Characteristics of solar radiation at Xiaotang, in the northern marginal zone of the Taklimakan Desert

Study area

Xiaotang is located at the northern margin of the Taklimakan Desert, northwestern China (Fig. 1). This area lies in the desert–oasis transition zone on the northern margin of the Taklimakan Desert. The underlying surface is a relatively flat, bare, wind-eroded ancient riverbed. It has a warm temperate desert climate that is extremely arid and has a high potential for evaporation. The annual average temperature, precipitation, and wind speed are 10 °C, 15.2 mm, and 2.5 m s⁻¹, respectively. The frequency of sandy and dusty weather is extremely high. Sandstorms and dusty devils occur in all seasons. Surface soils mainly comprise fine sand (125–250 µm) and very fine sand (62.5–125 µm) (Ma et al., 2020).

The observation site is in the northern margin of Taklimakan Desert. The Xiaotang land–air interaction observatory station (40.8°N, 84.3°E; altitude = 932 m) is on the south bank of the ancient riverbed, ∼2 km north of a Populus euphratica forest that is minimally affected by human activities. According to the meteorological data from the Xiaotang observatory station, the average temperature of the northern margin of Taklimakan Desert in 2018 was 11.3 °C, the highest temperature was 39.4 °C (in summer), and the lowest temperature was −40. 1 °C (in winter) (Fig. 2). In 2018, the annual average wind speed was 1.6 m s⁻¹, and the annual relative humidity was 34.95%.

Observation instrument

The Xiaotang land–atmosphere interaction observatory and experiment station used internationally recognized radiation detection sensors (Table 1). The measured parameters included total, direct, and scattered radiations. The model of A CR3000 micrologger (Campbell, United States) was used as the radiation data collector. The acquisition frequency was 1 s, and the output data were obtained at 1 s, 1 min, 30 min, and 1 h. In this paper, we used hourly data for our analysis. The clock of the collector adopted the local real solar time, which later Beijing time by 2 h 22 min 48 s.

According to the specifications, the radiometer was inspected and maintained every day, and the radiometer was wiped before sunrise. Some false data appeared due to instrument system errors, instrument failure, etc. during data transmission and recording. The following data must be corrected or eliminated: the output flux value is NAN.

Data and methods

The satellite data were obtained from the CERES_SYN1deg_Ed4A daily product data provided by the Atmospheric Science Data Center at NASA Langley Research Center. The accuracy of the CERES sensor for the atmospheric AOD measurements was 1°. Aerosol optical thicknesses were obtained using an aerosol transport model MATCH (Collins et al., 2001) that assimilated and spatially as well as temporally interpolated MODIS aerosol optical thickness. Additionally, MATCH provided aerosol types. The atmospheric AOD data were downloaded from the following link: http://ceres.larc.nasa.gov/.

The total, direct, scattered radiations’ data and ground meteorological observation data from the Xiaotang land–atmosphere interaction observatory and experiment station from January 1, 2018 to December 31, 2018 were used in this study. In this study, the months of March–May, June–August, September–November, and December–February are considered as spring, summer, autumn, and winter, respectively.

Kipp & Zonen’s A2P automatic tracker direct radiation meter was used to measure the direct solar radiation on the vertical plane; however, the direct solar radiation on the horizontal plane was required to analyze the direct radiation. Therefore, the direct radiation measured by the instrument was converted using the following formula (Mamtimin et al., 2014): (1)${\displaystyle R_b=R_{b}sinh,}$ where R_b is the direct horizontal radiation, R_b ’ is the direct normal radiation, and h is the solar height angle.

In (Eq. 1), the solar height angle was calculated using the following equation: (2)${\displaystyle sinh=sin\phi sin\delta +cos\phi cos\delta cost,}$ where φ (radian) is the local latitude, δ (radian) is the solar declination, and t is the hour angle.

The ratio of total radiation to total astronomical radiation is equivalent to the transmittance of the whole atmosphere. This parameter is called atmospheric transmittance in this paper (Tian et al., 2018).

The atmospheric transparency coefficient is an essential parameter for characterizing the degree of atmospheric turbidity. Therefore, the variation of the atmospheric transparency coefficient was used to discuss the influence of dust aerosols on direct solar radiation. The atmospheric transparency coefficient was calculated according to Eqs. (3) and (4). (3)${\displaystyle S=S_0{P}^m}$ (4)${\displaystyle P={\left(\frac{S}{S_0}\right)}^{1/m}}$ where S is direct radiation, S₀ is the solar constant, and m is the atmospheric mass. We obtained the value of S when m = 2, and calculated P ₂ according to (Eq. 5). (5)${\displaystyle P_2=\left[\frac{m/2P^m}{1-\left(\frac{1}{2}m\right)P^m}\right],}$ where P₂ isthe atmospheric transparency when the relative atmospheric mass is corrected to be 2 (that is, the solar altitude angle = 90°).

The characteristics of total, direct, and scattered radiations at Xiaotang during different seasons and sandy and dusty weathers were analyzed. Five typical days were selected for each clear, floating-dust, blowing-sand, and sandstorm weathers from spring to summer, when sandy and dusty days were more frequent. However, owing to limitations in the weather phenomena, four sunny days were selected for spring and summer and two floating-dust days were selected for summer. The daily changes in the sand and dust at Xiaotang were analyzed. Because dusty weather at Xiaotang was mainly observed during spring and summer, autumn and winter were not analyzed here.

A total cloudiness of <20% and no weather phenomenon indicated a clear day. When the dust and fine sand floated evenly in the air such that the horizontal visibility was <10 km, indicated a floating-dust day. Blowing-sand and sandstorm indicated the weather phenomenon in which the wind lifts the dust from the ground, making the horizontal visibility in the range of 1–10 km and less than 1 km, respectively.

In spring, the selected clear days were March 1, April 25, April 30, and May 12; the floating-dust days were March 6, March 7, March 9, March 16, and March 20; the blowing-sand days were March 17, March 29, April 5, April 12, and April 19; and the sandstorm days were March 3, March 19, April 2, April 11, and May 11. Their averages under these different weather conditions were then calculated.

In summer, the selected typical clear days were June 19, July 22, July 23, and August 21; the floating-dust days were June 1 and 2; the blowing-sand days were June 5, June 15, June 28, July 16, and July 24; and the sandstorm days were June 16, June 30, July 15, August 23, and August 29. Then, their averages were calculated under these different weather conditions.

Monthly variation of total, direct, and scattered radiations

In 2018, the total, direct, and scattered radiations at Xiaotang show fluctuating distributions with an evident seasonality (Fig. 3). The annual total, direct, and scattered radiations are 5781.8, 2337.9, and 3323.8 MJ m⁻², respectively. The total annual radiation at the Xiaotang area (5781.8 MJ m⁻²) is lower than that at the Tazhong Station (6515 MJ m⁻²), which is in the hinterland of the Taklimakan Desert.

Figure 4 shows that the maximum monthly total radiation concentration occurs from June to August and the total radiation peaks in July (679.8 MJ m⁻²), accounting for 11.7% of the total radiation for the whole year.

The total annual direct radiation is 2337.9 MJ m⁻², accounting for 42.4% of the annual total radiation. The peak value of the monthly total amount is seen in August (317.3 MJ m⁻²), accounting for ∼12.6% of the total annual direct radiation, and it reaches the lowest value (102.1 MJ m⁻²) in December (winter).

The scattered radiation begins to rise in January (138.1 MJ m⁻²) and reaches the peak value (455.7 MJ m⁻²) in May, which accounts for 13.7% of the total annual scattered radiation. Under high solar height angles observed in June, July, and August, the scattered radiation is 396.4, 353.4, and 314.9 MJ m⁻², respectively. The total, direct, and scattered radiations reach the lowest levels in December, with monthly totals of 242.4, 102.1, and 133.6 MJ m⁻², respectively.

Figure 5 shows that high daily variations of solar radiation reflect frequent changes in weather conditions and synoptic events. The amount of total radiation gradually increases from spring, and the peak value is reached in summer (1967.8 MJ m⁻²), which accounts for 34.03% of the annual total radiation; the values are mainly concentrated between 15 and 30 MJ m⁻². It reaches the lowest value in winter (862.9 MJ m⁻²). The values in December are mainly concentrated between 7 and 9 MJ m⁻² for 23 days, and the values in January–February are mainly concentrated between 9 and 12 MJ m⁻² for 31 days. The amount of total radiation follows an order of summer > spring > autumn > winter.

The amount of direct radiation is less in spring (449.5 MJ m⁻²). The total amount of direct radiation in summer is 864.4 MJ m⁻², accounting for 35.3% of the total annual direct radiation. The amounts in spring and summer are mainly concentrated between 0 and 15 MJ m⁻², with large fluctuations and lasting for 164 days. The value gradually decreases in autumn. In winter, the amount of direct radiation reaches the lowest value of 364.3 MJ m⁻², with the magnitude mainly concentrated between 2 and 5 MJ m⁻² for 54 days. The amount of direct radiation follows an order of summer > autumn > spring > winter.

During the year, the amount of scattered radiation reaches the maximum value in spring (1182.4 MJ m⁻²). The amount of scattered radiation in summer is 1064.7 MJ m⁻², and the total amount in spring and summer accounts for 67.61% of the total scattered radiation in the year. The amounts of scattered radiation in the two seasons are mainly concentrated between 8 and 18 MJ m⁻², with great fluctuations and lasting for 158 days. In spring and summer, the scattered radiation lasts for 141 days longer than the direct radiation. The scattering radiation in autumn is 619.1 MJ m⁻², and the value gradually decreases. The lowest value of scattered radiation is 457.7 MJ m⁻² (winter), accounting for 13.8% of the annual total amount of scattered radiation. The value of scattered radiation is mainly concentrated between 3 and 6 MJ m⁻² and lasts for 80 days in December. The amount of scattered radiation follows an order of spring > summer > autumn > winter.

Relationship between solar radiation and solar height angle

Linear regression is used to analyze the change in total radiation with the solar height angle under different weather conditions (Fig. 6). The square values of the correlation coefficients are 0.97 and 0.81 for clear and sandstorm days, respectively. In sandstorm weather, the total radiation slowly increases with the increase in the solar height angle, mainly due to the increase in dust content in the air. When the solar height angle is 10°, the total radiation amounts in clear, floating-dust, blowing-sand, and sandstorm days are 118.6, 19.7, 10.4, and 4.0 W m⁻², respectively. The higher the solar height angle, the more the radiation received by the ground. However, at the same solar height angle, the total radiation is the highest on clear days, while the total radiation in sandstorm weather is the lowest (Fig. 6). Solar radiation is affected by the solar height angle, atmospheric conditions, latitude, water vapor, cloud cover, and other factors. Here, this is mainly the influence of atmospheric conditions on solar radiation.

Characteristics of atmospheric transmittance

In total, 55.9% of astronomical radiation enters the ground, and the annual variation range is mainly between 30% and 74%. On clear days, the total radiation reaching the ground is 68.1%, with a variation range of 60%–70%. The total radiation reaching the ground on dusty days is 47.6%, with a variation range of 30%–60%. The variation range on clear days is mainly concentrated between 60% and 70% and that on the sandy and dusty days is mainly concentrated between 30% and 60%. The annual average atmospheric transmittances on clear, floating-dust, blowing-sand, and sandstorm days at the Xiaotang area are 0.67, 0.46, 0.44, and 0.30, respectively. Many studies show that the particle-size distribution, vertical distribution, and mineral composition of dust have important effects on radiation (Tegen & Fung, 1994; Sokolik & Golitsyn, 1993a; Sokolik & Golitsyn, 1993b; Sokolik & Toon, 1996).

As seen in Fig. 7, atmospheric conditions affect the total and scattered radiations. The higher the atmospheric transmittance, the weaker the scattered radiation and the smaller the ratio of the scattered radiation to the total radiation (R_d/R_s) (Fig. 7A). By contrast, as the atmospheric transmittance decreases, the scattered radiation and R_d/R_s increase. The ratio of the scattering radiation to the astronomical radiation (R_d/Q_d) indicates the contribution of the atmosphere to scattered radiation (relative to astronomical radiation). (Fig. 7B). However, when the atmospheric transmittance is larger than 0.5, R_d/Q_d begins to decrease.

Diurnal variation of radiation on dusty day in spring and summer

Figure 8 shows that the distributions of hourly total, direct, and scattered radiations are normal during midday on clear days. The amount of radiation increases from sunrise and reaches the maximum value at 12:00 LST or 13:00 LST in spring and summer. By contrast, the distribution becomes irregular owing to the reduction in total radiation and direct radiation associated with dust aerosols. The value of direct radiation decreases at the Xiaotang Station. In spring (Fig. 8A), the daily peaks of total, direct, and scattered radiations on clear days are 862.4, 504.5, and 339.6 W m⁻², while in summer (Fig. 8B), the daily peaks are 915.8, 643.4, and 253.5 W m⁻², respectively.

In spring (Fig. 8A), compared with the radiation values on clear days, the daily total values of total radiation decrease by 41.7%, 43.8%, and 62.8%; those of direct radiation decrease by 89.6%, 88.0%, and 97.6%; and the total daily values of scattered radiation increase by 9.1%, 6.4%, and 4.9% in floating-dust, blowing-sand, and sandstorm days, respectively.

Figure 8A shows that the daily variation curves of total and scattered radiations on blowing-sand and sandstorm days are similar, and the magnitudes are close. In summer (Fig. 8B), compared with the radiation values on clear days, the daily total values of total radiation decrease by 18.5%, 26.9%, and 57.5%, in floating-dust, blowing-sand, and sandstorm days, respectively. Those of direct radiation decrease by 81.5%, 76.4%, and 89.3%; and the total daily values of scattered radiation increase by 52.2%, 51.1%, and 9.2% in floating-dust, blowing-sand, and sandstorm days, respectively.

In general, the impact of sand and dust on radiation at the Xiaotang area is greater in spring than that in summer. According to the ground observation records (Table 2), the rainfall at the Xiaotang area is mainly concentrated in summer, with a total of 37 days. The occurrence of sandstorms is often accompanied by rainfall, which reduces the dust content in the air. In summer, there are nine sandstorms, five of which are accompanied by rainfall. In spring, there are more dusty days and less rainfall. Wang, Dong & Chen (2001) calculated the sand transport rate formula fitted using field-measured data and found that the annual sediment transport in the Xiaotang area was 3,800 kg m⁻¹, which is higher than the annual sediment transport in some oases on the edge of Tarim (Yang, Li & He, 2012), and the direction of sand transport was relatively scattered in the Xiaotang area. The underlying soil comprises fine (125–250 µm) and very fine (62.5–125 µm) sand. The smaller the particle size, the stronger the scattering effect on light and the larger the scattering angle.

Probability distribution characteristics of total, direct, and scattered radiations in different dust weathers

The total, direct, and scattered radiations at the Xiaotang area differed under the various weather conditions. Therefore, the distribution law was further analyzed, and the total, direct, and scattered radiations on clear, floating-dust, blowing-sand, and sandstorm days at the Xiaotang area were statistically examined. The probability distribution of the radiations is shown in Fig. 9.

The total radiation can reach 900–100 W m⁻² in clear, floating-dust, and blowing-sand weather, while the value varies between 700 and 800 W m⁻²on sandstorm days. In sandstorm weather, the probability of total radiation >600 W m⁻² is only 2.2%. On clear, floating-dust, blowing-sand, and sandstorm days, the magnitude in high-value of total radiation areas gradually decreases, gradually concentrating in low-value of total radiation areas. The weakening of total radiation by dust is mainly concentrated in high-value of total radiation areas.

The maximum value of direct radiation varies from 800 to 900 W m⁻² on clear days, while the value varies between 100 and 200 W m⁻²in sandstorm days. The probabilities of the occurrence of direct radiation <500 W m⁻² are 89.6%, 100%, 99.4%, and 100% on clear, floating-dust, blowing-sand, and sandstorm days, respectively. Compared with total radiation, sand and dust have a stronger weakening effect on direct radiation; especially in sandstorm weather, the magnitude is below 200 W m⁻². The probabilities of occurrence of direct radiation <200W m⁻² are 44.5%, 93.5%, 91.3%, and 100% in clear, floating-dust, blowing-sand, and sandstorm days, respectively.

On clear days, scattered radiation is mainly concentrated between 1 and 300 W m⁻². In sandy and dusty weather, the maximum scattered radiation can exceed 600 W m⁻². The probabilities of scattered radiation <300 W m⁻² occurring on clear, floating-dust, blowing-sand, and sandstorm days are 94.2%, 62.5%, 61.1%, and 72.1% and those for the occurrence of scattered radiation <600 W m⁻² are 100%, 99.1%, 98.1%, and 100%, respectively. Therefore, as the content of sand and dust in the air increases within a certain range, scattered radiation can be enhanced, especially in floating-dust and blowing-sand weather.

Characteristics of atmospheric transparency coefficient

The average P₂ values on clear, floating-dust, blowing-sand, and sandstorm days at the Xiaotang area are 0.57, 0.34, 0.17, and 0.08, respectively. The total, direct, and scattered radiations are closely related to P₂ (Fig. 10). P₂ has a positive correlation with the ratio of the direct radiation to the total radiation (R_b/R_s) (R² = 0.78) (Fig. 10A). R_b/R_s increases with an increase in P ₂, showing an asymmetrical relationship. When P₂ = 0, R_b/R_s tends to 0. P ₂ is inversely correlated with R_d/R_s (R² = 0.49) (Fig. 10B); R_d/R_s decreases with an increase in P ₂. When P₂ = 0, R_d/R _s tends to 1.

Direct radiation, scattered radiation, and AOD

Figure 11 displays the temporal changes in direct radiation, scattered radiation, and AOD in 2018. The figure shows that the annual variation of AOD is almost the same as that of scattered radiation, reaching the maximum in spring, May (0.71). The two variations have a good consistency. AOD is negatively correlated with direct radiation; there is an evident positive correlation between AOD and scattered radiation, while AOD is the most influencing and important factor for scattered radiation at the Xiaotang area (Huang et al., 2020). AOD is negatively correlated with direct and scattered radiations. Because the Xiaotang area is in the northern margin of Taklimakan Desert, the main aerosol in the air is dust, which leads to the enhancement of scattered radiation.