1 Indirect evidence of vertical humidity transport during very stable conditions at Cabauw Stephan de Roode 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 selectioncriterionheightdata fraction I.nighttime1m0.50 II.clear sky1m0.18 III.weak wind10m0.08 IV.stable5m0.07 V.surface cooling2m0.05

3 Monthly mean surface energy balance during stable conditions Month N blocks 10 minutes H (Wm -2 ) LE (Wm -2 ) -G 0 (Wm -2 ) LW net (Wm -2 ) Res (Wm -2 ) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

4 Monthly mean surface energy balance during stable conditions Month N blocks 10 minutes H (Wm -2 ) LE (Wm -2 ) -G 0 (Wm -2 ) LW net (Wm -2 ) Res (Wm -2 ) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

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 q sat,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 Floor Courtesy 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) Monthsfc2m10m20m40m80m140m200mLE (Wm -2 ) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec corresponding flux