Editor’s note: We would like to welcome Prof. Andrew Dessler of Texas A&M University as a guest blogger at Head in A Cloud! Please note this post was written by him and not Sean Davis.
The effects of convection on the summetime extratropical overworld
Much effort has been expended over the last decade in assessing the impact of deep convection on the extratropical lower stratosphere. It is now clear that summertime convection has an important impact on the chemical composition of the lowermost stratosphere (that part of the stratosphere with potential temperature Î¸ < 380 K).
Recently, Dessler and Sherwood [2004, hereafter DS04] showed that summertime convection plays an important role in the Northern Hemisphere (NH) extratropical H2O budget at 380 K, the boundary between the lowermost stratosphere and the so-called overworld (the stratosphere with Î¸ > 380 K). DS04 focused on the 380-K surface and did not look at altitudes above 380 K. I am now investigating how high summertime mid-latitude convection penetrates into the extratropical NH overworld.
To address this issue, I will use measurements of H2O made by the Halogen Occultation Experiment (HALOE), which was carried aboard the Upper Atmosphere Research Satellite [e.g., Dessler et al., 1998]. The measurements were obtained between 1994 and 2005, and are well validated [Harries et al., 1996; Park et al., 1996; Bruhl et al., 1996; SPARC, 2000], with systematic errors in the lower stratosphere of 20% for all three constituents. I use HALOE version 19 here, adjusted per instructions from the HALOE science team. The Î¸ of each measurement is determined using daily temperature and pressure fields from the National Center for Environmental Prediction (NCEP) that are provided with the HALOE data.
Depth of penetration of convection into the overworld
For context, I plot in Figure 1 the average H2O at 380 K measured by the HALOE between June 15 and August 15 of 1994 through 2005; similar plots appeared in DS04 and Randel et al. . The data show maxima in H2O between 30Â°N and 40Â°N over North America and Asia. DS04 connected these 380-K maxima to deep convection, arguing that convection moistens the lower stratosphere in regions where the relative humidity is low.
I now focus on the latitude range 30Â°N-40Â°N and calculate the summertime H2O anomaly as a function of Î¸ and longitude. For each Î¸ surface, HALOE data obtained between June 15 and August 15 of 1994 through 2005 and between 30Â°N and 40Â°N are interpolated to that Î¸ surface. These data are then separated into 30 longitude bins and the average of each bin is calculated. The anomaly is calculated by subtracting the value of the lowest-average bin from all of the bins. This process is repeated at each Î¸ surface and the resulting anomaly is plotted in Fig. 2.
Figure 2 shows that there are two longitudes where large H2O anomalies exist: around 50Â° longitude, over the Asian monsoon, and around 270Â° longitude, over North America. This is consistent with Fig. 1, and is anyway not surprising since these regions are well known to be the site of vigorous deep convection. Over North America (270Â° longitude), the anomaly decreases monotonically with height, from ~0.65 ppmv at 380 K (15.5 km) to ~0.1 ppmv at 410 K (17.5 km).
Over Asia (50Â° longitude), the anomaly also decreases monotonically with height, from ~0.60 ppmv at 380 K (15.5 km) to ~0.1 ppmv at 460 K (19 km). Thus, the anomaly over North American and Asia are of similar magnitude at 380 K, but the convective anomaly over the Asian monsoon extends about 50 K (1.5 km) higher than the North American convection anomaly. This is not unexpected because, by most metrics, convection over the Asian monsoon is far stronger than over North America[e.g., Dunkerton, 1995].
I am arguing in this post that upward advection of high H2O air by convection is responsible for the H2O anomalies in Fig. 2. One might ask whether horizontal advection of high H2O from lower or higher latitudes might also be playing a role. Figure 1 shows that this is impossible at 380 K: H2O is lower both poleward and equatorward of the mid-latitude maxima, so meridional advection cannot explain the maxima. To show this is also true at other Î¸, I plot in Figure 3 a latitude-height cross-section through the Asian monsoon. The crosses indicate the latitude of maximum H2O on selected Î¸ levels. This plot shows that between 380 and 450 K, H2O between 30Â°N-40Â°N is higher than air both poleward and equatorward. Thus, meridional transport cannot be responsible for the high H2O found there. In addition, H2O tends to decrease with increasing Î¸ in this season and over the Î¸ range of interest here, so downward transport also cannot explain the high values. I conclude that only upward transport can explain the H2O anomalies, and that the only possible explanation for this upward transport is convection.
While convection reaching the overworld, even as high as 460 K, might seem surprising, previous studies provide some evidence to support this conclusion. Fromm et al. reported several occurrences of forest fire smoke in the overworld, while Fromm and Servranckx  described observations of forest fire smoke as high as 460 K. Livesey et al.  observed a biomass burning product, methyl cyanide, at 100â€“68 hPa (16â€“19 km or 400-450 K) in the summertime NH mid-latitudes. In situ data analyzed by Jost et al. showed biomass burning products at altitudes between 380 and 400 K. Wang et al.  has modeled summertime continental convection and has simulated convection reaching Î¸ > 400 K, well into the overworld.
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