Steven K. Krueger | Searcharxiv

arXiv2018

Thermodynamic constraints on the size distributions of tropical clouds

Tropical convective clouds evolve over a wide range of temporal and spatial scales, and this makes them difficult to simulate numerically. Here, we propose that their statistical properties can be derived within a simplified time-independent co-ordinate system of cloud number $n$, saturated static energy $h^\star$, and cloud perimeter $λ$. Under the constraint that circulations around cloud edge compete for buoyant energy and air, we show that the product of cloud number and cloud perimeter $nλ$ is invariant with $λ$ and that cloud number follows a negative exponential with respect to cloud-edge deviations of $h^\star$ from the mean. Overall, the summed perimeter of all clouds scales as the square root of the atmospheric static stability, which suggests that the complexity of cloud field structures can be viewed statistically as an emergent property of atmospheric bulk thermodynamics. Analytically derived conclusions are compared with a detailed tropical cloud field simulation and found generally to agree to within $<$13%. For the sake of developing hypotheses about cloud temporal evolution that are testable in high resolution simulations, the shapes of tropical cloud perimeter distributions are predicted to be invariant as climate warms, although with a modest increase in total cloud amount.

physics.ao-ph↗

arXiv2012

Real time simulation of 2007 Santa Ana fires

There are many wildfire behaviors of increasing relevance that are outside the forecast capabilities of even the most sophisticated operational fire spread and fire behavior model. The limitations of the operational models are due primarily to their inability to represent coupled fire-atmosphere interactions. Coupled wildfire-atmosphere models are physics-based fluid-dynamical prognostic models of wildfire spread and behavior that attempt an almost complete representation of fire-atmosphere interactions. This level of fidelity however means that these models cannot be used operationally. The reason is that, despite ever increasing computational resources, the complexity and range of processes and scales (1 mm to 100 km) involved in this modeling approach make computational costs prohibitively expensive. In this study we propose an intermediate approach. A physics-based coupled atmosphere-fire model is used to resolve the large-scale and local weather as well as the atmosphere-fire interactions, while combustion is represented simply using an existing operational surface fire behavior model. This model combination strikes a balance between fidelity and speed of execution. The feasibility of this approach is examined based on an analysis of a numerical simulation of two very large Santa Ana fires using WRF-Sfire, a coupled atmosphere-fire model available at the Open Wildland Fire Modeling Community (OpenWFM.org); an earlier version is available as WRF-Fire in WRF release. The study demonstrates that a wind and fire spread forecast of reasonable accuracy was obtained at an execution speed that would have made real-time wildfire forecasting of this event possible.

physics.ao-ph↗