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Dynamics and Chemistry of the Summer Stratosphere



Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) is a NASA Earth Venture Suborbital research project to investigate the impacts of intense thunderstorms over the U.S. on the summertime stratosphere.

The scientific questions to be addressed by DCOTSS are discussed on the Science page. Facilities, instruments, and models are described on the Project page.

Project Overview

During the summer, strong convective storms over North America overshoot the tropopause into the lower stratosphere. These storms carry water and pollutants from the troposphere into the normally very dry stratosphere, where they can have a significant impact on radiative and chemical processes, potentially including stratospheric ozone. The photo below, taken from the International Space Station, shows one of these storms with an anvil, which is typically near the tropopause level; an overshooting top; and a plume of cirrus (ice) clouds injected into the stratosphere by the overshooting top. Overshooting tops can reach many kilometers above the tropopause into the stratosphere.

View of an overshooting convective storm from the International Space Station
This image is a photograph from the International Space Station of an intense overshooting convective storm with a cirrus plume in the lower stratosphere.

Material transported from the troposphere to the stratosphere by these storms may be trapped by the atmospheric circulation in the lower stratosphere. During the summer, the circulation over North America is dominated by a large high pressure system known as the North American Monsoon Anticyclone (NAMA). As the map below illustrates, air within the NAMA can circulate in a clockwise manner for a considerable period before it escapes.

Average July streamfunction in the lower stratosphere and circulation around the NAMA
Climatological streamfunction and circulation around the NAMA.

DCOTSS will use the NASA ER-2 high-altitude research aircraft to measure the composition of these convective plumes and determine their effects on the chemistry and composition of the stratosphere. ER-2 flights for DCOTSS will be based in Salina, KS, which offers an ideal location for sampling convective plumes in the stratosphere. The ER-2 will carry an extensive suite of instruments to measure trace gases and aerosol properties and can operate at altitudes as high as 70,000 feet. Commercial airliners, by comparison, typically fly at around 35,000 feet.

ER-2 aircraft
This image is a photograph of th eER-2 aircraft in flight.

The central U.S. offers an ideal place to investigate overshooting convection. The NEXRAD meteorological radar network, operated by the National Oceanic and Atmospheric Administration (NOAA), provides continuous, high-resolution observations of convective storms. NEXRAD is capable of detecting overshooting convective tops, as shown in this map and cross-section.

NEXRAD reflectivity map (left) and cross-section through an overshooting top (right) (Smith et al., 2017)
NEXRAD reflectivity map and cross-section through an overshooting top.

In addition, the NOAA GOES-16 geostationary weather satellite produces frequent high-resolution images of storm locations and cloud-top temperatures. Because atmospheric temperatures in the troposphere decrease with altitude, GOES observations can also be used to identify overshooting storm tops. This figure illustrates how visible cloud top texture and IR temperatures are used to identify overshooting tops.

GOES-16 visible and IR images; above-anvil cirrus plumes (arrows in B);
and overshooting tops (color shading in C and colored dots in D)

This image is a photograph of th eER-2 aircraft in flight.

Analysis of NEXRAD data reveals that, on average, there are more than 200 overshooting storms per day over the contiguous U.S. Overshooting storms are most common in the high plains of the central U.S., as can be seen in the figure below, which counts the number of overshooting events within 2 km by 2 km squares over ten summers. Salina, KS is located in the center of the region of highest occurrence of overshooting tops within the U.S.

Occurrence of overshooting events (Cooney et al., 2018)
Map of the number of overshooting occurrences in each 2 km x 2 km square from 2004 to 2013.

By combining NEXRAD and GOES-16 observations of overshooting tops with forecasts of atmospheric winds from the National Weather Service, we will be able to predict the paths of air injected by overshooting storms. ER-2 flights will be planned to intercept plumes from those storms hours to days after they occur. Flights on consecutive days will allow us to to visit the same plumes at different times to study how the chemicals within the plumes evolve in the stratospheric environment.

Forecast motion of material injected by overshooting storms
Forecasted motion of material injected by overshooting storms.

Three eight-week field deployments of the ER-2 are planned to take place during the summers of 2020 and 2021. The three deployments will observe early, middle, and late summer conditions.

Participating Institutions

Participants in the DCOTSS project come from the following universities and government laboratories. For information about the individuals taking part in DCOTSS, see the People tab.