Identifying Source Regions and the Distribution of Cross‐Tropopause Convective Outflow Over North America During the Warm Season

Abstract

We analyzed the interaction between the North American monsoon anticyclone (NAMA) and summertime cross‐tropopause convective outflow by applying a trajectory analysis to a climatology of convective overshooting tops (OTs) identified in GOES satellite images, which covers the domain from 29°S to 68°N and from 205 to 1.25°W for the time period of May through September 2013. With this analysis we identified seasonally, geographically, and altitude‐dependent variability in NAMA strength and in cross‐tropopause convection that control their interaction. We find that the NAMA has the strongest impact on the circulation of convectively influenced air masses in August. Over the entire time period examined the intertropical convergence zone contributes the majority of OTs with a larger fraction of total OTs at 370 K (on average 70%) than at 400 K (on average 52%). During August at 370 K, the convectively influenced air masses within the NAMA circulation, as determined by the trajectory analysis, are primarily sourced from the intertropical convergence zone (monthly average of 66.1%), while at 400 K the Sierra Madres and the Central United States combined constitute the dominant source region (monthly average of 44.1%, compared to 36.6% of the combined Intertropical Convergence Zone regions). When evaluating the impact of cross‐tropopause convection on the composition and chemistry of the upper troposphere and lower stratosphere, the effects of the NAMA on both the distribution of convective outflow and the residence time of convectively influenced air masses within the NAMA region must be considered.

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

“…Differences between June-AugustFrom June to August every year, deep convection frequently occurs over the central US (Fig. 3a-c; see alsoCooney et al, 2018). The water vapor mixing ratio over NA also shows positive anomalies relative to the zonal mean(Fig.…”

“…The GridRad dataset archives national NEXRAD WSR-88D radar data (version 3.1). Gridrad v3.1 has horizontal resolution 0.02 • longitude and 0.02 • latitude and covers 25 to 49 • N and 70 to 105 • W, i.e., most of the contiguous US (Homeyer, 2014;Homeyer and Kumjian, 2015;Solomon et al, 2016;Cooney et al, 2018). The vertical resolution of the GridRad data is 1 km, from 1 to 24 km a.s.l.…”

“…The vertical resolution of the GridRad data is 1 km, from 1 to 24 km a.s.l. (above sea level), and it has a temporal resolution of 1 h. Cooney et al (2018) used GridRad data to calculate the deep convective echo top and found out the highest level that the reflectivity over 10 dBz is a representative threshold that balances the sensitivity and noise. In our analysis, we also use this strategy and identify deep convection as observations of reflectivity over 10 dBz.…”

“…Previous studies based on the infrared satellite cloud-top height measurements are low biased (Ueyama et al, 2018;Schoeberl et al, 2019) owing to the isothermal nature of the lowermost stratosphere and the fact that convective clouds rapidly sublimate in the dry stratosphere and therefore may be missed in observations from polar-orbiting satellites. A better option for estimating cloud-top height over NA is ground-based radar (Liu and Liu, 2016;Cooney et al, 2018). The hourly interval GridRad data, derived from NEXRAD radar data, captures convective overshooting events over most parts of the contiguous US (Solomon et al, 2016;Cooney et al, 2018), sometimes extending as high as 22 km.…”

“…A better option for estimating cloud-top height over NA is ground-based radar (Liu and Liu, 2016;Cooney et al, 2018). The hourly interval GridRad data, derived from NEXRAD radar data, captures convective overshooting events over most parts of the contiguous US (Solomon et al, 2016;Cooney et al, 2018), sometimes extending as high as 22 km.…”