The role played by planetary boundary layer (PBL) in the development and
evolution of a severe convective storm is studied by means of meso-scale
modeling and surface and upper air observations. The severe convective
precipitation event that occurred on 14 September 1999 in the northeast of
the Iberian Peninsula was simulated by means of the mesoscale model MM5
(version 3) using three different PBL schemes. The numerical results show a
large impact of the PBL schemes on the precipitation fields associated to
the convective storm. The schemes are based on different physical
assumptions: the nonlocal first order Medium-Range Forecast (MRF) and
Blackadar (BLA) scheme and the local, one-and-a-half order ETA scheme.
Surface and radar observations are used to validate the model results. The
comparison focuses on three aspects: the evolution, the spatial distribution
and the 24-h accumulated precipitation. The comparison with rain gauge
observations shows that the MRF, BLA and ETA schemes predicted most of the
precipitation during the morning, whereas the rain gauge stations recorded
rainfall during the evening. The evaluation performed with the radar data
shows that all three numerical simulations produced a realistic spatial
accumulated precipitation distribution. According to the quantity
distribution, all three numerical simulations were able to predict
precipitation quantities comparable to the rain gauge measurements. The MRF
scheme predicted the largest average accumulated precipitation and the
largest average precipitation rate, whereas the ETA scheme predicted the
smallest accumulated precipitation and average precipitation rate. However,
the ETA scheme yielded the highest extreme precipitation rates.
The performance of the three schemes is analyzed in terms of the vertical
distribution of potential temperature, specific humidity and conserved
variables, like equivalent potential temperature and total water content. The
MRF scheme showed more evidence of enhanced mixing than did the other
schemes. Due to this process, more moisture was more efficiently transported
to the free atmosphere. Consequently, the MRF scheme predicts more
widespread precipitation. Furthermore, the enhanced mixing led to a less
sharp capping inversion. However, the stronger inversion resulting from
suppressed mixing processes in the case of the ETA scheme yielded higher
values of convective available potential energy (CAPE) than did the other
two schemes. Consequently, the more extreme precipitation rates are
simulated by MM5 when the ETA scheme is used. |