Influence of convection on stratospheric water vapor in the North American monsoon

Stratospheric water vapor, an important greenhouse gas, has a globally averaged mixing ratio of about 3-5 ppmv. However, observations from the Aura Microwave Limb Sounder (MLS) show that over North America (NA), water vapor mixing ratio higher than 8 ppmv is frequently observed especially during boreal summer. Evidence from several previous case studies indicates that deep convection brings high water vapor content to the lower stratosphere, providing a possible explanation for the observed high water vapor mixing ratios. In our research, we use the GridRad data derived from NEXRAD radar data as a convective proxy and a Lagrangian back trajectory model to show that high stratospheric water vapor mixing ratios over the NA region during boreal summer is always accompanied by deep convection penetrating into the stratosphere with relatively low temperature. According to our back trajectory model analyses, deep convection contributes an increase in monthly average water vapor by as much as 33%. Though the occurrence of convection is similar during June, July and August, the location of the NA anticyclone determines the net impact of deep convection, so the convective influence is lower during June than during July and August. These results demonstrate that deep convection over the NA region plays a non-negligible role in determining stratospheric water vapor in this region.