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Coupled vs. Decoupled Boundary Layers in VOCALS-REx ACP article describing these results:
http://www.atmos-chem-phys-discuss.net/11/8431/2011/
Chris Jones Department of Applied Math
Chris Bretherton
Department of Atmospheric Sciences
Dave Leon Department of Atmospheric Sciences
University of Wyoming
VOCALS RF05, 72W, 20S
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Importance of Decoupling • Cloud-topped boundary layers (CTBLs) can vary from simple, vertically
well-mixed structure to structure w/vertical gradients to cumulus regions • Vertical structure of CTBL important to …
– Cloud cover – Vertical mixing processes – Precipitation
• CTBL decoupling is the separation of BL turbulence into separate surface-driven and cloud-driven layers
(Albrecht et. al. 1995)
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October 2008-November 2008
SE Pacific
(http://www.atmos.washington.edu/~robwood/VOCALS/vocals_uw.html)
C-130 data available at http://www.eol.ucar.edu/projects/vocals/
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C-130 flight path (grey) Cloud base (lidar-derived) LCL (“well-mixed cloud base”)
Radar reflectivity (drizzle proxy)
(courtesy of Rob Wood)
We use vertical profiles and subcloud level legs
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Decoupling in VOCALS-REx Overview
• Use C-130 flight legs to measure extent of decoupling
– Profile-based decoupling index
– Subcloud leg decoupling index
• Dominant mechanism(s) for decoupling?
• Investigate relationship between inversion jumps, decoupling, and Sc breakup
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Profile data and selection criteria
• 110 profiles included: – North of 25 S (ignore 2 coastal aerosol flights) – Extend from ~150 m to above inversion
• Used 1 Hz C-130 in-situ atmosphere state measurements, averaged into 10 meter vertical bins (courtesy of Chris Terai)
• Inversion base (𝑧𝑖) defined as height of minimum T provided RH > 45%
• Decoupling classified using: – Total water mixing ratio (𝑞𝑇 = 𝑞ℓ + 𝑞𝑣)
– Liquid potential temperature (𝜃ℓ = 𝜃 −𝐿
𝑐𝑝𝑞ℓ )
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Profiles Decoupling metric(s)
Subcloud layer
Cloud layer
Δ𝑞 Δ𝜃ℓ Well-mixed Decoupled
Profile-based decoupling index: Δ𝑞 = 𝑞𝑡 𝑧∈[0,0.25𝑧𝑖] − 𝑞𝑡 𝑧∈ 0.75𝑧𝑖,𝑧𝑖
(bottom 25% - top 25%)
Δ𝜃ℓ = 𝜃ℓ 𝑧∈[0.75𝑧𝑖,𝑧𝑖] − 𝜃ℓ 𝑧∈[0,0.25𝑧𝑖]
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Subcloud data and selection criteria
• 89 level legs flown at 150 meters – 10 min segments (~60 km horizontal extent)
• In-situ T and q measurements used for LCL
• Wyoming Cloud Radar (WCR) – Cloud top
– Column-max radar reflectivity (drizzle proxy)
• Wyoming Cloud Lidar (WCL)-derived cloud base
• Decoupling classified by leg mean Δ𝑧𝑏 = 𝑧𝑏 − 𝐿𝐶𝐿
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Subcloud legs Decoupling metric: Δzb = 𝑧𝑏 − 𝐿𝐶𝐿
(actual cloud base – “well-mixed” cloud base)
drizzle
Profiles Decoupling metric(s)
Surface layer
Cloud layer
Δ𝑞 Δ𝜃ℓ Well-mixed Decoupled
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Decoupling Distribution
Δ𝑞 < 0.5g/kg Δ𝜃ℓ < 0.5 K
Δ𝑧𝑏 < 125m
Well-mixed criteria
Well-mixed (28%)
Profiles Subcloud legs
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Consistency of decoupling metrics
Least-squares fit
Thermodynamicargument
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Composite profiles
𝜃ℓ 𝑞𝑇
10 𝑞ℓ 𝑧𝑖
𝑧𝐿𝐶𝐿
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Organizing the results: can we deduce dominant physical mechanism(s)?
• Diurnal decoupling (solar absorption): warms the cloud layer, inhibits mixing b/w cloud and SC layer. – Not enough midday measurements
• Drizzle-induced decoupling: Latent heating of cloud layer and evaporative cooling of SC layer inhibits mixing.
• Wind speed / latent heat flux: Increased LHF => increased in-cloud buoyancy production of turbulence => more entrainment => more decoupling.
• Boundary-layer deepening: Deeper well-mixed cloud-layer => more buoyancy flux => more turbulence => more entrainment => more decoupling.
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POCs: Pockets of Open Cells
• Several VOCALS flights sampled POCs • POCs characterized by …
• Low droplet concentration • Enhanced drizzle • Broken clouds • Pronounced decoupling
Are decoupling processes in POCs statistically different than in other Sc regions?
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Decoupling somewhat correlated to drizzle, but drizzle not necessary for decoupling
Non-drizzling
Drizzling
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Decoupling uncorrelated to wind speed
Bretherton and Wyant (1997) suggested stronger latent heat fluxes should promote decoupling – not seen in our results.
𝐿𝐻𝐹 = 𝜌0𝐿𝑣 𝑤′𝑞𝑡′ = 𝜌0𝐿𝑣𝐶𝑇𝑉 𝑞𝑠𝑢𝑟𝑓
∗ − 𝑞𝑡𝑀
𝐶𝑇 ≈ 0.001 (transfer coefficient) 𝑉 = leg-mean 150 m horizontal wind speed 𝑞𝑠𝑢𝑟𝑓
∗ = saturation mixing ratio at
sea surface 𝑞𝑡𝑀 = subcloud leg-mean 𝑞𝑡
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“Well-mixed Cloud Thickness” Δ𝑧𝑀 best predicts decoupling
(this plot is the centerpiece of our results)
Δ𝑧𝑀 = 𝑧𝑖 − 𝐿𝐶𝐿: thickness the cloud would have if it was well-mixed
Subcloud Legs Profiles
Hollow marker = POC
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Mixed-layer model flux ratio condition for decoupling (Bretherton and Wyant, 1997)
• Decoupling occurs when Δ𝐹𝑅
𝐿𝐻𝐹< 𝑄 ≡ 𝐴𝜂
Δ𝑧𝑀
𝑧𝑖
• Too much uncertainty for meaningful quantitative test of this relationship, but qualitative agreement with observations
• In VOCALS-REx, 𝑄 varies more than Δ𝐹𝑅
𝐿𝐻𝐹. From east to west
–Δ𝐹𝑅
𝐿𝐻𝐹 varies from approximately 1.0 to 0.7
– 𝑄 / Δ𝐹𝑅
𝐿𝐻𝐹 increases from 0.3 to 0.9 (decoupling threshold at
approximately 0.4)
𝐴 ≈ 1.1 (Caldwell et al.,2005) 𝜂 ≈ 0.9 (thermodynamic variable)
Δ𝐹𝑅 (radiative flux divergence) ranges from 71 to 95 𝑊𝑚−2 𝐿𝐻𝐹 ranges from 70 to 135 𝑊𝑚−2
(restricted to morning, non-drizzling legs)
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Organizing the results: dominant decoupling mechanisms from VOCALS data
• Diurnal decoupling (solar absorption) – Not enough midday measurements
• Drizzle decoupling – No heavily drizzling well-mixed profiles, – Drizzle promotes decoupling, but is not primary cause
• Wind speed / latent heat flux – Not at any given longitude, but LHF does increase to west
along with decoupling
• Boundary-layer deepening – Well-mixed cloud thickness Δ𝑧𝑀 = 𝑧𝑖 − 𝑧𝐿𝐶𝐿 is the best
predictor of decoupling in VOCALS-REx data – Δ𝑧𝑀 > 500 𝑚 ⇒ decoupled
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Inversion Jumps
• Lock (2009) and others have suggested high values of
𝜅 = 1 +𝑐𝑝𝛿𝜃ℓ
𝐿 𝛿𝑞𝑡
induce strong entrainment and Sc cloud breakup.
• Strong entrainment might also favor decoupling.
𝛿𝜃ℓ
𝛿𝑞𝑡 Inversion base
Inversion “top”
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Calculating inversion jumps – Inversion base is already determined
– Inversion top: objective criteria based on RH and 𝜃ℓ profiles
– Complex POC inversion structure => identify jumps visually
POC Profile
𝛿𝜃ℓ
𝛿𝑞𝑡
Non-POC Profile
Inversion base
Inversion “top”
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Decoupling not correlated with inversion jump parameter
𝜅 = 1 +𝑐𝑝𝛿𝜃ℓ
𝐿 𝛿𝑞𝑡
• Use REx C-130 profiles to calculate jumps/decoupling, adjacent subcloud
legs to calculate cloud fraction. Restrict to flights before 10:00 LT in left panel.
• κ > 0.4 often (but not always) goes with broken cloud.
• For κ < 0.5 there is no obvious correlation of κ and decoupling.
• POC and non-POC distributions overlap
Blue = well-mixed Red = decoupled Hollow = POC Dash = Lock (2009) LES results
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Summary
• Well-mixed cloud thickness Δ𝑧𝑀 was best predictor of decoupling. – Δ𝑧𝑀 > 500 m: decoupled – Δ𝑧𝑀 < 500 m: well-mixed
• LHF and drizzle increase to the west. Both are likely contributing mechanisms in VOCALS decoupling, but no single parameter predicts decoupling as well as Δ𝑧𝑀.
• Inversion jump parameter is 𝜅 not a good predictor of decoupling, but qualitatively agrees with cloud cover predictions.
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Extra slides
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EPIC 2001 (Bretherton, et al.)
Some important mechanisms that come into play
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Consistency of decoupling metrics (thermodynamic argument)
0.5 g/kg
0.0048 g/(kg m)
Δ𝑧𝑏 ≈ 104 m
(+ a bit for the term we dropped)
Δ𝑞 = 𝑞𝑡1 − 𝑞𝑡2 = 𝑞𝑣 𝑧𝑆𝐶 − 𝑞𝑣 𝑧𝑏 = 𝑞∗ 𝑝𝐿𝐶𝐿 , 𝑇𝐿𝐶𝐿 − 𝑞∗ 𝑝𝑏, 𝑇 𝑧𝑏 = 𝑞∗ 𝑝𝐿𝐶𝐿 , 𝑇𝐿𝐶𝐿 − 𝑞∗ 𝑝𝑏, 𝑇𝑑𝑎 𝑧𝑏 + 𝑞∗ 𝑝𝑏, 𝑇𝑑𝑎 𝑧𝑏 − 𝑞∗ 𝑝𝑏, 𝑇 𝑧𝑏
𝑛𝑒𝑔𝑙𝑒𝑐𝑡
Δ𝑞 ≈ −𝑑𝑞∗
𝑑𝑧𝑑𝑎
𝑧𝑏 − 𝑧𝐿𝐶𝐿
𝑧𝑆𝐶
𝑧𝐿𝐶𝐿
𝑧𝑏
𝑞𝑡1 = 𝑞∗(𝑝𝐿𝐶𝐿, 𝑇𝐿𝐶𝐿)
𝑞𝑡2 = 𝑞𝑣 𝑧𝑏 = 𝑞∗ 𝑝𝑏, 𝑇 𝑧𝑏
𝑞𝑡1 = 𝑞𝑣 𝑧𝑆𝐶
z 𝑞𝑡
• Neglect weak p-dependence in 𝑞∗ • Use characteristic BL reference T,p
• 𝑝0 ~ 950 hPa • 𝑇 ~ 285 K
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Consistency of the two metrics (rough thermodynamic argument)
0.05 g/kg 0.0048 g/(kg m)
104 m (+ a bit for the term we dropped)
Assumed to follow approximately dry adiabat, s is conserved
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Calculating inversion jumps
• Want systematic approach to identify inversion jumps. – Inversion base is already determined – Inversion top:
• Relative humidity (RH) gradient below some threshold for at least 100 meters (to eliminate “jitters”):
𝑑 𝑅𝐻
𝑑𝑧< 0.3% per meter
• RH close enough to min value in free troposphere (to make sure it’s really the full inversion jump):
𝑅𝐻 < min 𝑅𝐻 + 10% • 𝜃ℓ gradient also below threshold (moisture and temp jumps
should be the same, but not always the case here): 𝑑𝜃ℓ
𝑑𝑧< 0.1 K per meter
– Identify inversion top visually for POC flights
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Decoupling when Δ𝐹𝑅
𝐿𝐻𝐹< 𝑄 ≡ 𝐴𝜂
Δ𝑧𝑀
𝑧𝑖
70∘ − 75∘W
Flux ratio Q Decoupled
Coupled
Decoupling due to increased Q more than change in flux ratio (consistent with “deepening/warming” mechanism)
75∘ − 80∘ 80∘ − 86∘