Evaluating the Effect of Heat Waves on Early Melting of Snow Covers of Karkheh Catchment in Iran

The present study aims to evaluate the effect of heat waves on the early melting of snow cover in the Karkheh catchment of Iran. After collecting daily data on the maximum temperature of meteorological stations in the catchment during the statistical period (2000-2019), three indices of WSDI, Baldi, and ocular method were used to determine the threshold of days with heat wave. By extracting the hot waves and applying programming, the snow cover maps were drawn in the Google Earth Engine system and the area of ​​snow cover zones was calculated in the Model Builder environment. Finally, the atmospheric data were received from the NCEP/NCAR site and their generating patterns were examined after drawing in Grads software environment after determining the atmospheric synoptic patterns. The results indicated that the slope trend of hot waves is increasing at the catchment level. The average annual frequency of heat waves extracted for the snow cover growth period (November to May) was equal to 24 days of hot waves during the year. Regarding the ground pressure conditions and synoptic conditions of high atmospheric levels, two dominant synoptic patterns of heat waves were identified in the region as follows: 1) The Siberian high-pressure synoptic pattern at the ground level and the Saudi-African high-pressure ridge at high atmospheric levels. This synoptic pattern reduces snow cover area by an average of 40% in the Karkheh catchment and the highest frequency of the occurrence of this synoptic pattern is in February and March. 2) Pakistan-Africa low-pressure synoptic pattern at ground level and Saudi-African high-pressure ridge at high atmospheric levels, the frequency of the occurrence of which is higher in April and May months and reduces the snow cover area in the Karkheh catchment by an average of more than 55%.

Integrated impacts of synoptic forcing and aerosol radiative effect on boundary layer and pollution in the Beijing–Tianjin–Hebei region, China

Rapid urbanization and industrialization have led to deterioration of air quality in the Beijing–Tianjin–Hebei (BTH) region due to high loadings of PM2.5. Heavy aerosol pollution frequently occurs in winter, in close relation to the planetary boundary layer (PBL) meteorology. To unravel the physical processes that influence PBL structure and aerosol pollution in BTH, this study combined long-term observational data analyses, synoptic pattern classification, and meteorology–chemistry coupled simulations. During the winter of 2017 and 2018, Beijing and Tangshan often experienced heavy PM2.5 pollution simultaneously, accompanied by strong thermal inversion aloft. These concurrences of pollution in different cities were primarily regulated by the large-scale synoptic conditions. Using principal component analysis with geopotential height fields at the 850 hPa level during winter, two typical synoptic patterns associated with heavy pollution in BTH were identified. One pattern is characterized by a southeast-to-north pressure gradient across BTH, and the other is associated with high pressure in eastern China. Both synoptic types feature warmer air temperature at 1000 m a.g.l., which could suppress the development of the PBL. Under these unfavorable synoptic conditions, aerosols can modulate PBL structure through the radiative effect, which was examined using numerical simulations. The aerosol radiative effect can significantly lower the daytime boundary layer height through cooling the surface layer and heating the upper part of the PBL, leading to the deterioration of air quality. This PBL–aerosol feedback is sensitive to the aerosol vertical structure, which is more effective when the synoptic pattern can distribute more aerosols to the upper PBL.

Integrated impacts of synoptic forcing and aerosol radiative effect on boundary layer and pollution in the Beijing-Tianjin-Hebei region, China

Rapid urbanization and industrialization have led to deterioration of air quality in the Beijing-Tianjin-Hebei (BTH) region with high loadings of PM2.5. The heavy aerosol pollutions frequently occur in winter, closely in relation to the meteorological conditions. To unravel the complicated impacts of large-scale atmospheric forcing and the local-scale planetary boundary layer (PBL) characteristics on the pollution there, this study combined long-term observational data analyses, synoptic pattern classification, and meteorology-chemistry coupled simulations. During the winter of 2017 and 2018, Beijing, Langfang, Tianjin, and Tangshan often simultaneously experienced heavy PM2.5 pollution, accompanying with strong thermal inversion aloft. These concurrences of pollution in different cities were primarily regulated by the large-scale atmospheric processes. Using the principal component analysis with the geopotential height fields at the 850-hPa level during winter, the typical polluted synoptic pattern in BTH was identified. The pattern was featured by westerly winds from upstream mountainous regions. By inducing warm advections from the west, the thermal inversion aloft in the BTH could be enhanced, leading to shallow daytime PBLs and high near-surface PM2.5 concentrations. In addition, the aerosol may also modulate the PBL structure through its radiative effect, which was examined using numerical simulations. The aerosol radiative effect can significantly lower the boundary layer height in the afternoon through cooling the surface layer and heating the upper part of PBL. Thus, more aerosols could be accumulated in the lower portion of PBL, bringing about heavy pollution in the BTH. This study has revealed the important roles played by the meteorology-aerosol interaction on the air quality.

How Sea Fog Influences Inland Visibility on the Southern China Coast

Sea fog can lead to inland fog on the southern China coast, affecting visibility on land. To better understand how such fog influences inland visibility, we observed two sea-fog cases at three sites (over sea, at coast, and inland) and analyzed the results here. Our analysis suggests four factors may be key: (1) The synoptic pattern is the decisive factor determining whether fog forms inland. First, sea fog and low clouds form when the synoptic pattern involves warm, moist air moving from a warmer sea-surface temperature (SST) region to a colder SST region near the coast. Then, inland fog tends to occur under this low-cloud background with relatively large horizontal-vapor transport. A greater horizontal-vapor transport results in denser fog with higher liquid-water content. Conversely, a strong horizontal advection of temperature with less horizontal-vapor transport can hinder inland-fog formation. (2) Local cooling (including ground radiative cooling) helps promote inland fog formation. (3) Fog formation requires low wind speed and small turbulent kinetic energy (TKE). The small TKE helps the vapor accumulate close to the surface and maintain the local cooling effect. (4) Fog formation is promoted by having the energy flux downward at night with the land surface cooling the atmosphere as well as having lower soil temperature and higher soil humidity.