Modeling Cold Pools in California's Central Valley Travis Wilson ∗ and Robert Fovell Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles ABSTRACT Despite our increased understanding in the relevant physical processes, forecasting radiative cold pools and their associated meteorological phenomena (e.g., fog and freezing rain) remains a challenging problem in mesoscale models. The present study is focused on California's Tule fog where the Weather Research and Forecasting (WRF) model's inability to forecast the event is addressed and substantially improved. An intra-model physics ensemble reveals that no current physics is able to properly capture the Tule fog and that model revisions are necessary. It has been found that revisions to the height of the lowest model level in addition to reconsideration of horizontal diffusion and surface-atmospheric coupling are critical for accurately forecasting the onset and duration of these events. 1. Introduction Planning and Standards (Baker et al. 2011). The meteo- rological conditions associated with cold pools create stag- Whiteman et al. (2001) defined a cold pool as a to- nant air that cannot mix out vertically due to capping in- pographically confined, stagnant layer of air overlaid by versions. This increases ozone or PM2.5 (particulate mat- warmer air aloft. A dramatic and well known example is ter with diameter <2.5 um) concentrations that can exceed the Tule fog of California's heavily populated Central Val- the National Ambient Air Quality Standard (NAAQS) in ley (CV), of which can persist for several days. If a cold many parts of the Western United States including, but pool last longer than one diurnal cycle it is classified as not limited to, the Pacific Northwest, the Central Valley of a persistent cold pool, whereas diurnal cold pools form at California, and Great Basin (Gillies et al. 2010). Of course, sunset and decay at sunrise (Whiteman et al. 2001). While elevated ozone and PM2.5 levels pose a large health risk to the Central Valley is a common example, cold pools are many Americans. Despite the impact they impose on air most prevalent in mountain valleys during periods of high pollution and weather, persistent cold pools have received atmospheric pressure, light winds and low solar isolation relatively little research attention (Zhong et al. 2001). (Daly et al. 2009). Due to their characteristic inversions, WRF, along with the Fifth-Generation NCAR/Penn cold pools are also conducive to freezing rain that creates State Mesoscale Model (MM5), have failed to reproduce a hazard to transportation and safety (Whiteman et al. the intensity and persistence of cold pools despite system- 2001). atic improvements to both model physical parameteriza- In this paper, an attempt to properly fix the cold pool tions as well as horizontal and vertical resolution (Baker modeling problem in California's CV will be undertaken. et al. 2011). Data assimilation and surface nudging have Most recently, Ryerson (2012) noted the Weather Research also been explored, both failing to add value to the al- and Forecasting (WRF) model's inability to produce fog in ready poor representation of cold pools (Avery 2011). Of and around the CV, which was largely traced to a overnight the cold pools that are resolved, they often mix out too warm bias. In order to fix this, he applied post-processing early, leading to erroneous surface temperatures, ozone, corrections which added significant overnight skill; how- and PM2.5 concentrations (Avery 2011). It is becoming ever, due to the non-linear evolution of fog, only modest increasingly clear that a cold pool-aware surface and/or skill increases were seen after sunrise. This paper is not an boundary layer schemes are needed in WRF to accurately attempt to amend the cold pool model problem via post reproduce these stagnant air quality episodes (Baker et al. processing techniques, but rather, by way of model physics 2011; Avery 2011). improvements. The process of cold air pooling can be enhanced by shel- Advancements in modeling cold pools need to be made tering from valleys and nearby trees that effectively reduce not only for forecasting fog and precipitation type, but also the vertical mixing that would otherwise bring warmer air for air quality as it is of great concern to the U.S. Envi- down to the surface (Gustavsson et al. 1998). In addition ronmental Protection Agency (EPA) Office of Air Quality 1 to this, Whiteman (2000) has demonstrated that further ing terrain of Oklahoma, Hunt et al. (2007) concluded that cooling in valleys is possible simply because of their shape; the cooling observed in cold pools occurred in situ which a cross sectional column of air over a valley is always less counters the results of LeMone et al. (2003). More recently, than that of flat terrain. That said, Whiteman et al. (2004) Bodine et al. (2009) found that cold pools were suppressed showed the cooling effect is counteracted by the the down- by the katabatic winds in the Lake Thunderbird Micronet, ward longwave radiation originating not only from the at- a dense collection of surface stations (28 stations) in the mosphere, but also from the valley walls. gently sloping terrain of Lake Thunderbird, Oklahoma. In The usual conceptual model of cold pool formation in fact, they went as far as stating that, \pooling of cold air valleys involves cool air drainage. Just after sunset, winds as a result of drainage flow can clearly be excluded as a diminish and a shallow, stable, boundary layer forms due factor causing the CP [cold pool] development at the mi- to the strong radiative flux divergence. Negatively buoyant cronet". Instead, cold pool formation was likely caused by air originating on the side slopes of the valley descends to the cooling that occurred in situ due to the radiative heat the stable layer, detaches from the side wall, and flows out loss and diminishing turbulent heat transfer in sheltered over the center of the basin (Clements et al. 2003). Essen- regions. tially, the air aloft is efficiently cooled by the basin walls Sometimes, drainage flows are not even observed, as before becoming detached; this acts to enhance the cooling Clements et al. (2003) has shown. When inspecting the above the surface due to the sensible heat flux divergence closed 1km Peters Sinks basin in Utah, the winds ceased (Whiteman et al. 1996). in the entire basin roughly 2 hours after sunset when they Nevertheless, Whiteman et al. (1996) found the sensible became \too weak to measure accurately". This led to heat flux at the surface to be near zero. Neff and King their conclusion that downslope flows in the Peters Sinks (1989) produced similar results in their research showing basin play only a minor role in the formation of cold pools. that drainage flows along the Colorado River basin overlay These weak flows were somewhat expected as Katurji and a stronger surface based inversion of lighter winds. This Zhong (2012) have shown that weaker drainage flows do suggests that the cooling above the surface can be due to correspond to smaller basins with larger slope angles in advective effects while the cooling at the bottom occurs two-dimensional idealized simulations. Additionally, they in situ. It should be noted that cooling aloft by drainage found the main cooling process near the basin floor (<200m) flows indirectly assists the in situ cooling at the surface by to be the longwave radiative flux divergence, while the ver- reducing the downward longwave radiation. Though this tical advection of temperature dominated the cooling pro- effect is suspected to be insignificant, it is currently unclear cess in the upper basin area. how important it may be. Although the production of cold pools may be slightly Drainage flows do not always become detached from controversial, the latest research agrees that temperatures the basin walls and have been well observed flowing in val- are not further reduced due to cold air drainage. This is ley locations (Hootman and Blumen 1983; Gudiksen et al. important because if they were, drainage flows of only a few 1992; Bodine et al. 2009). Yet, their role in the production meters would be extremely difficult to resolve and would of cold pools remains controversial. In the 1997 Cooper- somehow have to be parameterized. However, since this ative Atmospheric Surface Exchange Study (CASES-97) is not case, we believe cold pools in California's Central at the Walnut River watershed in Kansas, LeMone et al. Valley can be resolved with relatively coarse resolution. (2003) attributed cold temperatures at the low elevation sites to cold air drainage and radiative cooling. However, 2. Methodology the fast drainage flows observed at higher elevations were Using WRF version 3.5, cold pools in California's Cen- extremely weak lower down, causing the authors to suggest tral Valley will be modeled with and without several mod- that the flows were elevated over a more dense cold pool. ifications to the default WRF. Different atmospheric ini- Mahrt et al. (2001) examined the CASES-99 observations tializations were tested and their influence was deemed in south-central Kansas and found that while the drainage relatively unimportant; because of this, all atmospheric flows do exist, their influence on the surface fluxes were variables will be initialized from the North American Re- undetectable because the shallow flows decoupled the ob- gional Reanalysis (NARR). In contrast, simulations were servations (located at 3-10m) from the surface. It is worth sensitive to the soil initialization, so we have consequently noting that of the surface fluxes measured, the sensible chosen to extract these surface fields from a variety of heat remained downward-directed; that is, it acted as a sources including the NARR, North American Mesoscale heat source throughout the night.