Coastal lows along the subtropical west coast of South America: Numerical simulation of a typical case
Coastal lows along the subtropical west coast of South America: Numerical simulation of a typical case
复制标题
DOI：
10.1175/1520-0493(2003)131
复制
发表时间：
2003-05-01
影响因子：
3.2
通讯作者：
Rutllant, J
中科院分区：
文献类型：
--
作者：
Garreaud, RD;Rutllant, J
在南半球的冬季，沿副热带南美洲西海岸沿着经常观测到次天气系统的暖心低压区。这些所谓的沿海低气压（CL）往往是随着高空中纬度脊接近副热带安第斯山脉而发展起来的，因此，当高空和更远的南方气压增加时。这些CL对沿海天气产生深远影响，导致晴朗的天空和比平均水平更强的向赤道低空气流迅速转变为多云条件和缓和的向赤道（甚至向极地）气流。内陆的天气条件主要反映了沉降反转基底强度和高度的相关变化。使用数值天气预报模式[第五代宾夕法尼亚州立大学-NCAR中尺度模式（MM 5）]对典型CL事件进行中尺度模拟，与观测结果比较表明，模式模拟正确地捕捉了大的-尺度模式以及许多中尺度特征的CL。模型的结果被用来诊断CL。结果表明，对流层低层显著的绝热增暖（包括逆温的显著加强）是造成沿海槽形的主要原因。低层东风气流受到副热带安第斯山脉西坡的限制，使得高空脊轴前方的大范围沉降加强。副热带沿海的低空风场接近地转平衡，并由1-2天前位于对流层中层太平洋上空的气包提供。当沿岸气压梯度由于海外脊的作用而变为极向时，就形成了东风流。随着CL在副热带纬度的发展，这种梯度进一步增强。脊轴一穿过，低层东风气流就消失了，一片浅而窄的西北风和层积云从智利北方向极地传播。此后不久，被困的风逆转与传入的天气尺度对流层深气旋环流合并。
Subsynoptic, warm core low pressure areas are frequently observed along the west coast of subtropical South America during austral winter. These so-called coastal lows (CLs) tend to develop as an upper-air, midlatitude ridge is approaching the subtropical Andes and, therefore, while pressure is increasing aloft and farther to the south. These CLs have a profound impact in the coastal weather associated with a rapid transition from clear skies and stronger than average equatorward low-level flow to overcast conditions and relaxed equatorward (or even poleward) flow. Weather conditions inland mostly reflect the associated changes in the strength and height of the base of the subsidence inversion.In this work, a mesoscale simulation of a typical CL episode is performed using a numerical weather prediction model [the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5)] Comparison with observations reveals that the model simulation properly captures the large-scale pattern as well as many of the mesoscale features that characterize the CL. The model results were then used to diagnose the CL. It is found that the coastal troughing is largely due to the marked adiabatic warming of the lower troposphere (including a significant strengthening of the temperature inversion). The large-scale subsidence ahead of the incoming upper-air ridge axis is enhanced as the low-level easterly flow is constrained by the western slope of the subtropical Andes. The low-level wind off the subtropical coast is close to geostrophic balance and it is fed by air parcels that 1-2 days before had been located in the middle troposphere over the Pacific Ocean. The easterly flow is set up as the alongshore pressure gradient becomes poleward oriented because of the extratropical ridging. This gradient is further enhanced as the CL develops at subtropical latitudes. As soon as the ridge axis crosses, the low-level easterly flow vanishes and a shallow, narrow tongue of northwesterlies and stratocumulus clouds propagates poleward from northern Chile. Shortly thereafter, the trapped wind reversal merges with the incoming synoptic-scale, tropospheric deep-cyclonic circulation.