Cool Roofs Seen in Black, White, and Green

Science teaches us not to answer questions in black and white terms—the meaning of data usually has many shades of gray. So it is with figuring out the micro and macro effects that different roof coverings might have on global warming or energy efficiency. The answers are starting to look more complex, and more promising, than ever.

Assessing the indoor and outdoor climatic benefits of roofs with mixed cover.

For example, it would seem natural when studying rooftops and their effects on climate large and small to focus on extremes of albedo—in other words, black and white surfaces. This is sometimes the case in simplified modeling studies. But at this week’s AMS 9th Symposium on Urban Environment, in Keystone, Colorado, Adam Scherba of Portland State University submitted findings that moved beyond simple comparisons of white (“cool”) roofs versus black roof coverings. He and his colleagues mixed roof coverings, notably photovoltaic panels and greenery (which has the advantage of staying cooler during the day but not, like white roofs, getting much colder at night). The mix makes sense when you consider that roofs are also prime territory for harvesting solar energy. Scherba et al. write that

While addition of photovoltaic panels above a roof provides an obvious energy generation benefit, it is important to note that such systems – whether integrated into the building envelope, or mounted above the roof – can also result in an increase of convective heat flux into the urban environment. Our analysis shows that integration of green roofs with photovoltaic panels can partially offset this undesirable side effect, while producing additional benefits.

This neither-black-nor-white-nor-all-green approach to dealing with surface radiation makes sense from both energy and climate warming perspectives, especially given the low sun angles, cold nights, and snow cover in some climates. For instance, a recent paper, in Environmental Research Letters, uses global atmospheric modeling to estimate that boosting albedo by only 0.25 on roofs worldwide—in other words, not a fully “cool” scenario of all white (albedo: 1.0) roofs—can offset about a year’s worth of global carbon emissions.
And the authors of a recent modeling study (combining global and urban canyon simulation) published in Geophysical Research Letters simulated a world with all-white roofs but showed that:

Global space heating increased more than air conditioning decreased, suggesting that end-use energy costs must be considered in evaluating the benefits of white roofs.

Delving directly into such costs this week at the AMS meeting is a paper from a team led by Anthony Dominguez of the University of California-San Diego. They looked at the effects of photovoltaic panels on the energy needs in the structure below the roof, not the atmosphere above it.
Did they find that a building is easier to keep comfortable when covered by solar panels? Well, if you need to air-condition during the day, yes. If, however, like many people on the East Coast this summer, you need to keep cooling at night, then no. This is science—don’t expect a black-and-white answer.

Remaking Stable Boundary Layer Research, From the Ground Up

A recently accepted essay for the Bulletin of the American Meteorological Society by Joe Fernando and Jeff Weil is good background reading for this week’s AMS 19th Symposium on Boundary Layers and Turbulence in Keystone, Colorado.
Fernando and Weil point out that research into the lowest layer of the atmosphere where we all live and breathe will need to evolve to meet needs in numerical weather prediction. While progress is apparent in the modeling of the boundary layer when it is stirred into convection, those models have obvious shortcomings when the low-level air is not buoyant—the stable boundary layer typically encountered at nighttime. The stable boundary layer controls transport of pollution, formation of fog and nocturnal jets in the critical time before the atmosphere “wakes up” in daytime heating. Weil, in his presentation this Thursday at Keystone calls the still-flawed modeling of the stable situation “one of the more outstanding challenges of planetary boundary layer research.”
Fernando and Weil write in BAMS that study of the stable boundary needs to be retooled to embrace interactions of relevant processes from a variety of scales of motion. The weakness and multiplicity of relevant stable boundary processes means that investigations of individual factors will not be fruitful enough to improve numerical prediction. Scientists need to temper their natural tendencies to try to isolate phenomena in their field studies and modeling and instead seek

simultaneous observations over a range of scales, quantifying heat, momentum, and mass flux contributions of myriad processes to augment the typical study of a single scale or phenomenon (or a few) in isolation. Existing practices, which involves painstakingly identifying dominant processes from data, need to be shifted toward aggregating the effects of multiple phenomena. We anticipate development of high fidelity predictive models that largely rely on accurate specification of fluxes (in terms of eddy diffusivities) through computational grid boxes, whereas extant practice is to use phenomenological models that draw upon simplified analytical theories and observations and largely ignore cumulative effects/errors of some processes.

This new perspective, the authors argue, will be a “paradigm shift” in research and modeling.