The diurnal evolution of the atmospheric boundary layer—the lowermost part of the atmosphere where the majority of human activity and meteorological phenomena take place—is described by its depth. Additionally, the boundary layer height (BLH) and the turbulence intensity strongly impact the pollutant diffusion, especially during transition periods. Based on integrated observations from a 325-m meteorological tower and a Doppler Wind lidar in the center of Beijing, the entire diurnal cycle of urban BLH in December 2016 was characterized. Results highlight that the Doppler lidar exhibited it is well suited for monitoring convective BLH while it trudges in monitoring stable BLH, while a 325-m meteorological tower provided an important supplement for Doppler lidar under nocturnal boundary layer and heavily polluted conditions. For the diurnal cycle, under light wind condition, the pattern of urban BLH was largely modulated by thermal forcing of solar radiation and may partly be affected by wind speed. While under strong wind condition, the pattern of urban BLH was largely modulated both by thermal forcing and dynamical forcing. The present work also presented evidence for several new features in the morning and afternoon transitions of the urban boundary layer, showing the duration of the morning transition varied between 1 and 5 h, with the largest value occurring under weak wind with high PM_2.5 concentration; while the afternoon transition ranged from 3 to 6 h, which was positively (negatively) correlated to wind speed (PM_2.5 concentration). Our work highlights that weak wind speed (weak dynamic motion) and heavy aerosol pollution (weak thermal forcing due to the effect of cooling) can dramatically affect the evolution of the boundary layer.

In the Bay of Bengal (BoB) area, landfalling Tropical Cyclones (TCs) often produce heavy rainfall that results in coastal flooding and causes enormous loss of life and property. However, the rainfall contribution of TCs in this area has not yet been systematically investigated. To fulfil this objective, firstly, this paper used TC best track data from the Indian Meteorological Department (IMD) to analyze TC activity in this area from 1998 to 2016 (January–December). It showed that on average there were 2.47 TCs per year generated in BoB. In 1998, 1999, 2000, 2005, 2008, 2009, 2010, 2013, and 2016 there were 3 or more TCs; while in 2001, 2004, 2011, 2012, and 2015, there was only 1 TC. On a monthly basis, the maximum TC activity was in May, October, and November, and the lowest TC activity was from January to April and in July. Rainfall data from the Tropical Rainfall Measurement Mission (TRMM) were used to estimate TC rainfall contribution (i.e., how much TC contributed to the total rainfall) on an interannual and monthly scale. The result showed that TCs accounted for around 8% of total overland rainfall during 1998–2016, and with a minimum of 1% in 2011 and a maximum of 34% in 1999. On the monthly basis, TCs’ limited rainfall contribution overland was found from January to April and in July (less than 14%), whereas the maximum TC rainfall contribution overland was in November and December (16%), May (15%), and October (14%). The probability density functions showed that, in a stronger TC, heavier rainfall accounted for more percentages. However, there was little correlation between TC rainfall contribution and TC intensity, because the TC rainfall contribution was also influenced by the TC rainfall area and frequency, and as well the occurrence of other rainfall systems.

Previous studies on the impact of urbanization on the diurnal temperature range (DTR) have mainly concentrated on the intra-seasonal and interannual–decadal scales, while relatively fewer studies have considered synoptic scales. In particular, the modulation of DTR by different synoptic weather patterns (SWPs) is not yet fully understood. Taking the urban agglomeration of the Yangtze River Delta region (YRDUA) in eastern China as an example, and by using random forest machine learning and objective weather classification methods, this paper analyzes the characteristics of DTR and its urban–rural differences (DTR_U–R) in summer from 2013 to 2016, based on surface meteorological observations, satellite remote sensing, and reanalysis data. Ultimately, the influences of urbanization-related factors and different large-scale SWPs on DTR and DTR_U–R are explored. Results show that YRDUA is controlled by four SWPs in the 850-hPa geopotential height field in summer, and the DTRs in three sub-regions are significantly different under the four SWPs, indicating that they play a role in regulating the DTR in YRDUA. In terms of the average DTR for each SWP, the southern sub-region of the YRDUA is the highest, followed by the northern sub-region, and the middle sub-region is the lowest, which is most significantly affected by high-level urbanization and high anthropogenic heat emission. The DTR_U–R is negative and differs under the four different SWPs with variation in sunshine and rainfall. The difference in anthropogenic heat flux between urban and rural areas is one of the potentially important urbanization-related drivers for DTR_U–R. Our findings help towards furthering our understanding of the response of DTR in urban agglomerations to different SWPs via the modulation of local meteorological conditions.

The vertical profiles and trends of temperature and humidity at the South Pole up to 10 km above mean sea level (amsl) were investigated by using radiosonde data collected from March 2005 to February 2018. During an average year between 2005 and 2018, the highest (lowest) temperature in the lower troposphere was approximately −25 °C (−60 °C) in December (July) at a height of about 500 m above the surface (at the surface). A temperature inversion layer above the surface was found during the whole year but was weaker during the summer, while the inversion layers at the tropopause (about 8 km amsl) mostly disappeared during spring and winter. General warming trends were found at all heights and months, but in a few heights and months cooling trends still occurred (e.g., in September below 7 km amsl). Nevertheless, seasonal and yearly averaged temperatures all presented warming trends: 1.1, 1.3, 0.6, 1.5 and 1.1 °C/decade at the surface, and 0.7, 1.0, 0.3, 0.3 and 0.6 °C/decade for the layer average from the surface to 10 km amsl, for spring, summer, autumn, winter, and yearly average, respectively. Most of the water vapor was confined in the lowermost 3 km of the atmosphere with a maximum of 0.35 g kg⁻¹ in December at a 200 m height above surface, and the specific humidity had the similar characteristic of annual cycle and inversion layers as the temperature. At heights below 5 km amsl, increasing trends of specific humidity larger than 0.02 g kg⁻¹/decade occurred during summer months, including the late spring and early autumn, and the annual mean showed an increasing trend of about 0.01–0.02 g kg⁻¹/decade. Meanwhile, above 5 km amsl, the trends became small and generally less than 0.02 g kg⁻¹/decade in all the months, and beyond 7 km amsl the specific humidity remained almost invariant due to its small moisture content as compared with lower levels. From the surface to 10 km amsl, the specific humidity averaged trends of 0.0062, 0.019, 0.0013, 0.002 and 0.007 g kg⁻¹/decade for spring, summer, autumn, winter and yearly average, respectively.

The limited understanding of the structure of the urban surface atmospheric boundary layer can be attributed to its inherent complexity, as well as a deficiency in comprehensive measurements. We analyzed one year of meteorological data and Doppler wind lidar measurements in Beijing to explore how atmospheric stability is influenced by wind speed, radiation, turbulence, and pollution levels. Results indicate that the predominant state of the urban boundary layers in Beijing is an active condition (characterized by strong unstable and unstable stability regimes) throughout the day, attributed to the significant heat storage capacity of the urban canopy. Strong stable regimes are more frequently observed during winter and autumn, peaking during transitions from night to day. Furthermore, both strong unstable and strong stable regimes occur under very weak wind conditions (indicating weak dynamic instability), with strong instability associated with high net radiation levels while strong stability correlates with low net radiation conditions (indicative of robust thermal stability). The unstable regime manifests under strong winds (reflecting strong dynamic instability) alongside moderate net radiation environments, characterized by elevated values of turbulence kinetic energy and urban boundary height, highlighting the critical role of mechanical turbulence generation during periods of high wind activity. Additionally, six instances of pronounced stable conditions observed during daytime can be partially attributed to low radiation coupled with high pollutant concentrations near the surface, resulting from prolonged temperature inversions due to intense radiative cooling effects and weak dynamic forcing. Our findings presented herein are expected to have urban boundary layer climate and environment implications for other cities with high pollution and dense urban infrastructure all over the world.