O b fi ld d. CBLd. l d idl f l i. â¡ Over bare fields, a deep CBL developed rapidly from early morning. Thermal in the CBL was strong. The heat flux contributed to ...
Characteristics of convective boundary layer and cumulus clouds over humid terrestrial area Satoshi Endo Taro Shinoda, Hiroki Tanaka, Kazuhisa Tsuboki, Tetsuya Hiyama, Hiroshi Uyeda, Kenji Nakamura (HyARC, Nagoya Univ.)
Atmospheric Boundary Layer The earth surface p provides sensible heat flux and latent heat flux to the atmosphere. Surface energy budget :
R Rn Net radiation
=
H
+
lE
Sensible heat Latent heat flux ((SHF)) flux ((LHF))
+ G Ground or ocean heat flux
( F) (+ Heat storage
The atmospheric boundary layer (ABL) appears as a result of the surface forcings.
Importance of ABL
ABL consists of dry and moist convection
continuous surface flux
Trade wind cumulus boundary layer stores water vapor (latent energy), and feeds deep convection in the Inter-Tropical Convergence Zone (ITCZ).
ABL has an important function as a storage of water vapor.
Atmospheric Boundary Layer over terrestrial area The ABL over terrestrial area is marked by the diurnal change caused by the diurnal variation of the large surface flux (e.g. Angevine et al., 1994; Yi et al. 2001)
From Cohn and Angevine (2000)
In convective boundary layer (CBL), thermal updrafts and inter-thermal d downdrafts d f fform circulation i l i (e.g. Young 1988a, b).
Zi : inversion i i height h i ht (CBL top)
From Stull (1988) Surface flux
Atmospheric Boundary Layer over terrestrial area The ABL over terrestrial area is marked by the diurnal change caused by the diurnal variation of the large surface flux (e.g. Angevine et al., 1994; Yi et al. 2001)
From Cohn and Angevine (2000)
Buoyancy associated with the buoyancy flux fuels the thermal updrafts.
Difference of the buoyancy flux ( w’θv’ ) and heat flux ( w’θ’ ) is small (cf. Stull, 1988).
Cumulus boundary layer over terrestrial area Fair-weather F i th cumulus l clouds l d are classified l ifi d iinto t 3 ttypes (Stull 1985).
・ Forced cumulus: cumulus forced by dry convection ・ Active cumulus : cumulus reaching LFC ・ Passive cumulus : the decaying remnants of active cumulus (dynamically still active)
Cumulus onset occurs when
Min. LCL
Max. height of thermal exceed the LCL of the thermal i.e., RH at inversion height > 100% (Wilde et al. 1985; Ek and Mahrt 1994; Zhu and Albrecht 2002, 2003) Max. height of thermal
From Wilde et al. 1985
Land cover map
double crop including rice
and others (73 classes)
From the IWMI’s Global Map of Land Use/Land Cover Areas
Surface forcing over terrestrial areas Extensive studies of the CBL over terrestrial areas have been conducted for the large plains in North America. Crop land (wheat) (The South Great Plain, Zhu and Albrecht 2003)
H ∼ 300, lE ∼ 250 W m-2
at the maximum
Depending on soil moisture
Paddy fields dominates in monsoon Asian region. Paddy field (The Huaihe River Basin in China, Ikebuchi et al. 1998)
H ∼ 150, lE ∼ 500 W m-2 Water body on soil Small Bowen ratio (H / lE)
at the maximum
Few studies have been made on the CBL over such humid terrestrial areas.
Purpose
This study clarifies the characteristics of dry convective boundary layer and cumulus boundaryy layer y over humid terrestrial area. such as the paddy fields with a focus on the onset of active cumulus
Observation Shouxian,, Anhui Province,, China Monitoring observation by LAPS/CREST (2003/8 – 2006/1)
・ Middle stream of the Huaihe River ・ 23 m above sea level ・ Flat and nearly uniform surface ・ Double cropping (wheat and rice)
Observation IOP2004: 24 May – 16 July 2004
1290- MHz Wind Profiler Radar Signal to Noise Ratio (SNR) (SNR), Wind velocity (3 components) 32-m flux tower Temperature, Water vapor, Wind velocity (3 components) components), Radiation, Soil temperature, etc.
Manual observation Temperature of water, soil, surface, Cloud fraction, etc.
Time variation of the land surface during the IOP-2004 2004 5/24
7/16 time
Wheat field Bare field Paddy P dd fi field ld
Time variation of the land surface during the IOP-2004 2004 5/24
7/16
LHF ∼ SHF
LHF ≫ SHF
The two clear days are selected to study dry convective boundary layer 5/31: Dry-case
6/22: Wet-case
Surface flux and development of dry CBL
Over Bare field Inversion height Tanaka et al. (2007) Deep CBL d D developed l d rapidly from early morning. (2250 m at a maximum)
Max value of updraft ∼ 3 m s-1 Repetition of up up- and downdrafts Horizontal scale of tthe e circulation c cu at o ∼ 3 km
LHF SHF ≦ ∼ 200 Wm-2 ∼ 300 Wm-2
Surface flux and development of dry CBL
Over Paddy field Inversion height Tanaka et al. (2007) Shallow Sh ll CBL d developed l d slowly from late morning. (1400 m at a maximum)
Max value of updraft ∼ 1.5 m s-1 Repetition of up up- and downdrafts Horizontal scale of tthe e circulation c cu at o ∼ 1.3 km
LHF SHF >> ∼ 500 Wm-2 ∼ 100 Wm-2
Numerical model and Experimental setup
も
Model
CReSS 2.1 2 1 (Tsuboki and Sakakibara 2002)
Turbulence
1.5-order TKE
Cloud
Not used
Radiation
Only surface energy balance
Num of grid point
Horizontal : 200 x 200 (20 km x 20 km)
((Domain) o a )
Vertical : 120 (5888 m)
Grid size
Horizontal : 100 m Vertical : 30 m below 3000m
Time step
Non-acoustic wave : 1 s Acoustic wave : 0.1 s
Lateral boundaryy
Periodic boundaryy condition
Top boundary
Rigid boundary condition with a sponge layer
Bottom boundary (Surface model)
Bulk method (Louis et al. 1981) Prediction of soil temperature: 0.1 0 1 m x 30
Forcings
Diurnal change of shortwave radiation
Initial conditions
Sounding at 00 UTC (08 LST) at Fuyang
Integrated time
12 h
Development of dry convective boundary layer Simulation
Over Bare field
Observation
Rn SHF LHF
Rn SHF LHF
Vertical fluxes of heat, water, and buoyancy
Homogeneous heating of the CBL
Moistening near the CBL top
Contribution of moisture flux
Over Bare field
≪
Contribution of heat flux
Development of dry convective boundary layer Simulation
Over Paddy field
Observation
Rn SHF LHF
Rn SHF LHF
Vertical fluxes of heat, water, and buoyancy
Over Paddy field
The difference of air density caused by water vapor is considerable id bl source off b buoyancy.
Smaller heating & Larger moistening compared with the dry case
Contribution of moisture flux
∼
Contribution of heat flux
Characteristics of dry convective boundary layer
Over O bare b fields, fi ld a d deep CBL d developed l d rapidly idl ffrom early l morning. i Thermal in the CBL was strong. The heat flux contributed to almost all of the buoyancy flux. This characteristic is common in the relatively dry terrestrial areas. Over paddy fields, a shallow CBL developed slowly from late morning. Th Thermal l iin th the CBL was weak. k The Th contribution t ib ti off th the water t vapor flux fl was same order as that of the heat flux. The large contribution off moisture to the buoyancy is one off the main characteristics of the CBL over humid terrestrial areas.
Characteristics of cumulus boundary layer? To clarify Cumulus the development of cumulus BL over humid terrestrial area, boundary layer over terrestrial area idealized numerical experiments p were p performed . Focus on transition from forced to active cumulus boundary layer Sensitivity to evaporative efficiency, initial water vapor and static stability
large in humid terrestrial area
Onset process of active cumulus BL? Interaction between cumulus clouds and environmental condition?
Model case for the control experiment For the idealized simulations, several parameters are based on observational data obtained over paddy field on 20 June 2004.
LCL
Spike-shape Spike shape large SNR appeared above the LCL after around 12 LST.
Small SHF & Large LHF from paddy field
The spike-shape SNR was attributed to the fluctuation of refractive index of the atmosphere surrounding the cumulus clouds (Kollias et al., 2001).
Numerical model and Experimental setup Model
CReSS 2 2.1 1 (Tsuboki and Sakakibara 2002)
SGS Turbulence
1.5-order TKE
Cloud
Bulk warm rain
Radiation
Only surface energy balance
Num of grid point (Domain)
Horizontal : 200 x 200 (20 km x 20 km) Vertical : 230 (∼10000 m)
Grid size
Horizontal : 100 m Vertical : 30 m below 6000m
Time step
Non-acoustic wave : 1 s Acoustic wave : 0.1 s
Lateral boundary
Periodic boundary condition
Top boundary
Rigid boundary condition
Surface
Bulk method (based Louis et al al., 1981) Prediction of soil temperature : 0.1 m x 30
Forcing
Diurnal change of shortwave radiation
Initial conditions
Idealized profile
Integrated time
14 hrs (06 – 20 LST)
Initial profile Idealized initial profiles are used for the simulation ・ Pressure on the basis of hydrostatic equilibrium ・ Water vapor mixing ratio exponentially decreasing with height ・ Virtual potential temperature linearly g with height g increasing cf. Stevens (2007) The parameters for control experiment
q0
17 g kg-1
λ
2700 m
Γv
4.5 K km-1
θv0
301 K
Solid lines: Created θv and qv ●：Observed θv and qv at Fuyang at 08 LST
Development of cumulus boundary layer in the control experiment Total cloud fraction
Cloud fraction LFC
At 0920 LST, total cloud fraction began to increase.
LNB
The cloud layer developed deeper until sunset. LFC and LNB suddenly decreased at 1210 LST.
LCL
Contours: θv Colors: CF
inversion height
CF decreased above inversion height. After the LFC drop, θv showed a vertical gradient in the cloud layer layer.
Cumulus clouds before and after the LFC drop 11 LST
The cumulus clouds were negatively buoyant (θv' < 0)
Forced cumulus clouds existed.
qc = 0.01 g kg g-1
13 LST
The cumulus cloud was positively buoyant (θv' > 0)
Active cumulus cloud also existed.
Cumulus clouds before and after the LFC drop Profile of cloud fraction (CF) ( ) Profile of active CF (θv’ > 0) 11 LST
13 LST
On the basis of vertical fluxes of heat and water (not shown) ...
11 LST Small contribution of clouds to the increase in the inversion height
13 LST The evaporation of cloud water cooled and moistened the inversion layer.
Only forced cumulus clouds existed.
Active cumulus clouds also existed.
Large contribution L t ib ti off clouds l d tto the increase in the inversion height
The LFC drop was almost coincident with the active cumulus onset.
How does the LFC drop occur? Time variation of mean θe and θe* profiles
θe
θ e*
The LFC drop was attributed to a decrease of the local minimum value of θe* at the bottom of the inversion layer, i addition in dditi to t an increase of θe near the land surface.
The local minimum value of θe* at zib
θd ×
Lv qvs(T ) exp cpd T
θe*
Inversion layer
T
The local Th l l minimum i i value l was yielded i ld d as a naturall result l off well-mixed ll i d llayer and inversion layer. The value of θe*(zib) decreased with the increase of the inversion height.
zib
Experimental setup for the sensitivity experiment The sensitivity exp exp. were performed to demonstrate the impact of external
factors: initial amount of water vapor, static stability and evaporative efficiency. Water W t vapor mixing i i ratio, ti static t ti stability t bilit
are controlled by q0 and Γv, respectively.
Large evaporative efficiency β
results smaller Bowen ratio (SHF / LHF). LHF)
1. Control exp. Γv = 3.5 K km-1 q0 = 17 g kg g-1 β = 0.25
2. Sensitivity exp. to qv q0 = 13,, 14,, 15,, 16 g kg g-1
3. Sensitivity exp. to Γv and dβ 36 combinations of q0 = 14,, 15,, 16,, 17 g kg g-1, Γv = 3.5, 4.5, 5.5 K km-1, β = 0.10, 0.25, 0.40
Sensitivity to initial water vapor qv
control
■：Total CF > 0.01 （Forced Cu onset） ▲：LFC < zi (Active Cu onset)
Both B th fforced d and d active ti cumulus onsets delayed with the decrease of qv.
After the active cumulus onset, the inversion height kept large increasing rate.
Sensitivity to initial water vapor qv Time series of mean θe(zs) and θe* (zib)
z
θe*(z ( ib) θe(zs) θe*(zib)
▲
With the decrease of initial water vapor,
▲
▲
・ Initial value of θe (zs) decreased． ・ θe* (zib) increased because of the decrease of inversion height height．
θe(zs) Active cumulus onset delayed．
θ
Sensitivity to static stability Γv and evaporative efficiency β
Γv
Symbols: active Cu onset time
5.5
control
4.5
3.5
0.10
0.25
0.40
β
Sensitivity to static stability Γv
Γv 5.5
4.5 early onset
3.5
0.10
0.25
0.40
β
Sensitivity to static stability Γv
Γv
z
Mean θe((zs) and θe* ((zib)
θe(zs) θe*(zib)
5.5 Fast decrease of θe*(zib)
4.5 early onset
3.5
0.10
0.25
0.40
β
θ
Sensitivity to evaporative efficiency β
Γv 5.5 Fast decrease of θe*(zib)
4.5 early onset
3.5
0.10
early onset 0.25
0.40
β
Sensitivity to evaporative efficiency β
Γv
z
Mean θe((zs) and θe* ((zib)
θe(zs) θe*(zib)
5.5 Fast decrease of θe*(zib)
4.5 early onset
3.5 Fast increase of θe(zs)
0.10
early onset 0.25
0.40
β
θ
Development of Cumulus boundary layer In the control experiment, the boundary layer developed from dry convective
boundary layer, to forced cumulus boundary layer, and to active cumulus boundary layer. Since the evaporation of cloud water cooled and moistened the inversion
layer, the active cumulus boundary layer developed continuously. The active cumulus onset was attributed to a decrease of the local minimum
value of saturated equivalent potential temperature at the bottom height of the inversion layer (θe*(zib)), )) in addition to an increase of equivalent potential temperature near the land surface (θe(zs)). The active cumulus onset time became earlier with
・ the larger amount of water vapor, ・ the smaller static stability, and ・ the larger evaporative efficiency through the change in the time tendencies of θe*(zib) and θe(zs).
z
θe(zs) θe*(zib)
θ
Characteristics in the humid terrestrial area?
Small Bowen ratio (SHF/LHF) (large evaporative efficiency)
Large amount of water vapor in the lower troposphere
More likely to be active cumulus boundary layer
Change in water vapor profile Profiles of water vapor p mixing g ratio at 06 and 19 LST
Diff Difference b between t 06 and d 19 LST
The earlier onset
Higher inversion height Transport of water vapor to Higher level
Th later The l t onsett
Lower iinversion L i h height i ht Accumulation of water vapor in the lower atmosphere
Conclusions In comparison with the relatively dry terrestrial area area, over the humid terrestrial area, Dry convective boundary layer
・ Shallow dry convective boundary layer develops slowly from late morning, and the dry convection is weak. ・ The contribution of the water vapor flux to the buoyancy flux is the same order as that of the heat flux flux. Cumulus boundary y layer y
・ Because of small Bowen ratio and large water vapor mixing ratio, active cumulus onset (θe(zs) = θe*(zib)) is more likely to occur. ・ The onset time and vertical fluxes of the active cumulus boundary layer should be important for the change of the vertical water vapor distribution.
Bench warmers
Effect on of water precipitation vapor on systems Precipitation Over humid terrestrial area - Small Bowen ratio (SHF/LHF) - Large amount of water vapor
・More likely to be active cumulus boundary layer ・Large difference in water vapor profile
Specifically S ifi ll iin thi this region, i water t vapor profile fil iis one off th the essential ti l ffactors t
for the evolution of precipitation systems. - Yamada et al al. (2007) pointed out that water vapor evaporated from paddy field daytime could enhance the nighttime evolution of precipitation system. - Shinoda and Uyeda (2002) demonstrated that the precedent cumulus cloud moistened the mid-troposphere and make a favorable condition for the development of following deep precipitating cumulus cloud.
The regulation of the vertical water vapor distribution by cumulus boundary layer process should be important for the enhancement and/or development of precipitation systems in this region.
Impact on total cloud fraction
Γv 5.5
4.5 early onset
3.5
0.10
early onset 0.25
0.40
β
Histogram of θe’ with mean θe and θe* at the LFC-drop time θe* qc > 0.1gkg-1
The high-θe air mass 1) was generated near the land surface,, w > 0.5ms-1
2) rose through the subcloud layer and
θe
3) was included in the cloud at the height of the local minim of mean θe* (corresponds to the bottom off the th inversion i i llayer zib). ) At zib, the larger edge of the distribution reached mean θe*.
Cloud fraction and Vertical fluxes at 11 LST Cloud ffraction (C C (CF)) Active CF (θv’ > 0)
Only forced cumulus clouds existed.
Before the LFC drop
qt flux qv flux qc flux
θl flux (Enthalpy flux) Cont. of θ flux Cont. of qc flux
The contribution of qc flux is small.
qc flux contributed to θl flux.
The forced cumulus made cooling inversion layer and contributed to increase the inversion height.
Cloud fraction and Vertical fluxes at 13 LST Cloud ffraction (C C (CF)) Active CF (θv’ > 0)
Active cumulus clouds also existed.
qt flux qv flux qc flux
qc flux contributed to qt flux.
After the LFC drop
θl flux (Enthalpy flux) Cont. of θ flux Cont. of qc flux
qc flux largely contributed to θl flux.
The active cumulus clouds made heating the cloud layer and cooling and moistening the inversion layer.
Profiles of Cloud fractions and Vertical fluxes
Vertical fluxes and Components Total water flux
qv flux f
qc flux fl
∼ zero
Liquid water potential temperature flux (equivalent for enthalpy flux)
Cont. of θ flux
Cont. of qc flux
Buoyancy flux
Cont. of qv flux Cont. of qc flux
Cont. of θ flux
