Spatiotemporal variability of snowfall and its concentration in northern **njiang, Northwest China

Abstract

This study investigated the spatial and temporal variability of snowfall and its concentration across northern **njiang, Northwest China based on the corrected dataset (derived from 26 stations) using a wet-bulb temperature method during the snowfall hydrological years from 1961 to 2017. The snowfall concentration was analyzed by means of three indices: the snowfall concentration index (CI), snowfall concentration degree (SCD), and snowfall concentration period (SCP). The results demonstrated that the snowfall and temperature increased by 5.69 mm and 0.35 °C per decade, respectively, in northern **njiang during the time period from 1961 to 2017. Maximum snowfall occurred at a critical temperature ranging from − 2 to 1 °C. The increased snowfall mainly took place in northern **njiang during winter. The distribution of the high snowfall CI value indicated that the daily snowfall showed a high irregularity in the northern slope of the Tianshan Mountains and the northern Altay Mountains, in which more than 77% snowfall was contributed by the 25% snowfall days in a year. The results of SCP revealed that the snowfall concentrated in northern **njiang during December, with an earlier arrival in the north and east of northern **njiang than in the western part. The SCD results proved that the monthly snowfall was more concentrated in the northern Altay Mountains, Ili Valley, and Boertala Valley than over the eastern part of northern **njiang during a year. The snowfall in almost all stations increased in concentration throughout the year. The obtained study results could provide scientific reference for future water resource management and snow disaster prevention under a warming climate.

Appendices

Appendix 1: T_min, T_max, and T_w

The T_min and T_max are given below:

$$ {\mathrm{T}}_{\mathrm{min}}\left\{\begin{array}{l}{T}_0-\varDelta S\ast \ln \left[\exp \left(\frac{\varDelta T}{\varDelta \mathrm{S}}-2\ast \exp \left(-\frac{\varDelta T}{\varDelta S}\right)\right)\right],\kern0.5em \frac{\varDelta T}{\varDelta S}>\ln 2\\ {}{T}_0,\kern15.75em \frac{\varDelta T}{\varDelta S}\le \ln 2\end{array}\right.\kern0.5em $$

(12)

$$ {\mathrm{T}}_{\mathrm{max}}\left\{\begin{array}{l}2\ast {T}_0-{T}_{\mathrm{min}},\kern0.5em \frac{\varDelta T}{\varDelta S}>\ln 2\\ {}{T}_0,\kern4em \frac{\varDelta T}{\varDelta S}\le \ln 2\end{array}\right.\kern0.5em $$

(13)

where ∆T represents the temperature difference between the snow probability and the cumulative probability of snow and sleet. ∆S represents a temperature scale. T₀ is the temperature which determines the occurrence probability of snow and rain. ln2 stands for the equivalent to RH = 78%. If RH < 78%, T_min and T_max are equal; otherwise, T_min and T_max demonstrate different values (Ding et al. 2014). ∆T, ∆S, and T₀ depend on RH and could be calculated using the following equation:

$$ \varDelta \mathrm{T}=0.215-0.099\times RH+1.018\times {RH}^2 $$

(14)

$$ \varDelta \mathrm{S}=2.374-1.634\times \mathrm{RH} $$

(15)

$$ {\mathrm{T}}_0=-5.87-0.104\ast \mathrm{Z}+0.0885\ast {Z}^2+16.06\times RH-9.614\times {RH}^2 $$

(16)

where RH is the mean daily relative humidity (range [0, 1]) and Z the elevation (km) of the meteorological station.

T_W is the wet-bulb temperature (°C) and could be calculated below:

$$ {T}_w={T}_a-\frac{e_{sat}\left({T}_a\right)-\left(1- RH\right)}{0.000643{p}_s+\frac{\partial {e}_{sat}}{\partial {T}_a}} $$

(17)

where T_a represents the daily air temperature (°C) and P_s stands for the daily air pressure (hPa), and e_sat shows the saturated vapor pressure (hPa). According to Tetens’s empirical formula (Murray 1967), e_sat could be calculated as below:

$$ {e}_{sat}\left({T}_a\right)=6.1078\exp \left(\frac{17.27{T}_a}{T_a+237.3}\right) $$

(18)

Appendix 2: K = 1/catch ratio (CR)

CR is calculated from the air temperature and wind speed, expressed as follows (Ye et al. 2005):

$$ {CR}_{snow}=\exp \left(-0.056 Ws\right)\times 100\kern0.5em \left(0< Ws<6.2\right) $$

(19)

$$ {CR}_{rain}=\exp \left(-0.04 Ws\right)\times 100\kern0.5em \left(0< Ws<7.3\right) $$

(20)

$$ {CR}_{sleet}={CR}_{snow}-\left({CR}_{snow}-{CR}_{rain}\right)\times \left({T}_a+2\right)/4 $$

(21)

where Ws demonstrates the wind speed at a 10 m height (m s⁻¹).