Abstract: Abstract This study aims to find the typical growth rate of the temperature inversion during the onset of the stable boundary layer around sunset. The sunset transition is a very challenging period for numerical weather prediction, since neither accepted theories for the convective boundary layer nor those for the stable boundary layer appear to be applicable. To gain more insight in this period, a systematic investigation of the temperature inversion growth rate is conducted. A statistical procedure is used to analyze almost 16 years of observations from the Cabauw observational tower, supported by observations from two additional sites (Dome C and Karlsruhe). The results show that, on average, the growth rate of the temperature inversion (normalized by the maximum inversion during the night) weakly declines with increasing wind speed. The observed growth rate is quantitatively consistent among the sites, and it appears insensitive to various other parameters. The results were also insensitive to the afternoon decay rate of the net radiation except when this decay rate was very weak. These observations are compared to numerical solutions of three models with increasing complexity: a bulk model, an idealized single-column model (SCM), and an operational-level SCM. It appears only the latter could reproduce qualitative features of the observations using a first-order closure. Moreover, replacing this closure with a prognostic TKE scheme substantially improved the quantitative performance. This suggests that idealized models assuming instantaneous equilibrium flux-profile relations may not aid in understanding this period, since history effects may qualitatively affect the dynamics.

Abstract: There is a strong need to identify blowing snow events with and without concurrent falling snow and to estimate solid precipitation amounts in mountainous areas and polar regions. For these purposes, we first developed a method using the concomitant analysis of an anemometer and a drifting snow sensors (SPC-S7 and Wenglor/YH03PCT8-YH08PCT8). Photoelectric sensors, such as the SPC-S7 (Snow Particle Counter), specially designed for studying drifting snow, or a simpler photoelectric counter manufactured by Wenglor, were chosen because they had already been tested in previous studies for measuring solid precipitation. They were set up at Lac Blanc Pass, an experimental site dedicated to the study of drifting snow in the French Alps. The data set obtained was compared with the independent database of blowing snow events with or without falling snow collected at the same experimental site, i.e. data on the precipitation amount stemming from heated precipitation gauge and SAFRAN modeling output. The analysis of snow flux and mean diameter according to wind speed allowed us to separate blowing snow events with and without precipitation for moderate wind speed. To reduce the uncertainty at high wind speed, the SPC-S7 must be set up at least 4m above the snow surface. Similar preliminary results were obtained with the simpler Wenglor photoelectric counter, despite the minimum observable diameter being 200?m and the particle size distribution unavailable. These results must be confirmed by further experiments. The SPC-S7- estimated precipitation amount is in relatively good agreement with modeled precipitation given the many uncertainties due to the calculation hypotheses. Since the particle size distribution is not available for the simpler photoelectric counter and there are too many uncertainties and hypotheses in calculating solid precipitation, we concluded that the solid precipitation amount cannot be reliably estimated by the simple photoelectric counter.

Abstract: Abstract The atmospheric nocturnal stable boundary layer (SBL) can be classified into two distinct regimes: the weakly SBL (wSBL) with sustained turbulence and the very SBL (vSBL) with weak and intermittent turbulence. A hidden Markov model (HMM) analysis of the three-dimensional state-variable space of Reynolds-averaged mean dry static stability, mean wind speed, and wind speed shear is used to classify the SBL into these two regimes at nine different tower sites, in order to study long-term regime occupation and transition statistics. Both Reynolds-averaged mean data and measures of turbulence intensity (eddy variances) are separated in a physically meaningful way. In particular, fluctuations of the vertical wind component are found to be much smaller in the vSBL than in the wSBL. HMM analyses of these data using more than two SBL regimes do not result in robust results across measurement locations. To identify which meteorological state variables carry the information about regime occupation, the HMM analyses are repeated using different state-variable subsets. Reynolds-averaged measures of turbulence intensity (such as turbulence kinetic energy) at any observed altitude hold almost the same information as the original set, without adding any additional information. In contrast, both stratification and shear depend on surface information to capture regime transitions accurately. Use of information only in the bottom 10 m of the atmosphere is sufficient for HMM analyses to capture important information about regime occupation and transition statistics. It follows that the commonly measured 10-m wind speed is potentially a good indicator of regime occupation.

Abstract: Abstract The evolution of profiles of meteorological state variables during nights with and without transitions in the nocturnal stably stratified boundary layer (SBL) between weakly stable (wSBL) and very stable (vSBL) regimes, as classified by a hidden Markov model, is examined at nine different tower sites. During wSBL-to-vSBL transitions, inversion strengths increase, near-surface winds decelerate, and atmospheric layers vertically decouple. Turbulence kinetic energy (TKE) steadily decreases before wSBL-to-vSBL transitions and fluctuations of the vertical velocity become weak. In contrast to land-based sites where wSBL-to-vSBL transitions are normally caused by surface cooling, at sea-based stations the transitions generally are initiated by advection of warm air aloft. The vSBL-to-wSBL transition is characterized by a fast breakdown of the inversion strength, acceleration of wind profiles, and a restored vertical coupling of the atmospheric flow. TKE recovers on time scales of minutes first in atmospheric levels between 50 and 100 m. Profiles of state variables for the two different regimes during very persistent nights (nights without SBL regime transitions) are clearly separated and similar to structures during nights with transitions away from transition times. During very persistent nights the wind conditions stay relatively steady. Similarly, the temperature is steady after an initial adjustment time at sunset (wSBL) or shortly after sunset (vSBL). Even though nights with and without transitions are a common feature of the SBL, there is no clear indicator in Reynolds-averaged mean variables that distinguishes very persistent nights from nights with transitions.