Is a high‐altitude meteorological analysis necessary to simulate thermosphere‐stratosphere coupling?

Abstract

We compare simulations of mesospheric tracer descent in the winter and spring of 2009 with two versions of the Whole Atmosphere Community Climate Model (WACCM), both with specified dynamics. One is constrained with data from the Modern‐Era Retrospective Analysis for Research and Applications which extends from 0 to 50 km; the other uses the Navy Operational Global Atmospheric Prediction System‐Advanced Level Physics High Altitude (NOGAPS‐ALPHA) which extends up to 92 km. By comparison with Solar Occultation for Ice Experiment data we show that constraining WACCM to NOGAPS‐ALPHA yields a dramatic improvement in the simulated descent of enhanced nitric oxide (NO) and very low methane (CH4). We suggest that constraining to NOGAPS‐ALPHA compensates for an underestimate of nonorographic gravity wave drag in WACCM. Other possibilities, such as missing energetic particle precipitation or underestimated eddy diffusion, are less likely for the Arctic winter and spring of 2009.

Persistence of upper stratospheric wintertime tracer variability into the Arctic spring and summer

“…A similar offset was recently noted by (Siskind et al, 2015b) in their WACCM/NOGAPS simulation of the 2009 descent of mesospheric NO x . Since the summer CH 4 depletion is a consequence of the winter descent, this offset may reflect the small discrepancy seen by Siskind et al (2015b). Figure 4 shows that the effect of the CH 4 on ClO occurs over a relatively deep layer in the upper stratosphere; the detailed plots of the time behavior of CH 4 and ClO, specifi- cally Figs.…”

“…There may be a small vertical offset, perhaps one grid point, whereby the model profile is shifted slightly downward relative to both the MLS and SOFIE data. A similar offset was recently noted by (Siskind et al, 2015b) in their WACCM/NOGAPS simulation of the 2009 descent of mesospheric NO x . Since the summer CH 4 depletion is a consequence of the winter descent, this offset may reflect the small discrepancy seen by Siskind et al (2015b).…”

“…The figure shows very large variability in March and April, both intra-and interannually. This is largely driven by the large temperature variability, which itself is dynamically driven, as discussed by several authors (Siskind et al, 2015a;McCormack et al, 2006;Smith, 1995;Froidevaux et al, 1989). Of interest here is that after 1 May the interannual variability becomes very small, but is not zero.…”

“…However, as discussed by Siskind et al (2015b) and also by Salmi et al (2011) in their study of data from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer, there is no evidence that these enhancements penetrated down to altitudes where the NO x catalytic cycle affects ozone. Although SOFIE does not measure NO 2 , the excellent agreement between WACCM NO and SOFIE NO documented by Siskind et al (2015b) gives us confidence that the WACCM NO x results are an accurate reflection of reality. We suggest that the lower NO x from 1 to 8 hPa in 2009 is a legacy of greater winter/spring descent from the region of the NO minimum in the mesosphere near 60-75 km.…”

“…Our primary data come from the Solar Occultation for Ice Experiment (SOFIE) ) on the Aeronomy of Ice in the Mesosphere (AIM) satellite ) and the Microwave Limb Sounder (MLS) (Santee et al, 2008;Froidevaux et al, 2008) Bailey et al (2014) and (Siskind et al, 2015b) and were shown to vary in a manner consistent with the other tracers of mesospheric descent measured by SOFIE; ongoing validation studies (Rong et al, 2016) with the Atmospheric Chemistry Experiment suggest general agreement to within approximately 12 %. Here we emphasize the relative year to year variations.…”