Simulating boundary‐layer rolls with a numerical weather prediction model

Abstract

Boundary‐layer rolls have an impact on bushfires, pollution dispersion, the triggering of extreme convective weather events, the air–sea interaction and intensification of tropical cyclones and momentum, particle and gas fluxes. Previous numerical modelling studies that investigate the effects of boundary‐layer rolls and the processes that control their dynamics have relied upon high‐resolution, idealised simulations. Recently, however, numerical weather prediction (NWP) systems have increased in horizontal resolution to the point at which they are capable of explicitly resolving shallow convective circulations. Therefore, there is a need to assess the ability of NWP to accurately forecast the formation, development and breakup of boundary‐layer rolls. Here, we assess the ability of the UK Met Office Unified Model (UM), the NWP component of the Australian Community Climate and Earth‐System Simulator (ACCESS), to simulate the life cycle of boundary‐layer rolls. A suite of simulations is performed, over a range of horizontal resolutions, of boundary‐layer rolls observed across Victoria, Australia, on Black Saturday, 7 February 2009. In this case, it is found that a horizontal grid spacing of less than 0.6 km is required to adequately reproduce the scale and evolution of the observed rolls. We note that the boundary layer was particularly deep on this day and as a consequence the boundary‐layer rolls were wider than may be typically observed, meaning the grid spacing specified here lies at the upper end of what would be required with a shallower boundary layer. The model output is used to assess how the boundary‐layer rolls contribute to the high fire danger on Black Saturday. It is suggested that boundary‐layer rolls may increase fire danger by (i) causing wind‐direction variability at the surface, which increases the rate of spread of fires; and (ii) enhancing the process of long‐range spotting by augmenting the vertical lofting of embers.

A Case Study of Offshore Advection of Boundary Layer Rolls over a Stably Stratified Sea Surface

“…The cold front associated with this low pressure passed Sweden on 2 May, bringing in colder air. Data from ground stations maintained by SMHI (Swedish Meteorological Institute) showed that the daily maximum temperature on the mainland was [15][16][17][18][19] ∘ C on 2 May and 10-14 ∘ C on 3 May. SST charts acquired from SMHI showed a temperature of [4][5] ∘ C in the parts of the Baltic Sea which will be studied here.…”

“…Because the wind direction was southerly in the earlier part of the day it took a while before the rolls were advected out over the sea. During the time of full rolls the wavelength and amplitude did not change to a large degree, which justified using a time average, in contrast to, for example, the study by Thurston et al [19]. After the rolls started dissipating in the evening, there was a gradual transition until they completely disappeared.…”

“…However, boundary layer rolls with larger wavelengths have also been observed in many cases. For example, Thurston et al [19] observed boundary layer rolls with 7-9 km wavelength under strong convection over the Australian continent.…”

“…It is thus likely that these phenomena are real, and therefore the second problem is that the scale of the boundary layer rolls is unknown. Studies have shown that when increasing the resolution to subkilometre scale roll amplitude and growth rate grows [41], and roll circulations can turn into more unorganized convective patterns [18,19], which suggests that it can not safely be said that an increased resolution would improve the performance of the simulations. It is still unknown exactly how the boundary layer rolls interact with subgrid scale turbulence, and therefore it is not possible to say at which resolution the portrayal of rolls is most accurate.…”

“…Boundary layer rolls have been successfully simulated by LES models [1,5,8,[15][16][17] and in a few cases also by numerical weather prediction models [18,19]. The advantage of using weather prediction models is the ability to simulate a longer time period, to obtain the diurnal variations, and to include the large-scale forcing and realistic topography, which gives the horizontal variations over extended areas.…”