Long-term vegetation phenology changes and response to multi-scale meteorological drought on the Loess Plateau, China

Investigating the response of vegetation phenology to meteorological drought can provide insight into the feedback mechanisms of ecosystems to environmental and climate change. These key phenological parameters of vegetation are extracted on the China's Loess Plateau (LP) based on TimeSat using MODIS remote sensing data, and their trends are estimated using Sen's slope. Then, response of key physical parameters of vegetation phenology to multi-scale drought was investigated using Standardized Precipitation Evapotranspiration Index (SPEI). The results show that: 1) LP vegetation Start of Season (SOS) and Peak of Season (POS) are delayed from south (low elevation) to north (high elevation). End of Season (EOS) is delayed from northeast to southwest. Length of Season (LOS) show decrease trend with altitude and latitude increasing. Besides, temporal variation of different vegetation phenological periods is highest in farmland and is lowest in the forest. 2) Vegetation SOS and POS are advanced in the eastern part of LP and delayed in the western part. Meanwhile, EOS and LOS are advanced in the southern part of LP. 3) The POS and EOS of LP vegetation responded more significantly to drought. The western Longzhong Plateau of LP was susceptible to the negative effects of preseason drought, and the eastern Northern Shaanxi Plateau and Taihang Mountains were susceptible to the positive effects. Moreover, monthly- and seasonal-scale droughts have significantly effects on vegetation phenology than annual-scale drought. 4) Compared to forest and grassland, the vegetation phenology of farmland is more sensitive to monthly- and seasonal-scale droughts. However, the response of forest to annual-scale drought is more significant. The results of the study could provide scientific basis for crop yield stabilization and agricultural disaster warning in the LP. Keywords: Vegetation phenology; TimeSat; Drought response; Loess Plateau

Diurnal Variations in Different Precipitation Duration Events over the Yangtze River Delta Urban Agglomeration

Studying the characteristics of precipitation diurnal variation is beneficial for understanding precipitation formation and underlying mechanisms. In this study, using hourly rain gauge data from 108 stations in the Yangtze River Delta Urban Agglomeration (YRDUA) from 1980–2021, the diurnal variations of the precipitation amount (PA), precipitation frequency (PF), precipitation duration (PD), and precipitation intensity (PI) were analyzed. The effects of elevation, distance of the station from the east coastline, and urbanization on the characteristics of different precipitation duration events were determined. The results indicated that (1) the spatial distributions of PA, PD, and PF were similar in short-duration (SD), long-duration (LD), and ultra-long-duration (ULD), with high values in the south and low values in the north. Most of PA, PD, and PF showed an increasing trend after breakpoint in LD and ULD, but precipitation characteristics in SD showed a decreasing trend before and after breakpoint; (2) the diurnal cycles of PA presented two comparable peaks in the late afternoon and early morning, which occurred SD and ULD precipitation events, respectively. A single peak in the late afternoon (15:00 local solar time [LST]) occurred during the diurnal cycle of PI. The start and peak times occurred mainly in the afternoon for SD and LD. In contrast, the peak time of ULD mainly occurred in the early morning, accounting for 63% of the stations. The start and peak times of LD and ULD occurred in the early morning mainly along the Yangtze River; (3) from the plains to the mountains, the diurnal peaks of PA and PI had gradual variations from noon to afternoon. In addition, dominant diurnal peak values of PA and PI, which are affected by the distance from the east coast, were observed in the early morning in ULD. The effect of urbanization on the difference between urban and rural areas changed from negative to positive after 2000. In addition, urbanization had a significant impact on SD. After 2000, the increase of PA in urban areas was mainly due to the obvious increase of PD and PF in SD, while the increasing trend of LD and ULD in urban areas was smaller than that in rural areas

Diurnal Variations in Different Precipitation Duration Events over the Yangtze River Delta Urban Agglomeration

Studying the characteristics of precipitation diurnal variation is beneficial for understanding precipitation formation and underlying mechanisms. In this study, using hourly rain gauge data from 108 stations in the Yangtze River Delta Urban Agglomeration (YRDUA) from 1980–2021, the diurnal variations of the precipitation amount (PA), precipitation frequency (PF), precipitation duration (PD), and precipitation intensity (PI) were analyzed. The effects of elevation, distance of the station from the east coastline, and urbanization on the characteristics of different precipitation duration events were determined. The results indicated that (1) the spatial distributions of PA, PD, and PF were similar in short-duration (SD), long-duration (LD), and ultra-long-duration (ULD), with high values in the south and low values in the north. Most of PA, PD, and PF showed an increasing trend after breakpoint in LD and ULD, but precipitation characteristics in SD showed a decreasing trend before and after breakpoint; (2) the diurnal cycles of PA presented two comparable peaks in the late afternoon and early morning, which occurred SD and ULD precipitation events, respectively. A single peak in the late afternoon (15:00 local solar time [LST]) occurred during the diurnal cycle of PI. The start and peak times occurred mainly in the afternoon for SD and LD. In contrast, the peak time of ULD mainly occurred in the early morning, accounting for 63% of the stations. The start and peak times of LD and ULD occurred in the early morning mainly along the Yangtze River; (3) from the plains to the mountains, the diurnal peaks of PA and PI had gradual variations from noon to afternoon. In addition, dominant diurnal peak values of PA and PI, which are affected by the distance from the east coast, were observed in the early morning in ULD. The effect of urbanization on the difference between urban and rural areas changed from negative to positive after 2000. In addition, urbanization had a significant impact on SD. After 2000, the increase of PA in urban areas was mainly due to the obvious increase of PD and PF in SD, while the increasing trend of LD and ULD in urban areas was smaller than that in rural areas