Observations of flow and turbulence in complex terrain during evening transition

Flow transitions occurring around sunrise and sunset, where heat flux changes the sign, continue to pose challenges for mesoscale models, and these transitions are well-defined under conditions of no-synoptic forcing and when stable stratification is strong. The topic has received a great deal of attention in the last two decades, and progress has been made in identifying mechanisms of transition. Nevertheless, the lack of comprehensive datasets has limited the verification and application of existing theories and numerical findings. The focus of this paper is the evening period, which has received less attention than its morning counterpart. We use extensive measurements from IOP8 (30h -classified as super IOP) of the MATERHORN Fall campaign (www.nd.edu/~dynamics/materhorn) to analyse flow characteristics and turbulence decay over slopes and in valleys to identify mechanisms of evening transition. The dataset includes measurements from sonic anemometers as well as fast and slow temperature and humidity sensors mounted on several towers located in different slopes and valleys; full energy balance stations; PWIDS (Portable Weather Instrumentation Data Systems); SAMS (Surface Atmospheric Measurement Systems); MINI-SAMS and from two tethered balloons. RF data from a newly developed remote sensor were also used to detect changes in soil water content. Several techniques ranging from spectral investigations to proper orthogonal decomposition (POD) were employed, based on the type of the data, to detect the time of transition, study the vertical and horizontal flow structures, and obtain characteristic length scales during turbulence decay and evolution. Such analyses were complemented by detailed flow simulations using the Weather Research and Forecasting (WRF) model. Periods with no-synoptic influence, which facilitates the establishment of down-slopes and down-valley flows after well-defined flow stagnation, were of special interest. Those flows are highly time-dependent due to the nature of inhomogeneous topography, which promote the interaction of slope and valley flows arriving from different directions. This non-stationary nature was reflected from the evening-transition time; in valleys it occurs around sunset and develops vertically rather slowly (e.g. about 4 hours for the entire column to become stable) while the transition time is rather sharp on the slopes. As an example, Figure 1 shows the evolution of potential temperature (Fig. 1a) and wind direction profiles (Fig. 1b) measured by a tethered balloon operating in one of the valleys (i.e. Sagebrush area) before, during and after evening transition. Figure 1c and 1d show a comparison between a 1-km grid WRF model run based on YSU (YonSei University) Planetary Boundary Layer option (white vectors) with selected SAMS and MINI-SAMS (green vectors) around Granite Mountain (at the centre of the graph). The bottom left snapshot (Fig. 1c) shows wind vectors before the transition and the right snapshot (Fig.1d) after the transition. Note that the evening transition (time, wind speed and directions) is reasonably well captured in the valleys while the agreement becomes less satisfactory in the proximity of slopes. Such similarities and differences will be discussed in this presentation, together with verification of two possible scenarios for evening transitions. In one scenario the transition occurs as a slow moving front (as proposed by Hunt et al. 2003) with high turbulence in the eye of the front and in the other scenario the transition occurs over the entire air column as a sliding slab flow. A criterion based on Rayleigh number has been proposed for the occurrence of the latter (Fernando et al. 2013). Investigations using the MATERHORN dataset show that the second scenario is prevalent over most of the slopes, while the former is more appropriate for valley flows. References Fernando HJS, Verhoef B, Di Sabatino S, Leo LS, Park S (2013) The Phoenix evening transition flow experiment (TRANSFLEX). Boundary-Layer Meteorol. 147: 443-468 Hunt JCR, Fernando HJS, Princevac M (2003) Unsteady thermally driven flows on gentle slopes. J Atmos Sci 60:2169–2182. Figure 1 – Vertical profiles of air potential temperature evolution during transition a) and wind direction before, during and after transition b) measured using a tethered balloon (IOP8); Wind vectors from WRF simulations for IOP8 before the evening transition c) and a few hours after d). Green arrows are wind measurements from SAMS and MINI-SAMS and blue arrows are wind measurements from PWIDS.