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Journal of Hydrology: Regional Studies 5 (2016) 20–32

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Journal of Hydrology: RegionalStudies

jo ur nal homep age: www.elsev ier .com/ locate /e j rh

Discriminating types of precipitation in Qilian Mountains,Tibetan Plateau

Junfeng Liua,b,∗, Rensheng Chena,b

a Qilian Alpine Ecology and Hydrology Research Station, Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences, Lanzhou 730000, Chinab Heihe Key Laboratory of Ecohydrology and Integrated River Basin Management, Chinese Academy of Sciences, Lanzhou 730000, China

a r t i c l e i n f o

Article history:Received 4 May 2015Received in revised form 1 November 2015Accepted 15 November 2015Available online 28 November 2015

Keywords:Precipitation typeThreshold air temperatureHulugou river basinQilian MountainousTibetan Plateau

a b s t r a c t

Study region: Hulugou River Basin (HRB) of Qilian Mountains, eastern edge of TibetanPlateau.Study focus: Traditional manual observations only record point-scale precipitation ratherthan regional-scale precipitation. Automatic weather stations just record precipitationamount without discriminating by type of precipitation. This study observed precipita-tion types all over the HRB and analyzed air temperature and humidity conditions at dailyand half-hour resolution.New hydrological insights for the region: Combined observations of air temperature and pre-cipitation type indicate that, at daily resolution the threshold air temperature betweenrain and snow is 0 ◦C and the air temperature at rain/snow boundary is from 0 ◦C to 7.6 ◦C,which means the rain and mixed precipitation threshold air temperature can shift morethan 7.0 ◦C in the HRB. At half-hour resolution, air temperature is above 0 ◦C during rainfall,under 0 ◦C for snowfall, and above 0 ◦C at the rain/snow precipitation boundary, and couldeither be above 0 ◦C or fluctuate around 0 ◦C for mixed precipitation. Corresponding rela-tive humidity observations indicate that rainfall and mixed precipitation events correspondwith high humidity conditions in warm season of the HRB. Snowfall events correspond withlow humidity conditions in the HRB.© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC

BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Observation of precise precipitation amount and type of precipitation are critical for reliable stream flow forecasts(Fassnacht et al., 2006). Precipitation amount has been widely observed, but precipitation type is rarely observed by auto-matic weather stations. Type of precipitation has great impact on the land surface energy balance (Loth et al., 1993; Dinget al., 2014), as snowfall can increase the surface albedo, and rainfall can decrease the surface albedo (Box et al., 2012). Land

surface hydrological processes are also different for different precipitation types, i.e., rainfall usually converts into under-ground or surface water stream systems, while snowfall may accumulate during the cold season and melt in warm season.Observation of precipitation type is important, because some forms of cold season precipitation can pose a threat to humansafety or disrupt travel and commerce (Reeves et al., 2014). It has been widely recognized that gauge measured precipitation

∗ Corresponding author at: Qilian Alpine Ecology and Hydrology Research Station, Cold and Arid Regions Environmental and Engineering ResearchInstitute, Chinese Academy of Sciences, Lanzhou 730000, China

E-mail address: [email protected] (J. Liu).

http://dx.doi.org/10.1016/j.ejrh.2015.11.0132214-5818/© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

dx.doi.org/10.1016/j.ejrh.2015.11.013http://www.sciencedirect.com/science/journal/22145818http://www.elsevier.com/locate/ejrhhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.ejrh.2015.11.013&domain=pdfhttp://creativecommons.org/licenses/by-nc-nd/4.0/mailto:[email protected]/10.1016/j.ejrh.2015.11.013http://creativecommons.org/licenses/by-nc-nd/4.0/

J. Liu, R. Chen / Journal of Hydrology: Regional Studies 5 (2016) 20–32 21

Table 1Precipitation measurements intercomparison experiment.

Gauge Abbreviation Size Start date End date Measure time

China standard precipitation gauge CSPG � =20 cm, h = 70 cm June, 2009 April, 2014 20:00 and 8:00, local timeCSPG with Alter shelter Alter � = 20 cm, h = 70 cm June, 2009 April, 2014 20:00 and 8:00, local timePit gauge with a CSPG Pit � = 20 cm, h = 0 cm September, 2010 April, 2014 20:00 and 8:00, local timePit with a larger diameter bucket Pit500 � = 25 cm, h = 0 cm September, 2010 April, 2014 20:00 and 8:00, local time

htd

aMIatcrvo

vtitpbihmb

2

9tsantaa

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A

Double-Fence International Reference(Tretyakov wind shield + CSPG)

DFIR � = 20 cm, h = 3.0 m September, 2012 April, 2014 20:00 and 8:00, local time

as systematic errors mainly caused by wind-induced undercatch (Sugiura et al., 2003). When the wind speed is very high,he catch ratio of precipitation gauges depends on precipitation type (Yang et al., 1995). In this case, the precipitation typeata deficit will greatly affect measured precipitation calibration and precision.

However, most of the time precipitation type data are not available because these depend on manual observation andutomatic precipitation type discrimination sensors are not precise enough to be used widely. For example, the Worldeteorological Organization arranged an intercomparison of weather sensor abilities (Present Weather Sensors/Systems

ntercomparison; PREWIC, 1993–1995) in the field of precipitation detection and precipitation type discrimination (Leroynd Bellevaux, 1998). Sensors like the Vaisala FD12P performed the best of all optical sensors in PREWIC, but it is known thathe discrimination of precipitation type is not precise (Haji, 2007). Manual observations of precipitation type are limited atertain altitudes. A study by Thériault et al. (2010) indicated that small variations in temperature profiles and precipitationate can have major impacts on the types of precipitation formed at the surface. It was also found that precipitation typearies widely over space and time in mountainous regions (Harder and Pomeroy, 2013), where the rain/snow boundaryften formed at certain altitudes.

Little knowledge is available of how elevation impact the precipitation types, and no systemic precipitation type obser-ations have been undertaken at watershed scale in mountainous regions. To address this shortfall, a systemic precipitationype observation experiment was carried out in the Hulugou River Basin (HRB) of the Qilian Mountainous. The observationsnclude manual observations since 2009 as a reference at the outlet of the HRB. Two time-lapse cameras have been usedo photograph precipitation types in the HRB at two different locations since 2012. Through photographic interpretation,recipitation types which include rainfall, snowfall and mixed precipitation are acquired at basin scale. If the rain/snowoundary is visualized in a precipitation event, the elevation of the rain/snow boundary is obtained by georeferencing the

mage. On the basis of automatic land surface meteorological observations at different altitudes, the air temperature andumidity were determined for each precipitation event at daily and half-hour resolution. Specifically, in this study, theeteorological conditions at the rain/snow boundary were determined during each mixed precipitation event, which can

e used as the threshold air temperature to differentiate rain and mixed precipitation.

. Study area and data

The HRB is located in the Qilian Mountains along the northeastern margin of the Tibetan Plateau (38◦15′54.9′′N,9◦52′53.5′′E; Fig. 1). The basin forms the headwaters of the Heihe River and has a catchment area of 23.4 km2. Eleva-ions in the basin range from 2980 m to 4800 m above sea level. The landscape zones include glaciers at mountain top,ubnival, marsh meadows, alpine shrub, and mountain grassland. The annual precipitation ranges from 376 to 650 mm,nd the annual mean temperature varies from approximately 3.1 ◦C at 3000 m to −4.0 ◦C at 4200 m. The river basin facesorth with inclines from 5◦ to 85◦. The steep north facing slope experiences less solar radiation and lower ground surfaceemperatures, combined with the large elevation ranges, these conditions cause mixed precipitation events to frequentlyppear at high altitudes during the warm season. Consequently, the rain/snow boundary is often visualized on north facingspects of the HRB.

Field precipitation measurement intercomparison experiments (Fig. 1, Table 1) were conducted near the outlet of theRB (2980 m). Manual observations (rain, snow and mixed) are three time per day. In September 2008, four ENVISs (Envi-

onmental Information Systems) were installed at 2980 m, 3382 m, 3710 m, and 4166 m in the HRB. Measurements includeir temperature, humidity, wind speed and direction, radiation, amount of precipitation (weighing gauge), snow depth,umber of hours of sun, ground surface temperature, soil temperature at eight depths, and three ground heat fluxes. Theeighting gauge was used to determine the total rain using a weighting sensor TRwS500 with a resolution of 0.001 mm.ther sensors installed in each of the ENVISs include the temperature and relative humidity sensor of Campbell CS215-L,

he wind speed and direction sensor of Campbell Wind sonic Two-Dimensional Sonic Anemometer, the radiation sensor ofampbell four-component radiometer, the Campbell snow depth sensor of SR50 Sonic Ranging Sensor with a precision of

cm, the Campbell soil water content sensor of Enviro SMART Soil Water Content Profile Probes, the Campbell soil temper-ture sensor of 107 L Sensor, the CSD3sun time hour sensor, and the soil heat flux sensor of HFP01SC-L Self-Calibrating Soil

eat Flux Plate sensor.

In June 2012, two automatic weather stations (AWSs) were installed on the western tributary of the HRB (3839 m).nother AWS was installed at a higher site (4496 m) on the Shiyi glacier. Each AWS measures air temperature, relative

22 J. Liu, R. Chen / Journal of Hydrology: Regional Studies 5 (2016) 20–32

Fig. 1. Overview of the Hulugou River Basin. Shown are the locations of observation points of the manual precipitation, two automatic weather stations,four environmental information systems, the Shiyi glacier, the EOS 7D camera at 3140 m and the EOS 600D camera at 4550 m.

humidity, wind speed and precipitation once every 30 min. The sensors in these two AWSs for these measurements weresimilar to those in the ENVISs.

Since September 2011, manual shoot has been applied in the HRB at 3140 m every morning around 9:00 Beijing time,approximately 1 km from the catchment outlet. In April 2012, a Canon EOS 7D was equipped with automatic time-lapseaccessories and a protective device to replace manual shooting. This camera was installed at manual shooting site. Before

July 6 2012, pictures were taken automatically at 9:51 and 15:51 Beijing time, and after June 1, the pictures were taken onthe hour. The camera’s field of view covers the whole basin’s altitudinal gradient. This camera records the precipitation typeand snow cover distribution for the whole HRB.


Convert to orthophotos

Snow

Manual observa�on of p recipita�on type at the HRB o utlet at 2980 m

Snow for the HRB

Rain Formed n ew sn ow/rain bounda ry?No

Camera Hourly Photography

Extract al�tude of rain/snow boundary

Rain for the HRBYes

Rain under rain/snow boundary, mixed precipita�on above

Mixed precipita�on

Mixed precipita�on for the HRB

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Fig. 2. Flowchart describing the processes used to discriminate precipitation types in the Hulugou River Basin (HRB).

In July 2012, a Canon EOS 600D time-lapse camera was installed at 4550 m. The camera photographs the Shiyi glacier,hich covers 0.3 km2 and ranges from 4313 m at its terminus to 4800 m above sea level. In this study, this camera was used

o record high altitude precipitation types. The glacier surface temperatures are equal to or less than 0 ◦C, which meansrecipitation type is hardly affected by the glacier surface thermal conditions. In the warm season, clouds or fog persistentlyffect the view of the EOS 7D camera. Compared to the EOS7D camera, the EOS 600D camera has a closer view of the Shiyilacier and can sometimes avoid the clouds for fog. Sometimes, if the solid parts are low, the snow/rain boundary can hardlye observed by the EOS7D camera, but the EOS600D can detect it because of its close view. In this case, the EOS 600D cameraan observe more mixed precipitation than the EOS 7D camera.

. Methods

In the HRB cold season (October–April) observed air temperatures at 2980 m, in the basin outlet, are usually under◦C. Precipitation intensity and duration are low most of the time, and the precipitation amount is usually less than 30%f the annual total. The manual observation at 2980 m indicates that snowfall dominates the cold season precipitationhroughout the HRB. Precipitation is mainly concentrated in the warm season. Precipitation types can change from raino mixed precipitation or snowfall with increasing altitude. Even at the same altitude, precipitation may be preceded orollowed by mixed precipitation. In this case, precipitation type discrimination is a very difficult task. In the HRB, most of the

ixed precipitation or snowfall events happened at night, but precipitation type could only be monitored during daytimey observers or cameras after it happened.

Fig. 2 shows a flowchart of precipitation type discrimination methods used in this paper. The discrimination of precipi-ation is based on manual observations at the basin outlet at 2980 m and two cameras at 3140 m and 4550 m. In this study,f the manual observation at the basin outlet is snow, or mixed precipitation, the entire HRB is considered to have snowfallr mixed precipitation. If the basin outlet manual observation is rainfall, the two cameras were used to identify precipita-ion type at higher altitudes, where if either the EOS7D camera or EOS 600D camera captures the rain/snow boundary, therecipitation under the rain/snow boundary is considered to be rainfall and above the boundary is mixed precipitation. Theltitude of the rain/snow boundary was calculated based on georeferencing these pictures. The georeferencing method haseen presented by Liu et al. (2012, 2014).

In order to avoid subjective discrimination of precipitation type at high altitudes on the Shiyi glacier, we also used thebserved snow depth data, half-hour precipitation data, half-hour air temperature, and pictures taken by the two cameraso help in the precipitation type discrimination process.

The three TRwS500 weighing gauges at 3340 m, 3710 m and 4160 m are a cumulative record of both rainfall and snowfallvents. Phase was identified if the TRws500 recorded precipitation.

Snow depth was measured by the SR50 sensor. An increase in snow depth was used to identify snowfall, while no changer a decrease in snow depth was used to identify rainfall.

Ground surface temperature was used to identify snowfall and rainfall in the warm season. If the ground surface tem-erature was greater than 0 ◦C, then all precipitation was presumed to be rainfall.

If the two cameras captured snowfall, or the snow depth sensor recorded an increase in snow depth, the density of snowepth was calculated based on observed precipitation data and snow depth data. If snow density was over 0.2 g/cm3, therecipitation was identified as mixed, while a density under 0.2 g/cm3 was identified as snowfall.

Fig. 3 presents a mixed precipitation event starting on May 23, 2012 with associated meteorological data at 3340 m,710 m and 4160 m. At 3340 m, the snow depth was not captured by the SR50 snow depth sensors. At 3710 m and 4160 m,

he air temperature and ground surface temperature decreased with altitude, and snow depth and duration increased withltitude. During this precipitation event, the EOS 7D camera recorded the snow/rain boundary at 3185 m, which means thenow depth sensor could not detect the thin snow depth. At 3710 m and 4160 m, the density was over 0.2 g/cm3, whicheans mixed precipitation happened.


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Fig. 3. Observed snow depth and associated meteorological data at three different altitudes: (a) 3340 m, (b) 3710 m and (c) 4160 m.

4. Results

4.1. Precipitation type observation results at three locations

On the basis of observations at three different locations, great spatial and temporal differences in precipitation typeswere observed in the HRB (Fig. 4). Manual observations at the HRB outlet showed that rain accounted for 69% and 83% ofthe precipitation events in 2012 and 2013, respectively; mixed precipitation events accounted for 2.4% and 5%, snow eventsaccounted for 29% and 12% in 2012 and 2013. Rainfall was mainly concentrated from April to September, and snowfall mainlyoccurred from October to March. At 4550 m, observed rain, mixed and snow precipitation events occur respectively 33%,38%, and 29% in 2013. Compared with the manual observation results at the basin outlet, mixed precipitation events arevery common phenomena and rain occurs less often in the warm season at high altitudes above 4000 m.

Compared with the manual observations at 2980 m or the EOS 600D camera results at 4550 m, the EOS 7D cameraprovides a basin-scale perspective for observing precipitation type. For example, on 18 July 2013, the manual observation at2980 m was rain, and a heavy snowfall covered the whole Shiyi glacier and surrounding area at 4550 m (Fig. 5). The EOS 7Dcamera caught the rain/snow boundary at 3600 m (Fig. 5). Under this circumstance, the basin-scale precipitation is classifiedas mixed rather than rain or snow. Only when the whole HRB is experiencing rain or snow, the precipitation is classifiedaccordingly, otherwise it is classified as mixed precipitation. Fig. 6 shows the observed and monitored precipitation type atthree different locations in 2012 and 2013, based on this discrimination principle. Mixed precipitation appeared from Aprilto October in 2012, and accounts for approximately 50% of the precipitation events. Basin-scale rainfall events occurredonly 12 times in July and August of 2012, and snow events occurred 32 times in whole year of 2012. In 2013, rain eventshappened 36 times from June to August. Mixed precipitation happened 58 times from April to October, and snow occurred12 times in whole year of 2013.

After georeferencing the basin-scale mixed and snow precipitation events, we get the altitude of the rain/snow boundary(Fig. 7), where rain changed to mixed precipitation. In Fig. 7a it can be seen that from April to July 2012, the rain/snowboundary altitude increased to greater than 4000 m. The elevation of the rain/snow boundary then decreased to 3000 m inOctober. The change in elevation is closely related to the seasonal change in air temperatures. Fig. 7b shows that the warmseason rain/snow boundary altitude variation in 2013 is similar to 2012, except in July and August, during which the EOS 7Dcamera only caught the rain/snow boundary once in 2013. In July and August of 2013, more precipitation events occurred

than in the same months of 2012. The EOS 600D camera caught 14 and 13 mixed precipitation events from July and Augustin 2012 and 2013, respectively, but fewer rain/snow boundaries were caught by the EOS 7D camera in 2013, which meansthe rain/snow boundary had moved to even higher altitudes.


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Fig. 4. Observed number of events for rain, mixed precipitation, or snow at three different locations in 2012 and 2013. (a) and (b) show manual observationsat the HRB outlet at 2980 m in 2012 and 2013, respectively. (c) and (d) show EOS 7D camera observation results for the whole basin in 2012 and 2013,respectively. (e) and (f) show EOS 600D camera observation results for precipitation type on the Shiyi glacier surface at 4550 m in 2012 and 2013, respectively.

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From basin-scale comparative studies of precipitation type over two successive years, we find that mixed precipitationominated the HRB precipitation events from April to October. From October to March, snow dominated. Rain mainlyccurred in July and August. Precipitation type proportions were very different in 2012 and 2013. In 2012, rain, mixedrecipitation and snow accounted for 16%, 49% and 35% of precipitation events, respectively; but in 2013, the proportionshanged to 34%, 55% and 11%, respectively. Air temperature is generally the determining factor for the partitioning of rain,ixed and snow (Braun, 1985). More rain and less snow in 2013 indicated higher air temperatures. Comparative studies at

wo different altitudes in 2012 and 2013 indicated that monthly air temperatures in 2013 were higher than in 2012 in theRB (Fig. 8). In July and August 2013, the air temperatures were 0.4 ◦C and 1.4 ◦C higher than in the corresponding monthsf 2012. Consequently, the rain proportion was much higher in 2013 than in 2012.

Another difference from 2012 to 2013 is that basin-scale snow events lasted until late May in 2013, much later than in012 (when they stopped in April), and the mean rain/snow boundary altitude was lower in May of 2013 than in May of 2012Fig. 7). Detailed studies of these precipitation events indicated that in May of 2013, three heavy mixed precipitation eventsnd one small mixed precipitation event happened across the entire HRB, and the corresponding air temperatures fluctuatedround 0 ◦C. In May of 2012 only two small mixed precipitation events were noted in manual observations at the basin outlet.ven though the monthly air temperature was not different in May of 2012 than May of 2013, the higher rain/snow boundarynd fewer basin-scale mixed precipitation events in May of 2012 were related to the higher air temperatures during therecipitation period.


Fig. 5. Precipitation type observations at 3140 m and 4550 m on 18 July 2013. (a) and (b) were photographed by EOS 7D camera over basin scale. (a) wastaken before mixed precipitation occurred, and (b) was taken after mixed precipitation occurred. (c) and (d) were photographed by EOS 600D camera onthe Shiyi glacier. (c) was taken before snowfall occurred, and (d) was after snowfall occurred.

4.2. Daily mean threshold air temperatures in the HRB

Static or dynamic threshold air temperature has been widely applied to differentiate rain, mixed precipitation, or snow.We used observed air temperatures at different altitudes and precipitation type results to obtain the threshold air tempera-tures. Fig. 9 shows precipitation types and corresponding daily air temperatures. Manual observations at 2980 m indicatedthat rain appeared when the air temperature was above 3.6 ◦C, snow was not observed when the air temperature exceeded−2.7 ◦C, and mixed precipitation appeared when the air temperature was between 1.9 ◦C and 6.1 ◦C (Fig. 9a). Results from

the EOS 600D camera at 4550 m indicated that rain appeared when the air temperature was above 1.7 ◦C, snow appearedwhen the air temperature was under −0.5 ◦C, and mixed precipitation appeared when the air temperature was between 0 ◦Cand 7.6 ◦C (Fig. 9b).

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Fig. 8. Observed monthly air temperatures at (a) 2980 m and (b) 4550 m in 2012 and 2013 in the Hulugou River Basin.

Fig. 9. Rain, mixed precipitation and snow precipitation threshold mean daily air temperatures (a) air temperature at 2980 m, (b) air temperature at4550 m, (c) air temperature at rain/snow boundary for mixed rain, air temperature at 4765 m for rain, and air temperature at 2980 m for snow).


Fig. 10. Half-hour air temperatures during rain, mixed precipitation and snow events, and air temperatures at the rain/snow boundary during precipitationevents (a) half-hour air temperatures at 4550 m for rain events, (b) half-hour air temperatures during precipitation events at the rain/snow boundary, (c)half-hour air temperatures during mixed precipitation events, (d) half-hour air temperatures at 2980 m for snow events).

To calculate the threshold air temperature at the rain/snow boundary, first, the images were georeferenced into a digitalorthophoto with the same resolution as the available digital elevation model data (DEM). Then, the altitude at the rain/snowboundary is extracted based on the orthophoto and DEM data. Finally, two observed air temperatures that are close tothe rain/snow boundary were selected, and half-hour air temperatures were interpolated to the altitude of the rain/snowboundary based on the following equations:

Tb = T1 +(T2 − T1)(Hc − H1)

H2 − H1(1)

where Tb is the threshold air temperature at the rain/snow boundary (◦C); T1 and T2 (◦C) are observed air temperatures ataltitude H1 and H2 (m), respectively; and Hc is the altitude of the rain/snow boundary (m).

If the whole HRB is covered in rain, then the interpolated air temperature at 4765 m was selected as the correspondingair temperature for the rainfall event, and if the whole HRB is covered with snow, the air temperature at 2980 m wasselected as the corresponding air temperature for the snowfall event. The basin-scale precipitation types and correspondingair temperatures are shown in Fig. 9c. When rainfall occurred all over the HRB, the air temperature at 4765 m was greaterthan 1.0 ◦C; snowfall occurred when the air temperature was below 0.9 ◦C. Observations indicated that the air temperatureshifted from −0.3 ◦C to 7.4 ◦C at the rain/snow boundary, which means that the threshold air temperature at the rain/snowboundary varied.

Through manual precipitation type observation at 2980 m, EOS 600D camera assisted precipitation type discriminationat 4550 m, and basin-scale precipitation type observation, we conclude that the threshold air temperature between rain andsnow is generally 0 ◦C in the HRB. The rain and mixed precipitation threshold air temperature is not obvious in the HRB(Fig. 9). The mixed precipitation air temperature ranges from 0 ◦C to 7.6 ◦C.

4.3. Half-hour air temperatures for the three precipitation types

Precipitation is usually concentrated at a specific time of day, which means the precipitation type is determined moreby the air temperature at these specific times than by the daily resolution air temperature. Fig. 10 shows the half-hour airtemperatures during the three different precipitation types, in which the wide bars represent the upper quartile and thelower quartile of half-hour air temperature, the plus sign represents the median half-hour air temperature, and the finebars represent the maximum and minimum half-hour air temperatures during precipitation events. During rain events, thehalf-hour air temperatures were above 0 ◦C (Fig. 10a), the half-hour air temperature at rain/snow boundary were also above

0 ◦C (Fig. 10b). For mixed precipitation above the rain/snow boundary, the half-hour air temperature was either range frompositive to negative or above 0 ◦C (Fig. 10c). For the snow events, half-hour air temperatures are below 0 ◦C (Fig. 10d).

From these half-hour air temperature analyses, we conclude that if the half-hour air temperatures are below 0 ◦C, theprecipitation can be classified as snow; if half-hour air temperatures ranges from positive to negative during the precipitation


Fig. 11. Relative humidity during three precipitation types: (a) rainfall, (b) mixed precipitation and (c) snowfall.

ep

4

at

m8hhThHo

vent, the precipitation can be classified as mixed precipitation; and if the half-hour air temperatures are above 0 ◦C, therecipitation type can be either rain or mixed precipitation.

.4. Relative humidity for rainfall, mixed precipitation, and snowfall

Ding et al. (2014) and Harder and Pomeroy (2013) have shown that precipitation types are related to a combination ofir temperature and humidity. We selected the half-hour relative humidity during precipitation events at different altitudeso analyze the relationship of relative humidity and precipitation types.

Half-hour relative humidity observations in the HRB indicated that relative humidity is lower for snowfall events than forixed precipitation or rainfall (Fig. 11). Most of the observed relative humidity values at the rain/snow boundary were over

5%. Photographed weather conditions were cloudy or foggy during mixed precipitation or rainfall, which indicates veryigh humidity conditions. Mixed precipitation events were concentrated in the warm season, which corresponds with highumidity conditions. Some mixed precipitation events were actually shifts from rainfall to mixed or rainfall to snowfall.he falling rain adds moisture to the air, combined with falling air temperatures during precipitation event, this causesigh humidity conditions. In sub-saturated conditions, snowflakes will sublimate instead of melt into liquid water. In theRB, cloudy or foggy weather conditions indicate that sublimation or evaporation was not common. This explains why the

bserved threshold air temperature was 0 ◦C for mixed precipitation and rainfall.


y = 0.994xR² = 0.845

0

25

50

75

100

0 25 50 75 100

)%(

m0433 ta ytidimu

H evitaleR

Relative Humidity at 2980m(%)

a

y = 0.965xR² = 0.773

0

25

50

75

100

0 25 50 75 100

)%(

m0173 ta y ti dimu

H evitaleR

Relative Humidity at 2980m(%)

b

y = 0.917xR² = 0.542

0

25

50

75

100

0 25 50 75 100

)%(

m6614ta ytidimu

H evitaleR

Relative Humidity at 29 80m(%)

c

y = 1.038xR² = 0.432

0

25

50

75

100

0 25 50 75 100

)%(

m6944ta yti dimu

H e vi taleR

Relative Humidity at 29 80m(%)

d

Fig. 12. Comparison of observed relative humidity at four different locations: (a) between 2980 m and 3340 m; (b) between 2980 m and 3710 m; (c) between2980 m and 4166 m; (d) between 2980 m and 4496 m.

5. Discussion

5.1. Humidity at different altitudes

Observations in the HRB indicated that the correlation coefficients of humidity at four different altitudes decreased withincreasing distance (Fig. 12). A paired samples test indicates that the standard deviation for relative humidity is greater than25% between two different sites, and the standard error mean is more than 16%, and the differences are significant spatially.Estimation of humidity from other measurements will have a large uncertainty if the measured sites are not near to eachother. If the humidity was not available, the calculation of humidity based on observations from other site observationscould cause great uncertainty. Use of air temperature and estimated humidity to discriminate precipitation type could alsoinduce great uncertainty.

5.2. Liquid proportion in mixed precipitation

Discrimination of rain, snow and mixed precipitation and quantifying the liquid proportion in mixed precipitation arecritical to runoff estimations. In this paper, we monitored precipitation types with time-lapse cameras and manual observa-tions at point or basin scales, which enabled us to observe precipitation type at different altitudes in a mountainous region.Although we applied time-lapse photography to monitor rain/snow boundaries, and obtained threshold air temperaturesto differentiate rain, snow, and mixed precipitation, we need to quantify the liquid proportion in mixed precipitation. Therain/snow boundary is the upper limit of rain and lower limit of mixed precipitation. For other mountainous regions withoutbasin-scale observations of precipitation type, we need to find appropriate ways to discriminate precipitation types. In thiscase, a comparison of various precipitation discrimination approaches is needed in the HRB to evaluate precision at differ-

ent time scales. Future observations need to monitor the liquid proportion in mixed precipitation, which can improve theprecision of precipitation discrimination methods especially for mixed precipitation.

5

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.3. Threshold air temperature at different time scales

In this paper, we investigated daily resolution and half-hour threshold air temperatures for rainfall, mixed precipitationnd snowfall. At daily resolution, threshold air temperatures at two observation points (manual observation at 2980 m, andOS 600D camera at 4550 m) and basin-scale observation results are not identical. We found that rainfall happened if the airemperature was above 0 ◦C, but the precipitation type can be classified only unequivocally as rainfall if the air temperatures above 7.6 ◦C. Snow happened if the air temperature was below 0 ◦C. The air temperature at the rain/snow boundary shiftedetween 0 ◦C and 7.6 ◦C, so the differentiation of rainfall and mixed precipitation through a static threshold air temperature

s not appropriate.Half-hour air temperatures during the precipitation event combined with corresponding precipitation type indicated

hat rainfall happened when the half-hour air temperatures were above 0 ◦C and snowfall happened when the half-hour airemperatures were below 0 ◦C. When mixed precipitation happened, the half-hour air temperatures could be either positiver have crossed 0 ◦C during the precipitation event. The half-hour air temperatures at rain/snow boundary were above 0 ◦Curing the precipitation event.

At either a daily or half-hour resolution, 0 ◦C is the threshold air temperature between snow and rain. Discriminationf mixed precipitation and rain based on the threshold air temperature is not appropriate on a daily scale, because the airemperature at the rain/snow boundary fluctuated from 0 ◦C to 7.6 ◦C. At a half-hour resolution, when the half-hour airemperatures during the precipitation events are above 0 ◦C, the precipitation could either be rain or mixed precipitation.

hen the half-hour air temperatures fluctuated between negative and positive values during precipitation events, therecipitation type could be discriminated as mixed precipitation.

Past precipitation type observations in the Qilian Mountains usually identified two static threshold air temperatures toiscriminate snow, mixed and rain precipitation. For example, observations at the July 1st Glacier indicate that the twotatic threshold air temperatures are 2.3 ◦C and 7.2 ◦C. Within this range, the probability of rainfall increased with rising airemperature from 0 to 100% (Sakai et al., 2006). The two critical temperatures observed at the Yanglong River in the Qilian

ountains are 0 ◦C and 7.2 ◦C (Ding and Kang 1985). The lower and upper limits from our observation results are similar tohose of Ding and Kang (1985), except that in the HRB rainfall is likely to happen in this threshold temperature range.

. Conclusions

A comparative study of precipitation in two successive years in the HRB indicates that rising air temperatures inducedhifts from mixed precipitation to rain at high altitudes. Point and basin-scale observations indicated that precipitation typest high altitudes are more sensitive to air temperature changes than precipitation type at lower altitudes. Precipitation typebservations in the HRB indicate that at daily or half-hour resolutions, 0 ◦C can be used to discriminate rain and snow. Ataily resolution, air temperatures at the rain/snow precipitation boundary fluctuated between 0 ◦C and 7.6 ◦C, which meanshat precipitation can be discriminated as rain only when the daily resolution air temperatures were above 7.6 ◦C. From 0 ◦Co 7.6 ◦C, the precipitation type could be either rain or mixed precipitation. During a precipitation event, the half-hour airemperatures were above 0 ◦C at the rain/snow precipitation boundary, the half-hour air temperatures were above 0 ◦C whenainfall occurred, the half-hour air temperatures were below 0 ◦C when snowfall occurred, and the half-hour air temperatureuctuated around 0 ◦C when mixed precipitation occurred.

onflict of interest

None.

cknowledgments

This paper is supported by the National Basic Research Program of China (2013CBA01806) and the National Naturalcience Foundation of China (91025011, 41401078, 41222001, 91125002).

ppendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, atttp://dx.doi.org/10.1016/j.ejrh.2015.11.013.

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Discriminating types of precipitation in Qilian Mountains, Tibetan Plateau1 Introduction2 Study area and data3 Methods4 Results4.1 Precipitation type observation results at three locations4.2 Daily mean threshold air temperatures in the HRB4.3 Half-hour air temperatures for the three precipitation types4.4 Relative humidity for rainfall, mixed precipitation, and snowfall

5 Discussion5.1 Humidity at different altitudes5.2 Liquid proportion in mixed precipitation5.3 Threshold air temperature at different time scales

6 ConclusionsConflict of interestAcknowledgmentsAppendix A Supplementary dataReferences

contents lists available at sciencedirect journal of hydrology: regional … · 2017. 2. 15. ·...

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