Understanding Evaporation and Latent Heat Flux

(New) Methods of Observation

Henk A.R. de Bruin1

Session: Meet the expert in hydrology - The mystery of evaporation

European Geosciences Union Annual Meeting, 16 April 2015

Evaporation is about

a) Phase change at the surface: liquid into vapor (Thermodynamics, Schmidt) b) Vertical water vapor transfer in atmospheric surface layer (Dalton-Monin-Obukhov)

Surface, here applies Schmidt

Surface layer (here applies Dalton-MO)

Atmospheric Boundary layer

Free atmosphere height

1 mm

20 m

1 km

Measure the Myth

Avoid Parrotism

Science is not democratic

A laserscintillometer to measure 1-min fluxes of CO2 and H2O

Dual beam Laser

receivers

Seen from above

Light intensity at transmitter apertures

A B A’ B’

d

R

D

Light intensity at receiver apertures

Turbulent surface layer

A laserscintillometer to measure 1-min fluxes of CO2 and H2O

1-min fluxes of H2O (green) and CO2 (red) And global radiation (black)

overcast

clear sky

Measured response-curve for wheat

Global radiation wm-2

Challenge for young hydrologists: test novel166 Ghz Radio-wave scintillometer combined with an optical scintillometer

Allows measurements of evaporation at scales of kilometers

First prototype

RWS

Optical Large Aperture Scintillometers

Wageningen

Scintec BLS 900

John Dalton(1802) Vertical transfer in surface layer

( ) ( )[ ]ass eTeufE −=

Evaporation is driven by wind and atmospheric ‘demand’

Modern approach: Monin-Obukhov similarity theory (MOST): f(u) depends on stability, and thus on (Tsurface –Tair ) and u2 and roughness lengths for water vapor and sensible heat

Wilhelm Schmidt (1915) evaporation oceans

corATs

TsE ++

=γ

λ)(

)(

A = available energy T

s

dTdeTs

=)(

kgJ /10.5.2 6≈λ

..constpsych=γ

Physical picture: when water supply is not limited Evaporation is driven by thermodynamics, i.e. available energy and temperature

Cabauw, well-watered grass without advection De Bruin, Bosveld, Meirink and Trigo, 2015 (under review)

Estim

ated

Wm

-2

Measured with EC –EB method Wm-2)

Confirmation of Schmidt’s picture: If water supply is not limited available energy and air temperature determine latent heat flux: For very large fields (no advection) Global radiation is main external energy source. Allowing remotely sensed estimates

Schmidt-based estimate of daily latent heat flux using MSG derived global radiation and air temperature β

γλ +

+= A

TsTsE)(

)(

Did not change since 1976

Schematic picture well-mixed atmospheric boundary layer (daytime)

Surface Layer

z

h

q θ

θ∆q∆Atmospheric Boundary Layer

Free atmosfeer, warm en dry

Potential temp. Specific humidity

After sunrise ABL growth and at the top warm, dry air is entrained Into the ABL This explains why relative humidity is never 100% Over well-watered Surfaces This explains why

0>β

IFAPA Lysimeter and RIA site, Cordoba Sc

hmid

t est

imat

e W

m-2

Now besides global radiation horizontally advected sensible heat is additional energy source

IFAPA Lysimeter and RIA site, Cordoba

0 5 10 15 20

-400

-200

020

040

060

080

010

002009-07-30

Time

W/m

2

RnRn_calcHlysHcalcH_IRTH_radLETlysLETcalcLE_PMLET_Hrad

Typical advective day

Revised Schmidt-model with advection

Using MSG global radition and Air temperature only!

0 5 10 15 20

010

2030

4050

2009-07-18

Time

Cel

cius

TaTs_calcTs_IRTTs_rad

Surface and air temperature

Dalton -Monin-Obukhov

[ ]),;,( 002 hmasa

asp zzuTTr

TTcH−

−= ρ

MOSTnet HaRE −=λ

( ) 15.2731 41

−

−−=↓↑

σεε

s

ss

LLT

0 200 400 600 800

020

040

060

080

0

z0m = 0.025 z0h = 0.025

LET lysimeter W/m2

Da

lton-

MO

ST e

stim

ate

Wm

-2

Very sensitive to roughness lengths!

We proudly present a new method to measure actual ET from CNR 4 net radiometer

Step 1

Step 2

Step 3

Constant a = 0.9 at day-time a = 0.5 at night-time