Elena Tomasi1*, Lorenzo Giovannini1, Dino Zardi1, Massimiliano de Franceschi1,2
1 Gruppo di Fisica dell’Atmosfera, Dipartimento di Ingegneria Civile, Ambientale e Meccanica, Università degli Studi di Trento, via Mesiano 77, 38123
Trento (TN)
2 CINFAI - Consorzio Interuniversitario Nazionale per la Fisica delle Atmosfere e delle Idrosfere, Roma (RO)
*elena.tomasi@unitn.it
Abstract
High-resolution simulations are performed with the Weather Research and Forecasting (WRF) model, coupled with the Noah_MP land surface model (LSM), to reproduce specific meteorological conditions during the winter season in the Adige Valley. Model results are compared against data collected during a measurement campaign performed in the cropland surrounding the town of Aldeno, within the ALPNAP project, to assess the model ability to reproduce the 2-m temperature and the surface energy fluxes. Validation of model results highlights that WRF underestimates near-surface temperature over snow-covered terrain and fails to capture specific atmospheric processes, such as the development of ground-based thermal inversions. These errors are mainly caused by the wrong initialization of snow cover on the ground and the erroneous characterization of the cropland land use type. Modifications to the model are performed and a better reproduction of both the surface energy fluxes and the near-surface temperature is achieved.
Keywords
Meteorological modeling,WRF, land surface models, land use characterization, snow cover
Parole chiave
Modellazione meteorologica, WRF, schemi fisici al suolo, caratterizzazione uso del suolo, copertura nevosa
Introduction
Most of local meteorological phenomena typically occurring in mountainous areas are driven by energy exchanges between the atmosphere and the ground. Simulations of these meteorological processes are then strictly related to the appropriate modelling of the land surface processes, which becomes even more complex when a melting snow cover lies on the ground. For this reason it is essential to test and keep improving the performance of meteorological models in complex terrain and their ability to properly reproduce near-ground variables, such as 2-m temperature and the fluxes composing the surface energy balance.
Materials and Methods
High-resolution numerical simulations with the Weather Research and Forecasting (WRF, Skamarock et al., 2008) model were performed to reproduce meteorological conditions occurred during an intensive measurement campaign of the European ALPNAP Project (de Franceschi and Zardi, 2009). This campaign was carried out in February 2006 in the cropland surrounding Aldeno, a small village in the Adige Valley, with both conventional instruments and advanced sensors for the measurement of surface energy and mass budgets. The existence of such a detailed and broad dataset of meteorological measurements
allowed the accurate validation of the numerical results, especially as regards the meteorological variables near the ground and the interactions between soil and atmosphere. The length of the performed simulations is 4 days, from the 12th to the 16th of February, 2006: 4 nested domains were used to achieve a 400-m resolution in the inner domain, centred over Aldeno. Numerical results from the simulations with WRF were first compared with the available observations to evaluate the ability of the standard released version to reproduce 2-m temperature: some weaknesses of the model in the representation of the observed atmospheric processes were highlighted and therefore some modifications to the initialization of the model and to the Noah_MP land surface scheme (Niu et al., 2011) were performed. In particular, the attention focused on the treatment of the existing snow on the ground: on the basis of field observations, the initial amount of snow was halved; the initial density of the ground snow was changed from the default fresh-snow value to a denser-snow value, in order to properly reproduce the actual age of the snow cover, which was fallen 15 days before the simulation starting time; the surface temperature of each partially snow-covered cell was allowed to grow over 0°C if its snow cover is less thick than 15 cm. The other modification deals with the characterization of the local cropland land use type, which consists in apple orchards. Once these
modifications to the WRF code were implemented, a new simulation was run and improved results were obtained.
Results and Discussion
The results of two simulations are presented: the simulation run with the standard versions of WRF and Noah_MP LSM (Stnd. ver.) and the simulation run with the modified versions (Mod. ver.). The first panel of Fig. 1 shows the comparison between model results of both the simulations and the observed values of 2-m temperature in the valley floor: while the standard version of the model underestimates the 2-m temperature both during daytime and nighttime, the modified version succeeds in properly identifying the temperature minima and in increasing the maxima. The same result is also achieved in all the other available weather stations within the domain, as shown by the second panel of Fig. 1, where the comparison between 2-m temperature root mean square error (RMSE) before and after the modifications to the code is reported.
Fig.1 – 2-m temperature observed and modeled in the valley floor (top) and the RMSE registered in the other stations within the domain (bottom).
Fig.1 – Temperatura a 2 m misurata e calcolata nel fondovalle (sopra) ed errore quadratico medio calcolato nelle altre stazioni di misura nel dominio (sotto).
Fig. 2 shows the results in terms of outgoing shortwave radiation: it is clear how the implemented modifications reduce the loss of energy from the surface energy budget.
Fig.2 – Outgoing shortwave radiation observed and modeled in the valley floor.
Fig.2 – Radiazione ad onda corta in uscita misurata e calcolata sul fondovalle.
Finally, a comparison between the 2-m temperature measured and calculated at a station on the valley floor (~ 200 m a.s.l.) and at one located on the sidewall at 400 m a.s.l. is presented (Fig. 3): the standard version of the model is unable to reproduce the daily evolution of the observed strong ground-based thermal inversion but, thanks to the proposed modifications, both its developing during nighttime and its decaying during daytime are caught by the model.
Fig.3 – 2-m temperature observed and modeled in a station in the valley floor and in one at 400 m along the sidewalls. Fig.3 – Temperatura a 2 m misurata e calcolata in una stazione del fondovalle e in una a 400 m di quota.
Conclusions
WRF model results are very sensitive to the initialization of the snow cover and to the snow-related variables treated within the model land surface scheme. Moreover, results of the Noah_MP LSM may be improved thanks to the proposed modifications, which take into account the characteristics of the land use and of the snow covering the analyzed domain cell.
References
Skamarock W. C., Klemp J. B., Dudhia J., Gill D. O., Barker D. M., Duda M. G., Huang X.-Y. et al., 2008: A description of the advanced research WRF version 3. Tech. rep., NCAR Technical Note TN-475+STR, 125.
de Franceschi M. and Zardi D., 2009. Study of wintertime high pollution episodes during the Brenner-South ALPNAP measurement campaign. Meteorol. Atmos. Phys. 103: 237- 250.
Niu G.-Y., Yang Z.-L., Mitchell K.E., Chen F., Ek M.B., Barlage M., Kumar A., Manning K., Niyogi D., Rosero E., Tewari M.,Xia Y., 2011. The community Noah land surface model with multiparameterization options (Noah‐MP): 1. Model description and evaluation with local‐scale measurements. J. Geophys. Res. 116 (D12109): 1-19.