Boundary Layer Parameterization Sensitivity During Cold Surger on Southern Brazil

The goal of this work is study the differences between two parameterizations boudary layer (BL) in Weather Research and Forecast Model (ARW-WRF) version 3.5, during the cold front incursion on southern Brazil, on July 12-15 of 2013.

The cold surger selected has a classical synoptic system that has a strong termal and moisture gradients, increase in pressure on surface and inversion on meridional wind component as characteristics, the simulation time was between july, 12 (00UTC) and 16 (00UTC) of 2013.

Focused in two cities, Santa Maria (SBSM) and Porto Alegre (SBPA), on southern of Brazil, that have a good observing system, with radiossondes (00 and 12UTC) and automatic surface observations (one/hour). Was analysed the bahavior of simulated and observed boundary layer using two parameterization of BL: Yonsei University (YSU) and; Mellor-Yamada-Janjic (MYJ).

The YSU parameterization include a explicit treatment of entrainment processes at the top of the BL. Also, increases BL mixing in the thermally induced free convection regime and decreases it in the mechanically induced forced convection regime. This, alleviates the problems in the Medium Range Forecast (MRF) BL parameterization. Excessive mixing in the mixed layer in the presence of strong winds is resolved.

In the MYJ parameterization, the Mellor Yamada (1974), hereafter referred to as MY, level 2 and 2.5 BL schemes and the Betts and Miller (1986) were reexamined. A new marine viscous sublayer was proposed, so that the oceans there are two layers. The MY schemes were readjusted. The deep convective regimes are passed to be characterized by a parameter called cloud efficiency.

The others main parameterizations are fixed: Dudhia in shortwave; RRTM longwave radiation scheme; Kain-Fritsch (KF) in cumulus parameterization and; NOAH as soil model.

Each of the two experiments it was runned with two nested grids, the first domain with low resolution (12km in horizontal resolution) and the high resolution grid (3km) centered in southern Brazil, that was the analysed in this work. The simulations have 30 vertical levels with greater density in lower levels. Climate Forecast System Reanalysis version 2, with 0.5º horizontal resolution, 33 vertical levels and updated every 6h obtained from NCEP, are the lateral and initial conditions.

Was observed differences between two simulations, on temporal evolution of Θ and dew point temperature (Td) vertical profiles. Generally, the results show an increase in differences during time evolution. On Θ profiles, SBPA presented the greater differences than SBSM, over all, YSU simulation when compared with BYJ has a tendency to higher values on low levels that are inverse in upper levels. In Td the differences are most strong specially during cold displacement time (approximately 12Z july 14). Comparing the BL behavior simulated with observed is qualitatively agreement, a deeper and moisture layer in prefrontal and a dry, colder and stable layer in postfrontal conditions. In postfrontal time, the differences are most evident, specially on most continental station (SBSM) were on simulated profiles the moisture layer is most shallow than observed.

The results suggest that increase and propagation on differences between parameterization termodinamical conditions (q and Θ), probably in function of the surface processes that is modeled, here by NOAH model.