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1. Graphs with no even holes and no sector wheels are the union of two chordal graphs

   https://doi.org/10.1016/j.ejc.2024.104035  

   Abrishami, Tara; Berger, Eli; Chudnovsky, Maria; Zerbib, Shira (December 2024, European Journal of Combinatorics)

   Sivaraman (2020) conjectured that if G is a graph with no induced even cycle then there exist sets X1,X2⊆V(G) satisfying V(G)=X1∪X2 such that the induced graphs G[X1] and G[X2] are both chordal. We prove this conjecture in the special case where G contains no sector wheel, namely, a pair (H,w) where H is an induced cycle of G and w is a vertex in V(G)∖V(H) such that N(w)∩H is either V(H) or a path with at least three vertices. 

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2. Global Surface Temperature Response to 11-Yr Solar Cycle Forcing Consistent with General Circulation Model Results

   https://doi.org/10.1175/JCLI-D-20-0312.1  

   Amdur, T.; Stine, A_R; Huybers, P. (April 2021, Journal of Climate)

   ABSTRACT The 11-yr solar cycle is associated with a roughly 1 W m⁻²trough-to-peak variation in total solar irradiance and is expected to produce a global temperature response. The sensitivity of this response is, however, contentious. Empirical best estimates of global surface temperature sensitivity to solar forcing range from 0.08 to 0.18 K (W m⁻²)⁻¹. In comparison, best estimates from general circulation models forced by solar variability range between 0.03 and 0.07 K (W m⁻²)⁻¹, prompting speculation that physical mechanisms not included in general circulation models may amplify responses to solar variability. Using a lagged multiple linear regression method, we find a sensitivity of global-average surface temperature ranging between 0.02 and 0.09 K (W m⁻²)⁻¹, depending on which predictor and temperature datasets are used. On the basis of likelihood maximization, we give a best estimate of the sensitivity to solar variability of 0.05 K (W m⁻²)⁻¹(0.03–0.09 K; 95% confidence interval). Furthermore, through updating a widely used compositing approach to incorporate recent observations, we revise prior global temperature sensitivity best estimates of 0.12–0.18 K (W m⁻²)⁻¹downward to 0.07–0.10 K (W m⁻²)⁻¹. The finding of a most likely global temperature response of 0.05 K (W m⁻²)⁻¹supports a relatively modest role for solar cycle variability in driving global surface temperature variations over the twentieth century and removes the need to invoke processes that amplify the response relative to that exhibited in general circulation models. 

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3. Robust Power and Cadence Tracking on a Motorized FES Cycle with an Unknown Time-Varying Input Delay

   https://doi.org/10.1109/CDC42340.2020.9304066  

   Allen, Brendon C.; Stubbs, Kimberly J.; Dixon, Warren E. (December 2020, IEEE Conference on Decision and Control)

   null (Ed.)

   Functional electrical stimulation (FES) induced cycling is a common rehabilitative technique applied for those with a movement disorder. An FES cycle system is a nonlinear switched dynamic system that has a potentially destabilizing input delay between stimulation and the resulting muscle force. In this paper, a dual objective control system for a nonlinear, uncertain, switched FES cycle system with an unknown time-varying input delay is developed and a Lyapunov-like dwell-time analysis is performed to yield exponential power tracking to an ultimate bound and global exponential cadence tracking. Preliminary experimental results for a single healthy individual are provided and demonstrate average power and cadence tracking errors of -0.05 ± 0.80 W and -0.05 ± 1.20 RPM, respectively, for a target power of 10 W and a target cadence of 50 RPM. 

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4. Simulation and Scaling of the Turbulent Vertical Heat Transport and Deep-Cycle Turbulence across the Equatorial Pacific Cold Tongue

   https://doi.org/10.1175/JPO-D-21-0153.1  

   Whitt, D. B.; Cherian, D. A.; Holmes, R. M.; Bachman, S. D.; Lien, R.-C.; Large, W. G.; Moum, J. N. (May 2022, Journal of Physical Oceanography)

   Abstract Microstructure observations in the Pacific cold tongue reveal that turbulence often penetrates into the thermocline, producing hundreds of watts per square meter of downward heat transport during nighttime and early morning. However, virtually all observations of this deep-cycle turbulence (DCT) are from 0°, 140°W. Here, a hierarchy of ocean process simulations, including submesoscale-permitting regional models and turbulence-permitting large-eddy simulations (LES) embedded in a regional model, provide insight into mixing and DCT at and beyond 0°, 140°W. A regional hindcast quantifies the spatiotemporal variability of subsurface turbulent heat fluxes throughout the cold tongue from 1999 to 2016. Mean subsurface turbulent fluxes are strongest (∼100 W m −2 ) within 2° of the equator, slightly (∼10 W m −2 ) stronger in the northern than Southern Hemisphere throughout the cold tongue, and correlated with surface heat fluxes ( r 2 = 0.7). The seasonal cycle of the subsurface heat flux, which does not covary with the surface heat flux, ranges from 150 W m −2 near the equator to 30 and 10 W m −2 at 4°N and 4°S, respectively. Aseasonal variability of the subsurface heat flux is logarithmically distributed, covaries spatially with the time-mean flux, and is highlighted in 34-day LES of boreal autumn at 0° and 3°N, 140°W. Intense DCT occurs frequently above the undercurrent at 0° and intermittently at 3°N. Daily mean heat fluxes scale with the bulk vertical shear and the wind stress, which together explain ∼90% of the daily variance across both LES. Observational validation of the scaling at 0°, 140°W is encouraging, but observations beyond 0°, 140°W are needed to facilitate refinement of mixing parameterization in ocean models. Significance Statement This work is a fundamental contribution to a broad community effort to improve global long-range weather and climate forecast models used for seasonal to longer-term prediction. Much of the predictability on seasonal time scales is derived from the slow evolution of the upper eastern equatorial Pacific Ocean as it varies between El Niño and La Niña conditions. This study presents state-of-the-art high-resolution regional numerical simulations of ocean turbulence and mixing in the eastern equatorial Pacific. The results inform future planning for field work as well as future efforts to refine the representation of ocean mixing in global forecast models. 

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5. A scale-wise analysis of intermittent momentum transport in dense canopy flows

   https://doi.org/10.1017/jfm.2022.414  

   Chowdhuri, Subharthi; Ghannam, Khaled; Banerjee, Tirtha (July 2022, Journal of Fluid Mechanics)

   We investigate the intermittent dynamics of momentum transport and its underlying time scales in the near-wall region of the neutrally stratified atmospheric boundary layer in the presence of a vegetation canopy. This is achieved through an empirical analysis of the persistence time scales (periods between successive zero-crossings) of momentum flux events, and their connection to the ejection–sweep cycle. Using high-frequency measurements from the GoAmazon campaign, spanning multiple heights within and above a dense canopy, the analysis suggests that, when the persistence time scales ( $$t_p$$ ) of momentum flux events from four different quadrants are separately normalized by $$\varGamma _{w}$$ (integral time scale of the vertical velocity), their distributions $$P(t_p/\varGamma _{w})$$ remain height-invariant. This result points to a persistent memory imposed by canopy-induced coherent structures, and to their role as an efficient momentum-transporting mechanism between the canopy airspace and the region immediately above. Moreover, $$P(t_p/\varGamma _{w})$$ exhibits a power-law scaling at times $$t_{p}<\varGamma _{w}$$ , with an exponential tail appearing for $$t_{p} \geq \varGamma _{w}$$ . By separating the flux events based on $$t_p$$ , we discover that around 80 % of the momentum is transported through the long-lived events ( $$t_{p} \geq \varGamma _{w}$$ ) at heights immediately above the canopy, while the short-lived ones ( $$t_{p} < \varGamma _{w}$$ ) only contribute marginally ( $$\approx 20\,\%$$ ). To explain the role of instantaneous flux amplitudes in momentum transport, we compare the measurements with newly developed surrogate data and establish that the range of time scales involved with amplitude variations in the fluxes tends to increase as one transitions from within to above the canopy. 

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