Waves Non Linear Summer
Leoma Cianchetti
If you're using Pro Tools, AAX plug-ins (or RTAS for older versions) are what you need. Most other Mac DAWs support AU plug-ins. VST is compatible with a number of Mac and Windows programs. Be sure you choose plug-ins that work with your DAW!
Capturing the sound of three legendary consoles, Waves' NLS non-linear summing plug-in lets you easily and effectively add warmth, depth, and dimension to your DAW. This single amazing plug-in gives you the spot-on sound of three world-famous consoles. Inside NLS, you'll find Spike Stent's magical SSL 4000 G, Mike Hedges's unique EMI TG12345 Mk IV (the same one used on Pink Floyd's The Dark Side of the Moon), and Yoad Nevo's customized Neve 5116. Complete with over 100 individual channel models and three exquisitely detailed buss models, Waves' NLS non-linear summing plug-in makes adding color, character, and polish to your tracks easy.
To start their NLS non-linear summer plug-in off on the right foot, Waves got together with three of the world's top recording engineers/producers, each with his own unique and phenomenal-sounding recording console. These engineers include Spike Stent (Bjork, Maroon 5, Madonna), Mike Hedges (The Cure, Dido, U2), and Yoad Nevo (Bryan Adams, Pet Shop Boys, The Dandy Warhols). Working channel by channel, these engineers helped to ensure that each of NLS's models came out as accurate as possible. The results are absolutely astounding!
The key concept behind Waves' NLS non-linear summer plug-in is that the warmth and dimensional sound of these remarkable consoles is the result of many individual and channels (each with their own unique sound) interacting together. So, instead of modeling just a single idealized channel from each console, Waves modeled dozens of them. Not only does NLS allow you to re-create the same dynamic interaction between each channel as you'd get if you were using the original analog console, it also lets you create completely unique overall sounds by combining channels from multiple consoles. What's more, Waves also modeled each channel's self noise independently from its frequency response, giving you the option whether or not to include this realistic sonic element in your mix.
In addition to individual channels, the Waves NLS non-linear summer also includes a summing buss amplifier model plug-in for each console, which acts as a master control for each of the channel plug-ins. Together, these two plug-in types give you a ton of control over your sound. For instance, you can group multiple channel plug-ins (including channels from multiple console models) into eight VCA groups, which you can mix from the buss plug-in. Individual and buss drive controls let you add saturation to you sound, and each channel plug-in gives you a mic switch, which lets you add in the grit and harmonics from the consoles' preamp sections. However you use Waves NLS non-linear summer, we're sure you'll love the results you'll get.
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Linear-regime westerly waves that propagate across the South African domain are often linked to well-known rainfall producing systems such as tropical temperate troughs and synoptic scale tropical low-pressure systems, and ridging South Atlantic Ocean anticyclones at the surface. It is accepted that the baroclinic waves that propagate across the domain provide the lifting mechanism that causes the required vertical motion for rainfall to occur. This study shows that there exists a jet streak embedded in these waves that is located downstream of the trough axis, to the east of which vertically upward motion is expected to occur. The entrance of the jet streak passes just south of the country, as the waves propagate past the domain. The study further shows that for this class of waves, the vertical motion that causes rainfall to occur is induced by the thermally direct transverse ageostrophic circulation that is located at this jet entrance. This is instead of the conventional upper air divergence that is located at the inflection point east of the trough axis. Using a method of decomposing the Q-vector into its transverse (\(Q_n\)) and shear (\(Q_s\)) components, the divergence fields of which are used to decompose the vertical motion into the corresponding components, i.e \(\omega _n\) and \(\omega _s\), respectively; it was shown that the vertical motion over South Africa is explained more by the former than the latter. Therefore, the uplift over the country and that located at the infection point east of the trough are dynamically distinct processes. Taking the limitations of the quasi-geostrophic framework into consideration, the study concludes that during the passage of linear-regime waves vertical motion that might lead to rainfall is caused by the circulation at the jet entrance and not the divergence in the baroclinic wave.
The importance of westerly waves is not limited to the TTTs. Synoptic-scale ridging of the South Atlantic high (SAOH) pressure system eastward across the southern coast of Africa is crucial to coastal rainfall and regional moisture fluxes (Ndarana et al. 2021a, b) and is associated with westerly wave dynamics aloft (Ndarana et al. 2022). Further equatorward, Kuete et al. (2020) found that westerly waves may be linked to pulsing of the African Easterly Jet-South. Viljoen et al. (2023) suggested that the westerly wave is also associated with a tropical cyclone-like low pressure systems found over land, which they refer to as the Africne.
But, it is westerly waves that venture into non-linear regimes and break that have received the most detailed dynamical analyses in the literature. Wave breaking processes are most clearly seen in isentropic potential vorticity (PV) fields and is signalled by the PV contours becoming irreversibly deformed and turning back on themselves so that the PV gradient becomes negative (McIntyre and Palmer 1983; Thorncroft et al. 1993; Peters and Waugh 1996). When an isolated positive PV anomaly is found in the trough as a result of the wave breaking, a closed cyclonic circulation is then induced (Hoskins et al. 1985) and a cut-off low (COL) pressure system forms, which by definition is a closed circulation that has detached from the main westerlies (Palmn 1949; Palmn and Newton 1969). Numerous studies have considered COLs in the South African domain (Taljaard 1985; Fuenzalida et al. 2005; Pinheiro et al. 2017) and assessed their interannual variability (Singleton and Reason 2007a; Favre et al. 2012, 2013) due to the extreme rainfall and winds associated with these systems when they extend to the surface (Singleton and Reason 2006; Engelbrecht et al. 2015; Barnes et al. 2021a, b, 2022; Thoithi et al. 2022) due to depth of the associated PV anomaly (Barnes et al. 2022). At times a surface meso-low (Singleton and Reason 2007b; Thoithi et al. 2022) may develop during COLs, which may lead to strong onshore moisture fluxes (Thoithi et al. 2022), which may also be assisted by the long fetch from further into the South Indian Ocean, possibly caused by Type-S ridging events (Ndarana et al. 2022). Combining the moisture presence over the land with the large-scale upper-level divergence that is induced in strongly curved flow fields during wave breaking is often invoked to explain extreme rainfall associated with COLs and other weather systems such as TTTs (e.g. Hart et al. 2010).
This extensive analysis of non-linear wave breaking regimes associated with weather extremes has diagnosed the key role of downstream development (Gan and Piva 2013, 2016; Ndarana et al. 2021a, b; Pinheiro et al. 2022; Ndarana et al. 2023), which is largely facilitated by the strength of the jet streak located immediately upstream of ridge to the west of the main trough axis. Ndarana et al. (2023) diagnosed that during ridging SAOH events which are associated with a COL aloft, the upstream jet streak is stronger than the one located downstream of the trough axis. This configuration then increases strain rates so that the waves may break (Nakamura and Plumb 1994; Akahori and Yoden 1997). Why some waves in the South African domain enter non-linear regimes and others remain in more linear regimes is still an open question. However, Ndarana et al. (2023) showed that baroclinic conversion observed during the latter is less intense than in the case of former, meaning that when COLs (in general wave breaking) do not occur it is suppressed. A working hypothesis is that the barotropic governor mechanism (James and Gray 1986; James 1987) might be responsible for this. Moon and Feldstein (2009) showed that it leads to weaker upward fluxes of wave activity during the initial states of their weak barotropic wave life cycle case, which might explain the weaker increase in eddy kinetic energy generation during linear-regime waves in the South African domain (Ndarana et al. 2023).
It is clear from these studies that westerly waves in more linear regimes may play a substantial role in South African summer rainfall but are under studied. Indeed the majority of westerly disturbances do not enter a non-linear wave-breaking regime. For example in the case of ridging highs, Ivanciu et al. (2022) showed that only 44% of them were associated with wave breaking aloft. The dynamics associated with these linear waves are important to understand, especially as there may be different processes at play as compared to non-linear regime waves. As will be shown in this paper, the location of the upstream jet streaks west of the main wave trough might have a profound influence on the atmospheric dynamics influencing the country because they can be oriented such that jet entrances overlie the country as the disturbances propagate past. The conventional wisdom is that the dynamical ascent found over southern Africa during the passage of a westerly trough is caused by upper air divergence east of it. However, the presence of the jet streak and the location of its entrance could invoke a thermally direct transverse circulation, as described for straight zonal jets (Keyser and Shapiro 1986) and jet streaks greatly influenced by curvature (Moore and VanKnowe 1992). This might mean that vertical ascent over the country is caused by this transverse circulation, rather than the Dines compensation mechanism. Studies that consider such dynamics for linear-regime waves issue are missing in the literature for the region. In this respect, the research question raised in this study is: What is the relative importance of vertical motion associated with the jet entrance compared to that associated with upper-level divergence embedded in the wave trough of a westerly wave?
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