Terradellas, E; Bergot, TA comparison between two

single-column models designed for short-term fog and low clouds forecasting

Terradellas, E. and T. Bergot

6th. Annual Meeting of the E.M.S., Ljubljana 4-8 Sep. 2006

Introduction

Once analysed the results of the previously presented intercomparison, the Spanish INM and Météo-France agreed to undertake a deeper comparison between their respective SCM (H1D and COBEL-ISBA) in the field of fog and low-clouds forecast.The model outputs would be compared during a full winter season for three sites with different climate and geographic conditions: Paris-CdG, Warsaw-Okecie and Casablanca-Nouasseur airports.

The sites

Paris-Charles de Gaulle airport

Paris-Charles de Gaulle airport is located over relative flat terrain.

Test period

16 JAN. – 14 FEB. 2005

H1D runs: 0000, 0600, 1200, 1800Runs start 3 h 30 m after nominal runtime. 24-hour forecast

COBEL-ISBA runs: 0000, 0300, 0600, 0900, 1200, 1500, 1800, 2100. 12-hour forecast

Full season comparison

1 OCT. 2005 – 28 FEB. 2006

H1D runs: 0000, 0600, 1200, 1800Runs start 3 h 30 m after nominal runtime. 24-hour forecast

COBEL-ISBA runs: 0000, 0300, 0600, 0900, 1200, 1500, 1800, 2100. 6/8-hour forecast

Initialisation

Dedicated obs. system: 30-m tower: T, RH Soil T and humidity SW and LW radiation

Initialisation

The problem of fog forecasting

H1D/COBEL. Night BL temperature

Systematic difference in the cooling rate: nocturnal cooling is greater in H1D. Soil? Clouds?Lower part of the column is more stable in HIRLAM

H1D/COBEL. Daytime temperature

Daytime behaviour is “normal”:Bias is small and stable with timeRmse increases with time

H1D/COBEL. Night BL sp. humidity

“Normal” behaviour for the specific humidityH1D is slightly moister than COBEL

H1D/COBEL. Night BL IPW

H1D is slightly moister than COBEL. The difference comes from the initialisation.

H1D/COBEL. Night BL liquid water

Above 200 m, H1D presents less liquid water than COBEL higher cooling rate more liquid water at low levels (fog)

H1D/COBEL. Night BL wind

Wind: normal behaviour.H1D is less windy. Possible contribution to a more stable low-level column.

H1D/COBEL. Night longwave radiation

(downward positive)H1D: less liquid water more loss of longwave radiation

Intramodel diagnostics

Figures from the test periodH1D is more “consistent”.This fact has positive and negative implications...

T q

LW ws

Net radiation at ground

Figure on the right is from the test periodH1D overestimates both, the downward SW radiation and the upward LW radiation, probably because it underestimates cloudiness

Low C&V conditions

Low C&V conditions for LFPG:Visibility < 600 m orCeiling < 200 ft

7.2% of observations during the analysed period(0.4% only low visibility reported, 2.9% only low clouds reported and 3.9% both)That is 240 hours

Model skill

LOW C&V3-4 h FCST

CI H1D

POD 56 73

FAR 38 57

Similar performance for 3-4 h forecast.H1D: higher POD and FAR. Because its higher cooling rate?Before 3 h: COBEL performs better. Because its better initialisation?After 4 h: H1D performs better. Because its better treatment of horizontal unhomogeneity?

H1D: HH+06 / HH+07, that is, 3-4 h after ending the assimilation cycle.

ConclusionsSCM, in particular COBEL-ISBA and H1D, are useful tools for short-term C&V forecast.

The initialisation is very important. Future development of H1D should, probably, focus on it.

COBEL development should, probably, focus on its treatment of horizontal unhomogeneity.

Model intercomparison experiments are excellent tools to identify the weakest part of a model, to find out which aspect is worth to work on.