Structure of observable meteorological state variables during transitions in the stably stratified nocturnal boundary layer Carsten Abraham & Adam H. Monahan University of Victoria June 14, 2018 C. Abraham, A. H. Monahan (UVic) June 14, 2018 1 / 14
Why investigate transitions in the SBL? Inaccurate representations of turbulence in weather and climate models under stable conditions cause misrepresentations in the near-surface temperature and wind profiles: C. Abraham, A. H. Monahan (UVic) June 14, 2018 2 / 14
Why investigate transitions in the SBL? Inaccurate representations of turbulence in weather and climate models under stable conditions cause misrepresentations in the near-surface temperature and wind profiles: ◮ forecast of fog, frost, or dew formation C. Abraham, A. H. Monahan (UVic) June 14, 2018 2 / 14
Why investigate transitions in the SBL? Inaccurate representations of turbulence in weather and climate models under stable conditions cause misrepresentations in the near-surface temperature and wind profiles: ◮ forecast of fog, frost, or dew formation ◮ harvesting wind energy C. Abraham, A. H. Monahan (UVic) June 14, 2018 2 / 14
Why investigate transitions in the SBL? Inaccurate representations of turbulence in weather and climate models under stable conditions cause misrepresentations in the near-surface temperature and wind profiles: ◮ forecast of fog, frost, or dew formation ◮ harvesting wind energy ◮ assessment of pollutant dispersal and air quality C. Abraham, A. H. Monahan (UVic) June 14, 2018 2 / 14
Why investigate transitions in the SBL? Inaccurate representations of turbulence in weather and climate models under stable conditions cause misrepresentations in the near-surface temperature and wind profiles: ◮ forecast of fog, frost, or dew formation ◮ harvesting wind energy ◮ assessment of pollutant dispersal and air quality What can we learn about the physics of the transitions in the SBL from observational data? C. Abraham, A. H. Monahan (UVic) June 14, 2018 2 / 14
Two regime behaviour at Cabauw, Netherlands C. Abraham, A. H. Monahan (UVic) June 14, 2018 3 / 14
The hidden Markov model (HMM) ◮ estimate from observable state variables an unobservable Markov chain ◮ statistical model to systematically detect regime behaviour when state variables in state space overlap C. Abraham, A. H. Monahan (UVic) June 14, 2018 4 / 14
HMM analysis of two regime behaviour at Cabauw ; C. Abraham, A. H. Monahan (UVic) June 14, 2018 5 / 14
Frequency of transitions wSBL to vSBL transition 0.07 Boulder Transitions occur in about 0.06 Cabauw FINO-1 40-70 % of nights 0.05 FINO-2 probability FINO-3 0.04 Hamburg 0.03 Karlsruhe Los Alamos 0.02 0.01 0.00 vSBL to wSBL transition 0.07 0.06 0.05 probability 0.04 0.03 0.02 0.01 0.00 −2 0 2 4 6 8 10 12 14 time after sunset [h] C. Abraham, A. H. Monahan (UVic) June 14, 2018 6 / 14
Frequency of transitions wSBL to vSBL transition 0.07 Boulder Transitions occur in about 0.06 Cabauw FINO-1 40-70 % of nights 0.05 FINO-2 probability FINO-3 0.04 Hamburg 0.03 Karlsruhe wSBL to vSBL transitions Los Alamos 0.02 0.01 are most probable around 0.00 sunset whereas vSBL to vSBL to wSBL transition wSBL transitions lack 0.07 0.06 pronounced maxima 0.05 probability 0.04 0.03 0.02 0.01 0.00 −2 0 2 4 6 8 10 12 14 time after sunset [h] C. Abraham, A. H. Monahan (UVic) June 14, 2018 6 / 14
Frequency of transitions wSBL to vSBL transition 0.07 Boulder Transitions occur in about 0.06 Cabauw FINO-1 40-70 % of nights 0.05 FINO-2 probability FINO-3 0.04 Hamburg 0.03 Karlsruhe wSBL to vSBL transitions Los Alamos 0.02 0.01 are most probable around 0.00 sunset whereas vSBL to vSBL to wSBL transition wSBL transitions lack 0.07 0.06 pronounced maxima 0.05 probability 0.04 0.03 turbulence collapse and 0.02 recovery events are in 0.01 statistical equilibrium 4 0.00 −2 0 2 4 6 8 10 12 14 hours after sunset time after sunset [h] C. Abraham, A. H. Monahan (UVic) June 14, 2018 6 / 14
Frequency of timing between transitions Time between turbulence collapse to revovery Boulder 0.08 Cabauw FINO1 0.06 FINO2 subsequent transitions occur probability FINO3 Hamburg 0.04 in about 50-70 % of nights Karlsruhe Los Alamos with transitions 0.02 0.00 Time between turbulence recovery to collapse 0.08 0.06 probability 0.04 0.02 0.00 0 2 4 6 8 10 12 time [h] C. Abraham, A. H. Monahan (UVic) June 14, 2018 7 / 14
Frequency of timing between transitions Time between turbulence collapse to revovery Boulder 0.08 Cabauw FINO1 0.06 FINO2 subsequent transitions occur probability FINO3 Hamburg 0.04 in about 50-70 % of nights Karlsruhe Los Alamos with transitions 0.02 0.00 Time between turbulence recovery to collapse the pdfs of the time between subsequent 0.08 turbulence recovery or 0.06 probability collapse events have a very 0.04 similar shape independent of 0.02 the tower station 0.00 0 2 4 6 8 10 12 time [h] C. Abraham, A. H. Monahan (UVic) June 14, 2018 7 / 14
Structure of the stratification during transitions wSBL to vSBL transitions 200 150 height [m] 100 50 vSBL to wSBL transitions 200 150 height [m] 100 50 −80 −60 −40 −20 0 20 40 60 80 time [min] 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 mean(Θ h − Θ 2 ) [K] C. Abraham, A. H. Monahan (UVic) June 14, 2018 8 / 14
Structure of the stratification during transitions wSBL to vSBL transitions 200 150 radiative cooling leads to a height [m] 100 steady formation of stable stratification suppressing 50 vertical turbulent fluxes vSBL to wSBL transitions 200 150 height [m] 100 50 −80 −60 −40 −20 0 20 40 60 80 time [min] 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 mean(Θ h − Θ 2 ) [K] C. Abraham, A. H. Monahan (UVic) June 14, 2018 8 / 14
Structure of the stratification during transitions wSBL to vSBL transitions 200 150 radiative cooling leads to a height [m] 100 steady formation of stable stratification suppressing 50 vertical turbulent fluxes vSBL to wSBL transitions 200 very rapid break down of 150 stratification allowing for height [m] 100 enhanced turbulent vertical 50 fluxes which stays quite steady after turbulence −80 −60 −40 −20 0 20 40 60 80 time [min] recovery 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 mean(Θ h − Θ 2 ) [K] C. Abraham, A. H. Monahan (UVic) June 14, 2018 8 / 14
Structure of the wind shear during transitions Along-wind component Across-wind component wSBL to vSBL transitions wSBL to vSBL transitions 200 200 150 150 height [m] height [m] 100 100 50 50 vSBL to wSBL transitions vSBL to wSBL transitions 200 200 150 150 height [m] height [m] 100 100 50 50 −80 −60 −40 −20 0 20 40 60 80 −80 −60 −40 −20 0 20 40 60 80 time [min] time [min] 0.00 1.25 2.50 3.75 5.00 6.25 7.50 8.75 10.00 0.0 0.3 0.6 0.9 1.2 1.5 1.8 mean(U h parallel to U 200 ) [m s −1 ] mean(U h perpendicular to U 200 ) [m s −1 ] C. Abraham, A. H. Monahan (UVic) June 14, 2018 9 / 14
Structure of TKE during transitions wSBL to vSBL transitions 250 200 height [m] 150 100 50 vSBL to wSBL transitions 250 200 height [m] 150 100 50 −80 −60 −40 −20 0 20 40 60 80 time [min] 0.200 0.275 0.350 0.425 0.500 0.575 0.650 0.725 mean(TKE) [m 2 s −2 ] C. Abraham, A. H. Monahan (UVic) June 14, 2018 10 / 14
Change of wind power density during times of transitions wSBL → vSBL 0.14 vSBL → wSBL 0.12 0.10 0.08 pdf 0.06 0.04 0.02 0.00 −500 0 500 1000 1500 2000 ∂ t 0.5 ρW 3 80 [W m −2 s −1 ] C. Abraham, A. H. Monahan (UVic) June 14, 2018 11 / 14
Tendencies of geostrophic forcing during transitions wSBL to vSBL vSBL to wSBL 0.30 0.25 0.20 pdf 0.15 0.10 0.05 0.00 −4 −2 0 2 4 ∂ t U geo [m s −1 h −1 ] C. Abraham, A. H. Monahan (UVic) June 14, 2018 12 / 14
Tendencies of geostrophic forcing during transitions wSBL to vSBL vSBL to wSBL 0.30 0.25 0.20 pdf 0.15 0.10 0.05 0.00 −4 −2 0 2 4 ∂ t U geo [m s −1 h −1 ] systematic changes of the geostrophic wind in times of transitions is absent C. Abraham, A. H. Monahan (UVic) June 14, 2018 12 / 14
Evolution of low level cloud cover during transitions wSBL to vSBL transition 100 10-%ile low level cloud cover [%] 20-%ile 80 30-%ile 40-%ile 60 50-%ile 60-%ile 40 70-%ile 80-%ile 20 0 vSBL to wSBL transition 100 low level cloud cover [%] 80 60 40 20 0 −80 −60 −40 −20 0 20 40 60 80 time [min] C. Abraham, A. H. Monahan (UVic) June 14, 2018 13 / 14
Conclusions The evolution of state variables in times of transition (as classified by an HMM analysis) is physically reasonable and shows clear structures during turbulence collapse and recovery events C. Abraham, A. H. Monahan (UVic) June 14, 2018 14 / 14
Conclusions The evolution of state variables in times of transition (as classified by an HMM analysis) is physically reasonable and shows clear structures during turbulence collapse and recovery events A clear precursor in external state variables is not always evident, however, changes in low level cloud cover have the potential to initiate a transition C. Abraham, A. H. Monahan (UVic) June 14, 2018 14 / 14
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