HALO

PHILEAS

Probing High Latitude Export of air from the Asian Summer Monsoon

Mission status: No information available yet.

Persons in Charge

Mission-PI

  • Peter Hoor (Univ. Mainz)
  • Martin Riese (FZ Jülich)

Scientific Co-Leadership

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Mission coordinator​

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Contact point at DLR-FX for this mission:

Address

HALO Deployment Base

Time Period

June – October 2023

 

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Project description

Scientific rationale:

The composition of the upper troposphere and lower stratosphere (UTLS) with radiatively active trace gas species is a key factor for Earth‘s climate, as relatively small
changes in this composition cause large changes in surface temperatures. A particularly large radiative effect results for water vapour and aerosol (e.g., Solomon et al., 2010). Despite the urgency to correctly predict water vapour in numerical weather forecast and climate models, the distribution of water vapour across the tropopause and in the lower stratosphere (LS) is not well quantified and poorly constrained in numerical models.

In summer and early autumn East and South-east Asia is an important region for water vapour and other trace species to enter the lower stratosphere. Several dynamical features cause transport of trace species with near surface sources first into the UT and potentially further into the LS. This leads initially to an alteration of the composition of the UTLS over this region. A good example is the Asian Tropopause Aerosol Layer (ATAL) which is thought to be a direct consequence of the upward motions within the Asian summer monsoon (ASM). It has recently been shown that the ATAL consists to a large part of ammonium nitrate particles and affects the regional climate.

The ASM circulation during Northern summer is also thought to be the major pathway for tropospheric air masses, rich in water vapour, aerosol precursors and pollutants, into the UTLS. During summer (July-August), the large-scalenanticyclonic circulation in the UTLS traps lifted polluted air over Asia. Subsequently, when this circulation becomes weaker and more unstable, external forces disturb the temporally isolated system. Ultimately, frequent but irregular events of eddy shedding occur which lead to transport of polluted air from within the anti-cyclone into the extratropical LS. Observations and model simulations suggest a strong moistening effect of the eddy shedding transport on the Northern hemisphere UTLS, but with the strength of this effect being highly uncertain due to a lack of available observational data with sufficient precision and coverage. Very recent observations show evidence for a strong contribution of ammonium nitrate transport by the ASM to the UTLS aerosol budget and the ATAL, likely relevant for cirrus cloud formation.

Furthermore, the UTLS trace gas composition over the Eastern Pacific is affected by additional pathways in late summer and autumn: i) quasi-horizontal transport across the subtropical jet, exchanging air between the tropical UT and the extratropical LS, ii) vertical convective transport by tropical typhoons, which eventually move towards the extratropics, and iii) transport within extratropical weather systems, in particular, within warm conveyor belts and convection. All these features alter the composition of the UTLS over the eastern Pacific, thereby setting the initial chemical conditions before the air masses are transported further to middle and high latitudes and mix with LS background air. Initially, the respective chemical anomalies are relatively isolated within the shed eddy. Processes related to Rossby and gravity wave dynamics as well as diabatic forcings due to clouds in the UT and/or radiation lead to a mixing of these isolated anomalies into the background stratosphere. However, the time scales and the relative contribution of the underlying processes and their impact at different altitudes in the extratropical UTLS is largely unknown. Thus the temporal variability of the composition and in particular the gradients of radiatively active trace species are largely uncertain. This, in turn, is of importance for climate.
To investigate the relative importance of these processes contributing to the chemical composition of the UTLS we plan to characterize the evolution of the chemical composition of filaments during the full life cycle of eddy shedding process from the monsoon anticyclone and their effect particularly on the gradients of radiatively active species in the UTLS.

Key research questions:

1. What are the chemical composition and microphysical properties of the gas and particulate phase of large-scale eddies which are shed from the monsoon anticyclone?
2. What are the main transport pathways (advection and mixing) and time scales of polluted and moist air from the monsoon into the extratropical UTLS?
3. How do these shed eddies impact on the extratropical UTLS, and in particular on the water vapour and aerosol budget?

Fig. 1: Eddy shedding event from the Asian monsoon anticyclone in August 1997 as viewed by CRISTA ammonium nitrate measurements (colour shading).
The cyan line in (a) shows the 4.1 PVU contour, the black line the location of the cross-section in (b).
Within the proposed project the further fate of the shed eddy will be investigated.

Structure and Work Packages

To synergise the expertise of the involved partners in an optimal way for answering these questions, the project is structured into three main work packages as described in the following:

WP1: Different chemical species with different lifetimes will be measured (e.g., CO, O 3 , N 2 O, CO 2 , CH 4 , VSLS) to determine the chemical composition and to quantify the pathways and time scales for transport out of the monsoon anticyclone. In particular, „Lagrangian“ flights will be carried out to sample the eddy shedding process during different phases.

WP2: Water vapour and ice observations will be carried out and used to analyze the impact of transport from the monsoon on the extratropical water vapour budget. In particular, effects on the structure of the extratropical hygropause will be studied.

WP3: Measurements of aerosol precursor gases (e.g. NH 3 , HNO 3 ), of aerosol composition (e.g. ammonium nitrate, sulfate) and microphysical aerosol properties (size distribution) will be performed and formation and evolution of aerosols during transport from the monsoon to NH mid latitudes will be investigated.

Partners

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Scientific instruments and payload configuration

  • List of scientific instruments for the mission:

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Cabin and exterior configuration of HALO for the mission

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HALO flights for this mission

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More information

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Press releases, media etc

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