followed by an increase in near surface air temperature above (0) degrees. During this period there was a 16% increase in relative humidity. There was also a decline in net radiation during this period with a negative net radiation of (-9.0 W/m^2) at 14:30). High relative humidity with rising, but below freezing temperatures as well as a decline in net radiation, could indicate conditions that support some snow accumulation. The two-hour period from 15:45 to 17:45 is marked by a sharp increase in sensor voltage, ideally indicative of melting accumulated snow cover. During this time period, near surface soil temperature demonstrates a sharp increase, while simultaneously the net radiation is low, but remains positive. For a 3cm thick snow pack, there may have been a sufficient amount of radiation to melt it, supported by an increase in near surface soil temperature, possibly brought on by the onset of percolating warmer melt water. Assuming the equivalent amount of water within a 3cm depth snow pack for a square centimeter area is approximately 1gram and given that the conventional latent heat of fusion for ice would be (79.7 cal/g), an upper estimate for the amount of energy required to ripen the 3cm thick snow pack would be estimated as 18W/m2 over the 2 hour period. This is derived neglecting the volumetric water content. At 16:00h, the net radiation was about 13 W/m^2. There appears to have been a sufficient amount of energy to melt the existing snow pack during the time period that the instrument measured an increase in voltage.

Figure 5. Graph of results from 3-18-02 deployment at the ECm1 site as a function of time, averaged over 15 minute intervals: A) air temperature from ECm1 tower, B) soil temperature from ECm1 tower, C) relative humidity from ECm1 tower, D) net radiation from ECm1 tower, and E) Voltage measured by optical system at ECm1 site.

Conclusion

The application of an optical based instrument for the determination of snow accumulation in alpine environments seems plausible. The prototype instrument composed of a single coupled sensor-laser system, demonstrated a level of sensitivity required to monitor accumulation events in the Santa Catalina Mountain Range north of Tucson, Arizona. The laser, which operated between 630-680nm, was sufficiently attenuated by snow accumulation measured on January 29, 2002 and March 18, 2002. Ancillary meteorological data acquired during the optical instrument deployments, confirmed the timing and degree of accumulation and melt measure by the prototype system.

Development of the system from prototype to operational status will require several further steps. Short-term goals will involve laboratory/cold-room testing and calibration of the prototype, improvements in sensor and field design, as well as expanding the system field tests using more components. Additionally, a single sensor will be deployed to determine the influence of diurnal fluctuation of the incoming radiation field on measured voltages. Intermediate goals will involve a greater emphasis on the improvement of the circuit design, as well as more controlled field tests. Intermediate field exercises will involve acquisition of meteorological data in conjunction with an expanded sensor/laser network. Several questions could be addressed by the optical system: Can the optical instrument improve our understanding of how alpine vegetation canopy influences the spatial distribution of snow accumulation? How does the near-surface radiation budget vary as a function of snow accumulation in an alpine vegetated environment? The optical system coupled with meteorological data could address to what degree is soil moisture influenced by successive snow accumulation events?

Acknowledgments

I would like to thank the American Alpine Club, and the College of Social and Behavioral Science Department at the University of Arizona for their financial support of this project. I would like to also thank SAHRA for access to the meteorological data and facilities. I'd also like to thank Dr. S. Fassnacht for his support.

References

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Fassnacht, S.R., Innes, J., Kouwen, N., and Soulis, E.D., 1999. The specific surface area of fresh dendritic snow crystals, Hydrol. Process. 12: 2945-2962

Goodison, B.E., Ferguson, H.L., and McKay, G.A., Measurement and data analysis. In D.M. Gray and D.H. Male, (eds.), 1981. Handbook of snow: Principles, Process, Management and Use. Pergamon Press, Toronto, 191-274

Lillesaeter, O., 1965. Parallel-beam attenuation of light, particularly by falling snow, J. Appl. Meteor.,4: 607-613

Lundberg, A., and Johansson, R.M., 1994. Optical precipitation gauge-determination of precipitation type and intensity by light attenuation technique, Nordic Hydrology,25: 359-370

Ono, A., 1970. Growth mode of ice crystals in natural clouds. Journal of Atmospheric Science, 27: 649-658

Warner, C., and Gunn, K.L., 1968. Measurement of snowfall by optical attenuation, Journal of Applied Meterology, 8: 110-121

Please cite this paper as:
Lampkin, D.J. (2003) An Experimental Optical Instrument for the Determination of Snow Accumulation in Alpine Environments, International Journal of Fieldwork Studies, 1 (1), http://www.virtualmontana.org/ejournal/vol1(1)/snow.htm