Improving the representation of convective heat transfer in an urban canopy model

The urban street canyon has been widely recognized as a basic surface unit in urban micrometeorological studies. Urban canopy models (UCMs), which quantify the exchange of energy and momentum between the urban surface and the overlying atmosphere, often adopt this type of street canyon representation as the fundamental surface element. Since UCMs can be coupled to regional-scale weather and climate models such as the Weather Forecast and Research Model (WRF), parametrizations of the surface momentum and scalar fluxes in UCM are of paramount importance. However, many current single-layer UCMs rely on empirical relations that were obtained over 80 years ago and often invoke the exponential wind profile derived from the existing literature for vegetation canopy. In this study, we conducted wallmodeled large-eddy simulations (LES) to study the forced (very weak buoyancy) convective heat transfer over idealized two-dimensional street canyons. It shows that the transfer efficiency computed following commonly applied resistance formulations can be one order of magnitude lower than LES results. The main reasons for the deviation include inaccurate wind speed parameterization and the use of a log-law based formulation for turbulent heat exchange between canyon air and the flow above.

城市街道峡谷已被广泛认为是城市微气象研究中的一个基本地表单元。城市冠层模型（UCMs）用于量化城市地表与上层大气之间的能量和动量交换，通常将这种街道峡谷的表现形式作为基本地表要素。由于UCMs可与区域尺度的天气和气候模型（如天气预报研究模型（WRF））耦合，UCM中地表动量和标量通量的参数化至关重要。然而，许多当前的单层UCM依赖于80多年前获得的经验关系，并且常常引用现有植被冠层文献中得出的指数风廓线。在本研究中，我们进行了壁面建模大涡模拟（LES），以研究理想化二维街道峡谷上的强迫（极弱浮力）对流热传递。结果表明，按照常用的阻力公式计算的传递效率可能比LES结果低一个数量级。产生偏差的主要原因包括风速参数化不准确以及在峡谷空气与上方气流之间的湍流传热中使用了基于对数定律的公式。