1

Indirect evidence of vertical humidity transport during very stable conditions at Cabauw

Stephan de Roode (S.R.deRoode@tudelft.nl)Clouds, Climate and Air Quality Department of Multi-Scale Physics (TNW)

Stephan R. de Roode (TU Delft) &

Fred C. Bosveld (KNMI)

2

Stable boundary layers -Analysis of Cabauw data for the period 2001-2006

selection criterion height data fraction

I. nighttime 1m 0.50

II. clear sky 1m 0.18

III. weak wind 10m 0.08

IV. stable 5m 0.07

V. surface cooling 2m 0.05

€

SWnet <1 Wm-2

€

LWnet > 40 Wm-2

€

U tot < 3 ms -1

€

RiB > 0

€

dT

dt< 0

3

Monthly mean surface energy balance during stable conditions

MonthNblocks

10 minutes

H(Wm-2)

LE(Wm-2)

-G0

(Wm-2)LWnet

(Wm-2)Res

(Wm-2)

Jan 797 -4.9 -0.7 12.0 52.2 34.6

Feb 770 -7.7 0.0 13.0 55.6 34.9

Mar 1240 -5.9 0.8 12.7 52.2 34.4

Apr 1393 -6.1 1.6 14.1 49.8 31.2

May 1150 -6.5 0.4 14.5 48.5 27.9

Jun 1154 -8.4 1.8 15.9 48.3 25.9

Jul 996 -11.0 0.9 14.3 46.6 22.2

Aug 1297 -7.8 1.3 12.5 47.0 28.1

Sep 1755 -7.7 -0.4 14.8 47.7 24.7

Oct 1047 -8.2 -0.5 11.0 48.3 28.5

Nov 945 -4.7 -2.6 12.5 49.9 30.1

Dec 1222 -4.7 -1.2 13.4 52.5 33.3

4

Monthly mean surface energy balance during stable conditions

MonthNblocks

10 minutes

H(Wm-2)

LE(Wm-2)

-G0

(Wm-2)LWnet

(Wm-2)Res

(Wm-2)

Jan 797 -4.9 -0.7 12.0 52.2 34.6

Feb 770 -7.7 0.0 13.0 55.6 34.9

Mar 1240 -5.9 0.8 12.7 52.2 34.4

Apr 1393 -6.1 1.6 14.1 49.8 31.2

May 1150 -6.5 0.4 14.5 48.5 27.9

Jun 1154 -8.4 1.8 15.9 48.3 25.9

Jul 996 -11.0 0.9 14.3 46.6 22.2

Aug 1297 -7.8 1.3 12.5 47.0 28.1

Sep 1755 -7.7 -0.4 14.8 47.7 24.7

Oct 1047 -8.2 -0.5 11.0 48.3 28.5

Nov 945 -4.7 -2.6 12.5 49.9 30.1

Dec 1222 -4.7 -1.2 13.4 52.5 33.3

5

Dew formation downward vertical humidity transport

Problems

- eddy correlation humidity flux at 5 m is negligibly small

- large residual in surface energy balance

Relevance

- Testing schemes (TKE, EDMF, etc) against observations

- Site used for GABLS3 case

- Humidity important for fog formation

Question

- Are the measured latent heat fluxes in accord with observed humidity tendencies from

Cabauw tower?

6

Example: humidity tendencies from Cabauw tower

qsat,sfc

2m

10m

80 m

7

Mean humidity tendencies (all selected SBLs)

Significant mean tendencies near the surface

8

Mean humidity tendencies at 2m as a function of wind direction

Negative humidity tendency for any prevailing mean wind direction

9

Tendencies of the mean humidity at 2m and the surface saturation specific humidity

Surface cooling causes downgradient humidity flux

10

Mean humidity tendencies as a function of the bulk Richardson number

Tendencies even for very stable conditions

11

Ditches are another local source of moisture

Courtesy Kees FloorCourtesy Adriaan Schuitmaker

12

Dispersion of a passive scalar in a stable boundary layer at Cabauw

Field experiment (measuring vertical velocities with laser technique) by Petra Kroon, Harm Jonker, Adriaan Schuitmaker and others (TU Delft)

13

Sources dew formation

1. Vertical turbulent transport from atmospheric column

2. Condensation of moisture evaporated from ditches

14

Dew research in Wageningen (Jacobs et al.)

Wageningen dew fall corresponds to a mean latent heat flux of about 10 Wm-2

15

Conclusions

1. Surface energy imbalance partly due to "missing" latent heat flux

- instrument wetting

- measurement height 5 m

- small turbulent fluctuations

2. Dew fall from

- atmospheric column

- evaporated moisture from numerous ditches

3. We expect similar dew fall as in Wageningen (about 50 km from

Cabauw)

- latent heat flux LE≈-10Wm-2 (Jacobs et al. 2006)

16

Monthly mean humidity tendencies (g/kg/hr)

Month sfc 2m 10m 20m 40m 80m 140m 200m LE(Wm-

2)

Jan -0.23 -0.19 -0.07 -0.03 -0.02 -0.00 -0.01 0.00 -2.6

Feb -0.23 -0.15 -0.05 -0.04 -0.03 -0.03 -0.02 -0.04 -5.3

Mar -0.28 -0.18 -0.06 -0.04 -0.02 -0.02 -0.02 -0.02 -4.3

Apr -0.48 -0.20 -0.05 -0.02 -0.01 -0.01 -0.02 -0.02 -3.9

May -0.68 -0.22 -0.04 0.00 0.01 0.01 0.00 -0.04 -1.9

Jun -0.69 -0.17 -0.01 0.02 0.04 0.03 0.01 -0.02 1.2

Jul -0.85 -0.25 -0.08 -0.04 -0.01 0.02 -0.01 -0.06 -4.0

Aug -0.76 -0.31 -0.12 -0.09 -0.04 0.00 -0.05 -0.06 -8.3

Sep -0.71 -0.33 -0.09 -0.06 -0.03 -0.04 -0.06 -0.05 -9.4

Oct -0.64 -0.36 -0.15 -0.11 -0.08 -0.07 -0.06 -0.05 -12.5

Nov -0.49 -0.33 -0.13 -0.08 -0.05 -0.05 -0.06 -0.06 -10.5

Dec -0.27 -0.25 -0.09 -0.04 -0.03 -0.02 -0.03 -0.01 -5.4

correspondingflux