1. Matthias Raschendorfer 2007
T_2m diagnostics using a roughness length for a typical SYNOP station A case study
Matthias Raschendorfer
2007
Exponential canopy profiles represent to some extend the city effect.
According to this, temperature within the canopy is rather close to its surface value.
At SYNOP stations this effect is not represented (measurement above a lawn).
Thus SYNOP measurements of 2m temperature systematically have lower daily maxima.
SMA usually corrects this by increasing soil water content.
We test a modified diagnosis without these special canopy profiles, but with logarithmic Prandtl-layer profiles for a SYNOP lawn with an adopted roughness length z0d=0.02m for diagnostics only.

2. T_2M [0C] ANA FCT EXP minus ANA minus ANA city point z0d = 0.02 m
operational run: too high 2m-temperature at noon for points with large roughness length (in particular cities)
city point: ‘plcov<70%’ & ‘z0>0.3m’ & ‘z_surf<=800m’
modified diagnosis reduces 2m temperatures at noon for those points, but the effect seems to be too strong
EXP
minus ANA

3. SMA Daily cycle of PBL profiles of temperature and humidity
Difference of T2m forecast
compared to SYNOP-observations
Daily cycle of T2m, TD2m
Interpolation:
model levels to 2m
Turbulence model
SMA
Modification of Bowen ratio, i.e. relation between sensible and latent heat flux
Soil humidity
other evaporation parameters: stomata resistance, PLCOV, LAI, root depth, (sealing of the soil), soil type, …

4. TD_2M [0C] average over land points
test with reduced evaporation
ana
routi
exp
still too much evaporation
BL is too moist, due to overestimated evaporation.
One reason for this may be the SMA corrections of the period before.
We simulate reduced evaporation by increasing the laminar resistance for water transfer.
The reduction is not yet sufficient.

5. temporal evolution of TD_2M [0C] average over land points
test with reduced evaporation and z0d=0.02m
ana
routi
exp
still too much evaporation or too stable profile at noon
2m dew point temperature too high for day time.
This becomes better with modified diagnostics and reduced evaporation.
But it is still too moist at this level.

6. temporal evolution of T_2M [0C] average over land points
test with reduced evaporation and z0d=0.02 m
ana
routi
exp
more stable profile at night
more unstable profile at noon
more stable profile at night
Since there is still too much evaporation in particular for city points, 2m temperature is too low at noon with the modified diagnostics.
If we had the correct soil moisture content, temperature at noon could be OK.
This will be done by SMA later.
The whole system is in balance, if a soil moisture, tuned to provide proper 2m temperature values at noon, would provide a proper evaporation as well.
During night, 2m temperature does not fall deep enough and is even worse with the modified diagnostics, since 2m temperature is more decoupled from the surface.
We hope that this can be corrected by introducing a stronger dependency on stratification into the transfer scheme.
This leads for stable stratification (for night time) to 2m temperatures being closer to the surface value, and to lower surface values due to reduced transport of heat from BL. Thus 2m temperature at night time should fall.
For labile stratification (for day time) a stronger dependency on stratification leads to a stronger decoupling of 2m temperature and to stronger transport of heat into the BL. Thus 2m temperature during day time should also fall to some extend. This may reduce the pronounced peak of 2m temperature at noon and generate a more flat maximum during the whole afternoon.
more stable profile at night
still too much evaporation
still too much evaporation

7. Summary:
BL is generally too moist and cold, but diagnosed T_2m is too high at noon.
The problem is most visible at grid points with large z0 and unrealistically moist soil.
T_2m is too high during night.
TD_2m is too high throughout the whole day.
T_2m is too close to surface temperature at noon, in particular for rough surfaces.
SMA alleviates this defect by moistening the soil until saturation is reached.
If evaporation is reduced in order to obtain more realistic BL profiles,
T_2m at noon will be overestimated in general, but mainly for large z0.
SYNOP-station profiles pushes T_2m farther away from the surface value for large z0.
To get lower T_2m mainly at night, we need a stronger dependency on stability.
Use an inverse linear function for turbulent velocity scale at stable stratification.
Don’t use turbulent properties above the lowest full model level.
This is work in progress now.

8. GME Verification of T2M: Routine, i. e
GME Verification of T2M: Routine, i.e. z0=z0(grid average) for T2M diagnosis

9. GME Verification of T2M: Experiment, i. e
GME Verification of T2M: Experiment, i.e. z0=z0(SYNOP station) for T2M diagnosis