Modification Tests for the Coefficient of Turbulent Mixing Length Scale in QNSE Scheme in the WRF Model
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摘要: 边界层参数化方案中湍流混合对数值模拟起着重要的作用,湍流混合作用的恰当描述对于温度、湿度、风场以及降水的准确模拟至关重要。我国长江中下游流域人口密集,暴雨灾害频发,很有必要寻找一种适合该地区降水模拟的边界层参数化方案。本文运用WRF(Weather Research and Forecasting)中尺度数值模式,以QNSE(Quasi-Normal Scale Elimination)边界层参数化方案为基础,将其中湍流混合长度尺度系数调整为可变参数。本文将
Noh et al.(2003) 提出的Prandtl公式与Janjić提出的修正湍流长度尺度系数的方法相结合,通过考虑非局地项的强迫、地表稳定度与边界层高度对湍流长度尺度系数的影响,强调大气的动力结构特征与热力结构特征对湍流混合的共同影响,从而提高QNSE边界层参数化方案在不同地理环境下的模拟能力。文章通过进行长江中下游地区的典型暴雨试验,将调整参数后的QNSE方案与原方案进行比较,重点分析调整参数后的方案与原方案对关键基本气象要素场、边界层结构特征以及降水的模拟能力,并将模拟结果与观测结果进行对比,结果表明:调整参数后的方案一定程度上改进了地表温度、边界层结构以及降水的模拟效果。进一步研究表明,调整参数后的方案主要通过改变边界层混合缓解水汽混合比、位温模拟方面的误差。Abstract: Appropriate description of turbulence in PBL (planetary boundary layer) is essential for numerical weather prediction and simulation. Proper treatment of turbulence may have an important influence on the simulation of meteorological fields, such as temperature, moisture, wind speed and precipitation. Heavy rainfall event is frequent in the lower reaches of the Yangtze River and it is necessary to obtain a suitable PBL scheme for this region due to its dense habitability. In this study, based on the QNSE (Quasi-Normal Scale Elimination) planetary boundary layer scheme in the WRF (Weather Research and Forecasting) Model, the coefficients of the TLS (turbulent mixing length scale) in the Mellor-Yamada formulation were modified to be varying based on the application of the Prandtl formula. The MCTLS (modified coefficients of TLS) are dependent on the PBL height and surface stability, and a nonlocal term was imposed on MCTLS, thereby emphasizing the comprehensive impacts of the atmospheric dynamic and thermal structures on turbulent mixing. WRF model simulations using the original PBL scheme and the PBL scheme with the MCTLS were conducted over the lower reaches of the Yangtze River and results were compared to measurements. Improvements in the near-surface temperature, the planetary boundary layer structure and the rainfall simulations have been found. More specifically, the simulations with the MCTLS were shown to alleviate biases in the potential temperature and water vapor mixing ratio by altering turbulent mixing. -
图 3 调整参数化后的方案 (MCTLS试验) 与原方案模拟的 (a, b, c) 地面温度、(d, e, f) 地面温度露点差的统计检验结果:(a, d) 平均偏差;(b, e) 平均绝对偏差;(c, f) 均方根误差随积分时间 (从2009年7月23日12时到7月24日12时) 的变化情况。虚线是控制试验,实线是MCTLS试验,横坐标为积分12~36小时,纵坐标是偏差值, T2和Td分别代表地面温度与地面露点
Figure 3. Statistical tests of (a, b, c) surface temperature and (d, e, f) surface dew point depression: (a, d) Biases; (b, e) absolute biases; (c, f) RMSEs (root-mean-square errors) between mesurements and WRF simulations at national weather stations (NWS) during the 12-36 h forecast period from 1200 UTC 23 July to 1200 UTC 24 July 2009. Dashed lines indicate control run, solid lines indicate MCTLS run. Horizontal ordinate is integration time from 12 to 36 h, T2 and Td denote surface temperature and surface dew point, respectively
图 4 (a, b, c) 区域平均位温与 (d, e, f) 水汽混合比廓线:(a, d)2009年7月24日00时 (协调世界时,下同); (b, e)2009年7月24日06时; (c, f)2009年7月24日12时
Figure 4. Profiles of (a, b, c) potential temperature and (d, e, f) water vapor mixing ratio averaged over the sounding sites: (a, d) At 0000 UTC 24 July 2009; (b, e) at 0600 UTC 24 July 2009; (c, f) at 1200 UTC 24 July 2009
图 5 (a, b) 地面风速的均方根误差 (单位:m s−1) 与 (c, d) 上海嘉定站风廓线 (单位:m s−1)。(a) MCTLS试验与 (b) 原方案模拟的地面风速与实况在积分第24~36小时的均方根误差;(c)2009年7月23日09时;(d)2009年7月23日13时
Figure 5. Comparisons of root-mean-square error (RMSE) of 10-meter wind speed and PBL wind speed between observations and simulations (units: m s−1). (a) RMSE between observations and MCTLS run, (b) RMSE between observations and control run for the 24-36 h forecast period from 0000 UTC 24 July to 1200 UTC 24 July 2009, (c) wind speed profile at Jiading of Shanghai at 0900 UTC 23 July 2009, (d) wind speed profile at Jiading of Shanghai at 1300 UTC 23 July 2009
图 6 2009年7月23日12时至7月24日12时模拟的累计降水与实况 (单位:mm):(a) 实况;(b) 控制试验;(c) MCTLS试验模拟结果
Figure 6. Observed and simulated 24-hour accumulated rainfall in the lower reaches of Yangtze River for the 12-36 h forecast period of the 23 Jul 2009 run initiated at 0000 UTC (units: mm): (a) Observations; (b) control run; (c) MCTLS run
图 7 2009年7月23日12时至24日12时降水区 (28°N~32°N, 114°E~121.5°E) 区域平均降水 (单位:mm) 的时间序列:(a) 区域平均逐3小时降水量;(b) 区域平均逐小时降水量
Figure 7. Time series of simulated and observed domain-average rainfall (units: mm) over the strong rainfall region (28°N-32°N, 114°E-121.5°E) from 1200 UTC 23 Jul to 1200 UTC 24 Jul 2009: (a) Time series of 3-hour accumulated rainfall; (b) time series of hourly rainfall
表 1 模拟试验方案设计
Table 1. Design summary of different numerical experiments
试验 边界层参数化方案 地面层参数化方案 MCTLS试验 湍流长度尺度系数调整后的QNSE方案 QNSE方案 控制试验 QNSE方案 QNSE方案 -
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