Abstract
Abstract. The Asian summer monsoon anticyclone (ASMA) is a major meteorological system of the upper troposphere–lower stratosphere (UTLS) during boreal summer. It is known to contain enhanced tropospheric trace gases and aerosols, due to rapid lifting from the boundary layer by deep convection and subsequent horizontal confinement. Given its dynamical structure, the ASMA represents an efficient pathway for the transport of pollutants to the global stratosphere. A detailed understanding of the thermal structure and processes in the ASMA requires accurate in situ measurements. Within the StratoClim project we performed state-of-the-art balloon-borne measurements of temperature, water vapor, ozone and aerosol backscatter from two stations on the southern slopes of the Himalayas. In total, 63 balloon soundings were conducted during two extensive monsoon-season campaigns, in August 2016 in Nainital, India (29.4∘ N, 79.5∘ E), and in July–August 2017 in Dhulikhel, Nepal (27.6∘ N, 85.5∘ E); one shorter post-monsoon campaign was also carried out in November 2016 in Nainital. These measurements provide unprecedented insights into the UTLS thermal structure, the vertical distributions of water vapor, ozone and aerosols, cirrus cloud properties and interannual variability in the ASMA. Here we provide an overview of all of the data collected during the three campaign periods, with focus on the UTLS region and the monsoon season. We analyze the vertical structure of the ASMA in terms of significant levels and layers, identified from the temperature and potential temperature lapse rates and Lagrangian backward trajectories, which provides a framework for relating the measurements to local thermodynamic properties and the large-scale anticyclonic flow. Both the monsoon-season campaigns show evidence of deep convection and confinement extending up to 1.5–2 km above the cold-point tropopause (CPT), yielding a body of air with high water vapor and low ozone which is prone to being lifted further and mixed into the free stratosphere. Enhanced aerosol backscatter also reveals the signature of the Asian tropopause aerosol layer (ATAL) over the same region of altitudes. The Dhulikhel 2017 campaign was characterized by a 5 K colder CPT on average than in Nainital 2016 and a local water vapor maximum in the confined lower stratosphere, about 1 km above the CPT. Data assessment and modeling studies are currently ongoing with the aim of fully exploring this dataset and its implications with respect to stratospheric moistening via the ASMA system and related processes.
Highlights
Large-scale deep convection associated with the Asian summer monsoon (ASM) during boreal summer induces a strong and persistent anticyclonic vortex in the upper troposphere– lower stratosphere (UTLS), known as the ASM anticyclone (ASMA) (e.g., Hoskins and Rodwell, 1995) or, previously, as the Tibetan high (e.g., Krishnamurti and Bhalme, 1976)
We focus on analyzing the thermodynamic structure of the UTLS during the ASM season and how it relates to the vertical distributions of water vapor, ozone and aerosols
Thin cirrus clouds were observed in more than half of the COBALD soundings in NT16AUG (9 out of 17); they occur very frequently in the ASMA, and they were often found embedded within the Asian tropopause aerosol layer (ATAL) (Fig. 10a, e)
Summary
Large-scale deep convection associated with the Asian summer monsoon (ASM) during boreal summer induces a strong and persistent anticyclonic vortex in the upper troposphere– lower stratosphere (UTLS), known as the ASM anticyclone (ASMA) (e.g., Hoskins and Rodwell, 1995) or, previously, as the Tibetan high (e.g., Krishnamurti and Bhalme, 1976). Lagrangian trajectories were used to investigate the origin of the air masses in the ASMA (Bergman et al, 2013; Vogel et al, 2015), this approach is limited by the convective nature of the transport These studies are consistent with satellite observations (Fu et al, 2006), regional weather forecasting (Heath and Fuelberg, 2014) and global atmospheric circulation models (Fadnavis et al, 2013; Pan et al, 2016) which all indicate that southern slopes of the Himalayas (i.e., latitudes approximately 25–35◦ N south of the Tibetan Plateau) are a hot spot for the transport of boundary layer pollutants to the ASMA. All variables are binned in pressure intervals of 1 hPa for p > 300 hPa, and 0.5 hPa for p < 300 hPa, yielding an improved signal-to-noise www.atmos-chem-phys.net/18/15937/2018/
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
