Waves and coherent flows in the tropical atmosphere: New opportunities, old challenges

Abstract

Atmospheric science relies on numerical models to simulate the complex multiscale nature of atmospheric variability, but our confidence in weather and climate predictions relies on theory and simplified models that describe scale interactions at a mechanistic level and can provide causal accounts of atmospheric behaviour. Global simulations at kilometre‐scale resolution are now feasible and offer new opportunities to the atmospheric science community for testing and expanding our understanding of climate variability and change. Taking full advantage of this new tool requires smart strategies for evaluating and analysing the output, especially as kilometre‐scale climate modelling will be limited to relatively short simulations with a rather small number of realizations. We here review some of the available tools for diagnosing and studying the dynamics of waves, coherent flows, and the interactions between them in terms of their ability to provide causal accounts of the behaviour seen in observations and in comprehensive simulation models. We describe their successes but also some of their limitations. The limitations are seen to be especially pronounced in the Tropics, where clouds, convection and atmospheric circulation are inextricably linked. The lack of a natural spatial truncation scale in the Tropics has given rise to many theoretical challenges, but it is for precisely this reason that the Tropics are where we might expect the largest gain from global kilometre‐scale models.

Environment of severe storm formations over West Africa on the 26‐28 June 2018

“…In addition, the presence of a low‐level jet such as the AEJ, served as a significant barrier for the trapping of the waves, aided by the increasing mean windspeed (U), and a strong decay of the $$$ \frac{d^2U}{dz^2} $$$ term with height (not shown). In an environment of sufficient moisture (as observed in Figure 2), the trapped wave acted to modify boundary layer processes to promote the initiation of severe convection and the growth and longevity of the thunderstorms on 26 June 2018, similar to findings by Howard (2013); Sachsperger et al (2015); Stephan et al (2021). The rear of the propagating gravity wave was a significant region for storm initiation (Figure 3), through the modification of tropospheric heating (Halliday et al, 2018).…”

“…The implication of this tropospheric stability increases the possibility for a wave to travel along with the mean wind ( U ) into the upper troposphere, aided by the moderately strong low‐level wind shear (below 700 hPa, Figure 4a). It follows that, if U varies with height inside the stable SAL layer (see Figure 4c), then waves of short horizontal wavelengths can become trapped (Stephan et al, 2021). Additionally, the decay in I 2 should be such that, if it occurs suddenly at the mid‐troposphere, it should divide the troposphere into two distinct regions; a lower layer with large values (high stability) and an upper layer with small values (low stability).…”

“…The implication of this tropospheric stability increases the possibility for a wave to travel along with the mean wind (U) into the upper troposphere, aided by the moderately strong low-level wind shear (below 700 hPa, Figure 4a). It follows that, if U varies with height inside the stable SAL layer (see Figure 4c), then waves of short horizontal wavelengths can become trapped (Stephan et al, 2021).…”

The intricacies of identifying equatorial waves

“…Amplitudes at 850 hPa are slightly larger than for the 2DS-PCF projection, but again dominated by higher k values. The peak of MRG activity at synoptic zonal scales evident from Figure 11f is a property of the average MRG variance spectra in the troposphere, as discussed by Stephan et al (2021).…”

“…The peak of MRG activity at synoptic zonal scales evident from Figure 11f is a property of the average MRG variance spectra in the troposphere, as discussed by Stephan et al . (2021).…”