Low ozone and high water vapour mixing ratios are common features in the Asian summer monsoon (ASM) anticyclone; however, low ozone and low water vapour values were observed near the tropopause over Kunming, China, within the ASM using balloon-borne measurements performed during the SWOP (sounding water vapour, ozone, and particle) campaign in August 2009 and 2015. Here, we investigate low ozone and water vapour signatures in the upper troposphere and lower stratosphere (UTLS) using FengYun-2D, FengYun-2G, and Aura Microwave Limb Sounder (MLS) satellite measurements and backward trajectory calculations. Trajectories with kinematic and diabatic vertical velocities were calculated using the Chemical Lagrangian Model of the Stratosphere (CLaMS) trajectory module driven by both ERA-Interim and ERA5 reanalysis data.
All trajectory calculations show that air parcels with low ozone and low water vapour values in the UTLS over Kunming measured by balloon-borne instruments originate from the western Pacific boundary layer.
Deep convection associated with tropical cyclones over the western Pacific transports ozone-poor air from the marine boundary layer to the cold tropopause region.
Subsequently, these air parcels are mixed into the strong easterlies on the southern side of the Asian summer monsoon anticyclone.
Air parcels are dehydrated when passing the lowest temperature region (
The Asian summer monsoon (ASM) anticyclone plays an important role in transporting air masses from the troposphere into the stratosphere
Convection in tropical cyclones (e.g. typhoons in the western Pacific) can lift air masses from the marine boundary layer into the tropopause layer
The lowest temperatures in the TTL are often found over the western Pacific.
When air parcels are transported horizontally via the lowest temperature tropopause region (cold trap) over the western Pacific in winter, dehydration occurs
In situ observations from the Soundings of Ozone and Water in the Equatorial Region (SOWER) campaign in winter show a clear correspondence between dry air parcels and low temperatures during advection in the TTL over the western Pacific
Low water vapour mixing ratios below 2 ppmv were observed at the cold point tropopause (370–380 K) during the Stratospheric-Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere (SCOUT-O3) aircraft campaign in November and December 2005 over Darwin, Australia
Vertical profiles of temperature, ozone, and water vapour were measured over Kunming (25.01
The (saturation) water vapour mixing ratio is calculated from the (ambient) frost point temperature using the Hyland–Wexler equation
Ozone profiles for Naha (26.21
The upper air soundings of Chenzhou, Ganzhou, Xiamen, Taipei, and Ishigakijima (also known as Ishigaki Island) in August 2009 and Haikou, Wuzhou, Hong Kong, Shantou, Laoag, and Ishigakijima in August 2015 were used in this paper. The data were downloaded from the University of Wyoming. Balloons were launched routinely twice a day at 00:00 and 12:00 UTC. They provide profiles of temperature, pressure, relative humidity, and wind vector from the surface to 30 km.
Microwave Limb Sounder (MLS) measurements are used to validate ozone and water vapour profiles in the tropopause layer over the western Pacific.
MLS level 2 version 4.2x standard atmospheric product data are used for 4 August 2009 and the period of 31 July–10 August 2015.
The MLS provides ozone with 10 pressure levels ranging from 261 to 46 hPa (9.5–21.5 km) along the orbit track
The FengYun-2D (“Feng and Yun” means “winds and clouds” in Chinese), or FY-2D in acronym form, and FY-2G are the geostationary meteorological satellite series of China, organized and operated by the National Satellite Meteorological Center of the CMA (China Meteorological Administration).
FY-2D was launched on 8 December 2006 and carried a payload with a five-channel Stretched Visible and Infrared Spin Scan Radiometer (S-VISSR) to track cloud motion.
Long-wave infrared (10.3–11.3
Diabatic and kinematic backward trajectories along two balloons' ascending paths over Kunming on 8 August 2009 and 10 August 2015 were calculated using the CLaMS trajectory module
Geopotential height (black lines, gpkm) and vertical
All vertical profiles of temperature, ozone, and water vapour mixing ratios over Kunming in August 2009 are shown in Fig.
All vertical profiles of
Figure
A balloon was launched in Naha, Japan, on 4 August 2009 before the Typhoon Morakot passed through this site.
Low ozone values (35 ppbv) appeared at the layer between 360 and 370 K (Fig.
Profiles of ozone (blue), mean ozone (grey), relative humidity from CFH (RH
Figure
ERA-Interim kinematic
The time series of the lowest temperature in the tropopause layer for Chenzhou, Ganzhou, Xiamen, Taipei, and Ishigakijima.
The time series of the lowest temperature from Chenzhou, Ganzhou, Xiamen, Taipei, and Ishigakijima are shown in Fig.
Brightness temperature (K) from the IR channel of FY-2D on
Figure
The time evolution of
In order to investigate the variation of water vapour mixing ratios at the top of the typhoon convection, Fig.
Vertical profiles of temperature, ozone, and water vapour measured over Kunming in August 2015 are shown in Fig.
As Fig.
Figure
As Fig.
Figure
A balloon was launched at the Naha site, Japan, on 5 August 2015 before Typhoon Soudelor passed through.
As Fig.
A comparison of diabatic and kinematic trajectory calculations is shown based on ERA-Interim and ERA5 reanalysis in Fig.
As Fig.
The main difference between the backward trajectories based on ERA-Interim and ERA5 originates from the vertical transport over the western Pacific, where Typhoon Soudelor occurred (Fig.
As Fig.
Brightness temperature from the FY-2G satellite on
Figure
As Fig.
Figure
The low ozone and low water vapour mixing ratios near the tropopause measured on 8 August 2009 and 10 August 2015 in Kunming are investigated using balloon measurements, satellite measurements, and CLaMS simulations.
MLS ozone and water vapour measurements and trajectory calculations from the CLaMS model confirm that the vertical transport is largely caused by tropical cyclones, whereas the horizontal transport is caused by the ASM anticyclone.
The interplay between tropical cyclones and the ASM anticyclone exerts a major influence on transporting the ozone-poor western Pacific boundary air to the tropopause layer and even to the ASM anticyclone region.
This interplay is consistent with an earlier study by
The lowest temperatures at the tropopause over the western Pacific ocean mainly drive the dehydration process.
As a result, air parcels become dry when they pass the low-temperature region before entering the stratosphere.
Our observations further confirm previous studies
The interplay between tropical cyclones and the ASM anticyclone has the potential to impact the long-term trends of ozone, water vapour, and even optically thin cirrus near the tropopause, particularly under climate change conditions, when the occurrence of tropical cyclones is expected to increase.
Figure
Geopotential height (black lines, gpkm) and horizontal wind speeds (shaded,
Figure
Ozone and water vapour relative difference on 8 August 2009 for the climatological mean value for August 2009.
ERA-Interim diabatic trajectories for air parcels started along the measured balloon profile between 364 and 367 K colour-coded by temperature in (top) a longitude–latitude cross section and (bottom) as a function of time vs. potential temperature.
The time evolution of the temperature (blue dots), the saturation
water vapour mixing ratio (SMR), and relative humidity over ice (RH
As Fig.
The authors calculated an ensemble of trajectories for nine points around the observation site for cases 1 and 2. The minimum SMR for each trajectory is selected before the air parcel met the cloud as Fig.
Water vapour profiles (green line) measured by CFH on
ERA-Interim and ERA5 meteorological reanalysis data are freely available from the web page
DL prepared for the first draft. BV, RM, GG, FP, and MR provided effective and constructive comments and helped improve the paper. JB and HV gave useful comments. JB, ZB, QL, JZ, and DL made the balloon-borne measurements in Kunming. HV supported the measurements. All coauthors contributed to writing the paper
The authors declare that they have no conflict of interest.
This article is part of the special issue ”StratoClim stratospheric and upper tropospheric processes for better climate predictions (ACP/AMT inter-journal SI)”. It is not associated with a conference.
This research was supported by the National Key Research and Development Program of China (2018YFC1505703) and the National Natural Science Foundation of China (grant no. 91837311, 41975050, 41605025, 41675040, 91637104, and 41705127). We wish to thank three anonymous reviewers for very constructive suggestions.
Our activities contribute to the European Community’s Seventh Framework Programme (FP7/2007-2013) as part of the StratoClim project (grant agreement no. 603557). The authors thank the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDA17010102), the International Postdoctoral Exchange Fellowship Program 2017 under grant no. 20171015, and the China Postdoctoral Science Foundation (grant no. 2015M581153). The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association.
This paper was edited by Timothy J. Dunkerton and reviewed by three anonymous referees.