Emergence of a Nocturnal Low-Level Jet from a Broad Baroclinic Zone
Emergence of a Nocturnal Low-Level Jet from a Broad Baroclinic Zone
复制标题
夜间低空急流从广阔的斜压区出现
DOI：
10.1175/jas-d-21-0187.1
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发表时间：
2022
影响因子：
3.1
通讯作者：
Parsons, David B.
中科院分区：
文献类型：
--
作者：
Shapiro, Alan;Gebauer, Joshua G.;Parsons, David B.
本文提出了一个在宽斜压区产生类似Blackadar的夜间低空急流的分析模式。该流动由表面浮力梯度从下方（平坦地面）和由恒定压力梯度力从上方（自由大气）推动。指定日变化的混合系数在日出时突然增加，在日落时突然减少。由于注意力仅限于随水平坐标线性变化的表面浮力，Boussinesq近似的运动方程、热能方程和质量守恒方程简化为可以解析求解的一维方程组。对偏南急流的敏感性试验表明：（i）较强的急流与日落时涡动粘度的较大减小有关（2）夜间地面浮力梯度对喷流强度的影响很小;纯斜压强迫（无自由大气地转风），夜间涡动扩散率对急流强度影响不大，但白天涡动扩散率是非常重要的，并且比白天涡动粘性的影响更大。该模型被应用于2020年5月1日在德克萨斯州南部到南达科他州北方的大平原上在晴朗天气条件下发展的喷气式飞机。ECMWF再分析v5（ERA 5）在急流形成前的下午显示，一个广泛的南北向斜压带覆盖了该地区的大部分地区。峰值模型预测的风与来自俄克拉荷马州中北部的大气辐射测量（ARM）南部大平原（SGP）中心设施的ERA 5风和激光雷达数据非常一致。
An analytical model is presented for the generation of a Blackadar-like nocturnal low-level jet in a broad baroclinic zone. The flow is forced from below (flat ground) by a surface buoyancy gradient and from above (free atmosphere) by a constant pressure gradient force. Diurnally varying mixing coefficients are specified to increase abruptly at sunrise and decrease abruptly at sunset. With attention restricted to a surface buoyancy that varies linearly with a horizontal coordinate, the Boussinesq-approximated equations of motion, thermal energy, and mass conservation reduce to a system of one-dimensional equations that can be solved analytically. Sensitivity tests with southerly jets suggest that (i) stronger jets are associated with larger decreases of the eddy viscosity at sunset (as in Blackadar theory); (ii) the nighttime surface buoyancy gradient has little impact on jet strength; and (iii) for pure baroclinic forcing (no free-atmosphere geostrophic wind), the nighttime eddy diffusivity has little impact on jet strength, but the daytime eddy diffusivity is very important and has a larger impact than the daytime eddy viscosity. The model was applied to a jet that developed in fair weather conditions over the Great Plains from southern Texas to northern South Dakota on 1 May 2020. The ECMWF Reanalysis v5 (ERA5) for the afternoon prior to jet formation showed that a broad north–south-oriented baroclinic zone covered much of the region. The peak model-predicted winds were in good agreement with ERA5 winds and lidar data from the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) central facility in north-central Oklahoma.