Problems of HIRLAM in wintertime stable boundary layer

- analysis of forecasts and observations

Timo Vihma, Evgeni Atlaskin, and Laura Rontu

Model validation against Sodankylä sounding data

Periods: January and March 2005

Model versions: H635E (Gollvik-Rodriques soil-snow-forest schema), H637 and H640

Model product validated: 24 h forecasts

In 2005,

January was very mild in Sodankylä

but March was colder than usually

Results for January 2005: focus on the errors in the air temperature

- At the heigths of 30 and 90 m, HIRLAM has a large positive bias in cold conditions.

- In warm conditions, the bias is often negative but much smaller in magnitude

- H635N with the snow schema does not produce better results

- The largest errors in the lowermost 100 m occur under conditions of a strong inversion, as estimated from the temperature difference between 30 and 1100 m

- The temperature error at the heights of 30 and 90 m depends much more on T(170-30m) than on T(1100-170m), i.e., large errors are related to near-surface-based inversions, but not so much to clearly elevated inversions.

The largest temperature errors at the height of 30 are not associated with saturation (neither observed nor modelled).

- Near the surface, HIRLAM has a positive bias in cold conditions and a negative bias in warm conditions. This bias in q is qualitatively similar to that in T, but now magnitudes of the positive and negative bias are approximately equal.

Errors in the air specific humidity

Errors in q occur also without any temperature inversion, but in conditions of a large T(170-30m), the bias in q(30m) is always positive

The largest errors in q(30m) typically occur when the observed RH(30m) = 0.85-0.95.

H335N yields much lower values of RH than H640

March 2005

Everything told before holds also for March 2005, except:

- the error in specific humidity at the heights of 30 and 90 m depends neither on T nor on RH

- considering the spoecific humidity, H635N performs better than H637

Conclusions

- largest temperature errors occur in cold conditions with a large T(170-30m)

- in conditions of a large T(170-30m), the bias in q(30m) is always large

- the largest errors in temperature and humidity were typically not related to saturation

- the presence of solar radiation does not have a large effect on the temperature error in HIRLAM

- Although January 2005 was mild and March 2005 was cold, the differences in the model performance with respect to the air temperature were small

- H635N with the new snow-forest scheme does not show improvement, but this may also be related to problems in snow analysis and the digital filter initialization

- much more analyses are needed: next application of the tower data, then analysis on the relative importance of factors controlling T2m (a) in reality and (b) in HIRLAM.