The North American summer monsoon region exhibits a strong seasonal cycle in lower-stratospheric water vapor concentrations. However, quantifying the contribution of high-reaching convection to the stratospheric water vapor budget is still challenging, mainly due to the complex relationship between convection and monsoon dynamics.
11 years of water vapor concentrations provided by the Aura Microwave Limb Sounder (MLS) are combined with ground-based radar observations of high convection over the contiguous United States, as well as 5-day backwards trajectories for individual MLS profiles. This data set shows that deep convection over North America is associated with an increase in lower-stratospheric water vapor concentrations of up to 1 parts per million, especially during July and August. The magnitude of the water vapor anomaly is mainly dependent on the location of the North American anticyclone and the frequency with which air masses encounter high convection. The geographical distribution of water vapor anomalies is not aligned with areas of frequent convection. Instead, convective moisture is transported to the region of high water vapor by the dynamics of the monsoon.
Technical Description of Figure:
(a) Geographical distribution of MLS 100-hPa water vapor anomaly (after removal of the zonal mean), averaged over 2005–2016 in August. Thick black lines illustrate the 10−5 contours of GridRad convective occurrence, averaged over August. (b) Geographical distribution of the convection influence ratio over North America during August. The convection influence ratio is the fraction of MLS observations in each grid box that encountered deep convection during the previous 5 days. (c) Locations where convectively influenced parcels associated with individual MLS profiles sampled in August encounter convection over 2005–2016.
Scientific significance, societal relevance, and relationships to future missions:
Stratospheric water vapor has a significant impact on the atmospheric radiation budget. In order to reliably predict long-term climate trends, it is important to understand the relevant processes that control the water vapor budget. The annual cycle in stratospheric water vapor is primarily caused by the poleward advection of high humidity and the occurrence of strong convection reaching into the lowermost stratosphere. The relative importance of the two mechanisms is the subject of ongoing studies. This study relates the occurrence of high-reaching convection associated with the North American summer monsoon to long-term stratospheric water vapor anomalies.
Daily near-global observations of upper tropospheric and lower stratospheric water vapor from the Aura Microwave Limb Sounder provide a reliable 16-year data record to facilitate similar studies. While the planned Swedish SIW mission is expected to provide comparable measurements, there is currently no NASA sensor planned to continue such observations.
Water vapor profiles from the Aura Microwave Limb Sounder are available from https://mls.jpl.nasa.gov/products/h2o_product.php. National NEXRAD WSR-88D radar data are archived by the GridRad dataset, which can be found at http://gridrad.org/data.html. Winds, temperatures, and heating rates are provided by the ECMWF Reanalysis-Interim (ERA-interim) at https://www.ecmwf.int/en/forecasts/datasets/.
References: Yu, W., Dessler, A. E., Park, M., & Jensen, E. J.: Influence of convection on stratospheric water vapor in the North American monsoon region. Atmos. Chem. Phys., 20, 12153–12161, https://doi.org/10.5194/acp-20-12153-2020, 2020. .