Introduction HIRLAM 7.2

Transcription

1 Introduction HIRLAM 7.2 Differences between the current operational HIRLAM version 7.0 and the next operational version 7.2 are described in this document. I will try to describe the effect of this change on the output of the model, so these changes are known and can be taken into account in the operational applications. In this document I will also point to two web sites that contain more information on the change from HIRLAM version 7.0 to 7.2. What remains the same? The upgrade to HIRLAM version 7.2 is not going to be accompanied by changes to the model grid on which the products are available for the KNMI customers. Due to time constraints at our high performance computer and the increased cost of HIRLAM 7.2 in comparison with HIRLAM 7.0 we have to decrease the total calculation area with 50 points along all the edges. But that still means that the HIRLAM products that are delivered by KNMI fall well inside this calculation domain. Therefore, for end users of the HIRLAM model output, nothing changes technically, except for the model version number which is increased by 1 to 8 for the large domain and 108 for the smaller domain. What does change? The results of 2 years HIRLAM research are included in HIRLAM 7.2. This version of HIRLAM was officially released in the Autumn of 2008 and KNMI started running this version already during summer of Therefore we already have quite some experience with the output from this model version and are able to share this information with the users of the HIRLAM output. The most important differences between HIRLAM 7.2 and HIRLAM 7.0 are: - Change in convection and condensation scheme from STRACO to Kain-Fritsch Rasch-Kristjansson - Improved mixing in of ECMWF analysis - Improved boundary condition at the model top that prevents crashes in Winter - Different tuning of the vertical diffusion scheme and the removal of the turning of the surface stress - 6-hour assimilation cycle instead of 3-hour cycle - Increased use of satellite data in analysis - Extended and improved postprocessing Impact of changes for the behaviour of the model With a few examples I will try to make clear the impact of the changes. Most changes are not easily seen in the model output. The largest impact is coming from the change in convection and condensation scheme. This changes the model behaviour 1

2 for precipitation as well as for the dynamics of the model, especially in cases with a combination of strong winds and convection (e.g. in a northwesterly flow of cold air over a warm sea). Impact of Kain-Fritsch Rasch-Kristjansson In 2007, after a large intercomparison exercise, it was decided to Kain-Fritsch Rasch-Kristjansson the reference HIRLAM convection and condensation schemes. In this intercomparison two months were used in every season and the final decision was made based on a objective composite score. Subjectively it was already clear that there are very large differences between STRACO and KF-RK. One of the main differences between KF-RK and STRACO is that STRACO allows the model to generate convection cells on the model grid scale, cells that have their impact on the pressure and wind pattern. Especially in situations with unstable air masses and strong winds this is found. Figure 1 shows the vertical motion (omega, hpa/hour) in HIRLAM 7.0 (left) and HIRLAM 7.0 with KF-RK (right). This figure show a cross section from 20 W to 4 E along 60 N. At around 5 W a cold front is positioned. Cold air is being advected and the cold layer increases in depth going towards the West and in this cold air open cell convection is found, as can be seen in satellite images. In the STRACO run cells with relatively strong upward and downward motion can be found in the area between 18 W and 8 W. Figure 1: Cross section of omega (hpa/hour) in HIRLAM 7.0 with STRACO (left) and with KF-RK (right) from 20 W to 4 E along 60 N based on the run of 12 UTC on 8 February 2008 valid 24 hours later. The fact that strong vertical motions are found in an area where you would like the convection scheme to do all the vertical exchanges is not so much of a problem, when it would not have any consequences for the pressure and wind. In HIRLAM with STRACO, however, small low pressure areas that have a clear impact on the wind can be found, with sometimes strong local increases in the wind speed. The wind in these areas often is stronger than observed and especially in extreme conditions, close to weather alert criteria, this leads to a overforecasting of extreme events. 2

3 Figure 2 shows the impact of these convective cells on the wind speed, in this case the wind gusts based on the average wind speed and Turbulent Kinetic Energy. On 29 February 2008 a low pressure system passed north of the Netherlands and behind this system a strong northwesterly flow developed. The average wind speed reached values of 21 m/s. In the run with STRACO wind speeds up to 25 m/s were found and wind gusts up to 34 m/s were forecasted (figure 2 left). In the KF-RK run the wind speed was much weaker, around 20 m/s, resulting in wind gusts up to 28 m/s (figure 2 right). The observed maximum gust was 29 m/s in the North of the Netherlands in this case. Farther to the South the wind gusts were stronger, especially in the southwestern part of the Netherlands, but these wind gusts occurred on a line with strong convection. For these type of gusts the TKE method is not applicable. Figure 2: Wind gust forecast (m/s) based on TKE valid for 29 Februari UTC based on HIRLAM 7.0 with STRACO (left) and KF-RK (right). Note the different scales in the plots. Figure 3 is an example of the differences between HIRLAM 7.0 and HIRLAM 7.2, whereas the previous figures were both based on HIRLAM 7.0. In this figure the difference between HIRLAM 7.0 (left) and HIRLAM 7.2 (right) is also clear. In the area west of France the HIRLAM 7.0 wind field looks much more irregular than the 7.2 version. Where HIRLAM 7.2 has large areas with e.g. wind force 7 or 8 (lavender and purple colors), is the wind in HIRLAM 7.0 changing from wind force 6 to 9 in small spots. The differences in the behaviour of the wind and surface pressure are caused by the different convection schemes that are used in both runs. The STRACO scheeme 3

4 HiRLAM version H7.2 / 15 October 2009 (Smooth TRansition COnvection) somehow is not able to remove the instability in a proper way and allows the model to produce its own resolved convection. This gives rise to the development of long lived convective cells that develop into small scale lows that have their own wind fields. Figure 3: +24 forecast of PMSL (hpa) and wind speed (m/s) from HIRLAM 7.0 (left) and HIRLAM 7.2 (right) alid on 4 March 2009, 00 UTC. The difference in convective character between HIRLAM 7.0 and 7.2 is visible also in the precipitation fields and in the division between the convective and stratiform part of the precipitation. With STRACO both precipitation forms are almost always visible in the time series of precipitation. KF-RK makes a much clearer distinction between the two. In the unstable air behind a cold front KF-RK will mainly give convective precipitation while STRACO often also gives stratiform precipitation there. 4

5 Figure 4: +24 forecast of the 1-hour accumulated precipitation in HIRLAM 7.0 (left) and 7.2 (right) valid on 2 December 2008 at 00 UTC. Another difference is the horizontal distribution of the precipitation. Figure 4 shows that HIRLAM 7.0 has many small scale convective cells west of France and Ireland, whereas HIRLAM 7.2 has convective precipitation organized more in bands. Another difference is the behaviour of the model when the surface forcing of the convection is taken away, e.g. when air flows from warm water to a cold land surface. HIRLAM 7.0 has the tendency to let this convection get inland too far. HIRLAM 7.2 more often confines the precipitation to the coastal areas. Figure 5: +7 forecast of the one hour precipitation accumulation in HIRLAM 7.0 (left) and HIRLAM 7.2 (right) valid on 20 August 2009, 7 UTC. Figure 5 shows the precipitation forecast on the very warm 20 August 2009, when there were some small showers developing in the morning due to mid level convection with one stronger thunderstorm that developed over Limburg and moved northeast over Germany. HIRLAM 7.0 did not give this precipitation at all while HIRLAM 7.2 gave an indication of light rain with one stronger shower embedded (in the wrong place). HIRLAM 7.2 therefore gave a much better warning for the light rain than HIRLAM 7.0. The forecast for the afternoon was quite similar in both runs, they both overforecasted the intensity of the thunderstorms on the convergence zone, where showers only developed in the North of the Netherlands and the strong gusts that were forecasted did not materialize. Another problem of HIRLAM 7.0, that is improved in HIRLAM 7.2, is the sometimes significant precipitation from showers that are too shallow and warm (cloud top 5

6 HiRLAM version H7.2 / 15 October 2009 close to or above 0 C) to give more that a few drops of precipitation. Figure 6 shows such a situation. Figure 6: +12 forecast of the 1 hour precipitation accumulation in HIRLAM 7.0 (left) and 7.2 (right) valid on 21 August 2009, 12 UTC. Figuur 7: +6 (links) en +12 (rechts) verwachting van CAPE voor HIRLAM 7.0 (links) en HIRLAM 7.2 (rechts) geldig voor 12 UTC op 24 augustus

7 Other improvements in HIRLAM 7.2 compared to HIRLAM 7.0 are to be found in the postprocessing. One example is the calculation of CAPE. In HIRLAM 7.0 there is a problem with the stop criterium of these calculations. In areas with a thicker CINlayer (more than 2 layers thick), no CAPE is calculated. In HIRLAM 7.2 this is improved, causing the different air masses and frontal zones to be much more coherent than in the old HIRLAM version. HIRLAM 7.2 also gives the opportunity to get extra postprocessing from the model like the minimum and maximum temperature and wind speed over certain time intervals, plus the instantaneous and maximum mechanical wind gust, based on TKE (so without the convective component that can give the gusts extra strength) What is worse in HIRLAM 7.2 compared to HIRLAM 7.0? So far I have only mentioned the good things about HIRLAM 7.2, but as with any large change there are also aspects that become worse with the introduction of HIRLAM 7.2. One of the things that become worse with the new version of HIRLAM is the precipitation, especially in summer. There are more occasions with small precipitation amounts in HIRLAM 7.2 than in HIRLAM 7.0. In winter the opposite is the case, but overall there is an increase in small precipitation amounts. Another difference is the inland penetration of convection in case of unstable air traveling over a warm sea and reaching a cold land surface. In HIRLAM 7.0 this precipitation tended to get inland too far. In HIRLAM 7.2 the opposite may be the case with the precipitation getting inland not far enough. However, it still is the case that HIRLAM 7.2 is quite quick with convection developing, especially in comparison with other models. Therefore it still can be used for warnings on the possibility of convective activity. Verification On the webpage there is quite some material on the verification of HIRLAM 7.2 (CIS in these plots) against HIRLAM 7.0. Here near surface scores for Europe (EWGLAM stations and the Netherlands) and scores against profiles (all radiosoundings in the domain), precipitation contingency tables and frequency plots can be found. This information shall be extended in the future with months that are complete then. The verification for the Netherlands shows that the pressure in this HIRLAM version is in general a little bit better than in HIRLAM 7.0. This also is true for the temperature, except in the months of April and May (see figure 8), that are worse as they are too warm and too dry. It looks like there is too little evaporation, something that may be improved by changes in the vegetation characteristics. 7

8 Figure 8: Verification of the temperature as function of the forecast length for HIRLAM 7.0 (red) and HIRLAM 7.2 (blue) for the Netherlands and direct surroundings for May The dewpoint is better in HIRLAM 7.2 than in HIRLAM 7.0 so far for all the months that we have available in the verification. The wind speed is a little bit better in HIRLAM 7.2 due to a smaller bias but the wind direction bias is increased a little bit compared to HIRLAM 7.0. Looking at the verification over all EWGLAM stations the PMSL has improved a little bit, but especially the 2m temperature and the 2m dewpoint temperature are much better in spring. In some months the bias has been reduced by almost 1 C. For the upper air (visible in the profiles) the standard deviation has been reduced in general. The contingency tables of the precipitation show that HIRLAM 7.2 has more dry forecasts in winter, while the opposite is true in summer. Still, both models are much too wet in winter and in summer. Especially in summer there are more cases with the correct amount of precipitation in HIRLAM 7.2, there are larger numbers on the diagonals in the contingency tables for precipitation amounts larger than 2 mm per 12 hours for HIRLAM 7.2 (CIS). 8

9 Figure 9: Verification of the temperature as function of the forecast length for HIRLAM 7.0 (red) and HIRLAM 7.2 (blue) for the EWGLAM stations for May Webadresses with information about HIRLAM 7.2: Comparison of todays runs (as long as both model versions are running): Verification HIRLAM 7.2: Older verification (from last year with disfunctional archiving system, therefore only verification of first hours): 9