Background error covariance (BEC) is one of the key components in data assimilation systems for numerical weather prediction. Recently, a scheme of using an inhomogeneous and anisotropic BEC estimated from historical forecast error samples has been tested by utilizing the extended alpha control variable approach (BEC-CVA) in the framework of the variational Data Assimilation system for the Weather Research and Forecasting model (WRFDA). In this paper, the BEC-CVA approach is further examined by conducting single observation assimilation experiments and continuous-cycling data assimilation and forecasting experiments covering a 3-week period. Additional benefits of using a blending approach (BEC-BLD), which combines a static, homogeneous BEC and an inhomogeneous and anisotropic BEC, are also assessed.
Single observation experiments indicate that the noise in the increments in BEC-CVA can be somehow reduced by using BEC-BLD, while the inhomogeneous and multivariable correlations from BEC-CVA are still taken into account. The impact of BEC-CVA and BEC-BLD on short-term weather forecasts is compared with the three-dimensional variational data assimilation scheme (3DVar) and also compared with the hybrid ensemble transform Kalman filter and 3DVar (ETKF-3DVar) in WRFDA. The results show that BEC-CVA and BEC-BLD outperform the use of 3DVar. BEC-CVA and BEC-BLD underperform ETKF-3DVar, as expected. However, the computational cost of BEC-CVA and BEC-BLD is considerably less expensive because no ensemble forecasts are required.
On July 13, 2012, a bow echo was observed over the lee side of the Mt. Halla (1,950 m above sea level) on Jeju Island, Korea. Three-dimensional (3D) wind-field and surface observation analyses were carried out to understand the structure and evolution of convective systems with a bow echo on a bell-shaped terrain. A northeastward-moving convective system passed over the approximately bell-shaped isolated mountain with a mean speed of 17 m s−1. On the windward side of the mountain, the convective system developed by the inflow of unstable warm air from the ocean and terrain-induced upward motion, even with a low convective available potential energy value of 511 J kg−1. When passing the lee side of the mountain, a bow echo was formed in the convective system by the strongest winds behind the bow echo. Behind the leading edge of the bow echo, the strengthened rear-inflow jet descended with relatively dry air along the surface, resulting in enhancing evaporative cooling. The precipitation-induced downdrafts generated a cold pool on the lee side of the mountain. The development of an rear-inflow jet and cold pool formation both contributed to the evolution of the bow echo. In addition, the isolated bell-shaped terrain had a major indirect influence on the evolution of the convective system with a bow echo in this event.
There exists a minor, secondary early-morning peak in mei-yu rainfall along the western coast of Taiwan, and this work investigates one such event on June 8, 2012 in southwestern Taiwan under weak synoptic conditions through both observational analysis and numerical modeling, with the main focus on the triggering mechanism of the convection. Observations indicate that the convection developed offshore around midnight near the leading edge of a moderate low-level southwesterly wind surge of 15-20 kts and intensified and moved onshore to produce rainfall. The cold outflow from precipitation also led to new cell development at the backside, and the rain thus lasted for several hours until approximately 07:00 LST.
Numerical simulation using a cloud-resolving model at a grid size of 0.5 km successfully reproduced the event development in close agreement with the observations, once a time delay in the arrival of the southwesterly wind surge in initial/boundary conditions (from global analyses) was corrected. Aided by two sensitivity tests, the model results indicate that the convection breaks out between two advancing boundaries, one from the onshore surge of the prevailing southwesterly wind and the other from the offshore land/mountain breeze, when they move approximately 40 km apart. Additionally, both boundaries are required, as either one alone does not provide sufficient forcing to initiate deep convection in the model. These findings on the initiation of offshore convection in the mei-yu season, notably, are qualitatively similar to some cases in Florida with two approaching sea-breeze fronts (in daytime over land).