Large-eddy Simulation Of The Flow Patterns In The Convective Boundary Layer Over Heterogeneous Surfaces And The Impacts Of Urban On Sea Breeze Circulations

Using large-eddy simulation, investigations are conducted on three issues including the effects of small-scale heterogeneous surfaces, the developing characteristics and the initiating mechanism of sea breeze circulations (SBC) and the impacts of urban on SBC respectively. Main contents and conclusions are as follows:(1) By means of Dutch Atmospheric Large-Eddy Simulation, we investigate the impact of the idealized one-and two-dimensional heterogeneous surface heating on the turbulence characteristics in the convective boundary layer (CBL) developing in the background of light wind. The wavelength λ of surface heterogeneity is 4.8 km. For the one-dimensional heterogeneous cases, the roll-like circulations are triggered, and higher boundary-layer height zi is observed when the wind blows along the stripe-like patches. Significant changes occur correspondingly to the profiles of turbulent velocity variances, not only in shape but also in magnitude, such as a reduced u-variance near the surface, two-peak profile of v-variance, and the height of the maximum w-variance reaching up to 0.6-0.7z;. When the wind blows across the stripe-like patches, the roll-like circulations can also be found, but the rolls’structure is barely visible. Its turbulent variances are similar to those in the homogeneous case.The results of the two-dimensional mosaic heterogeneity are analogous to those in the one-dimensional cases respectively. The across-side wind results in a well mixed CBL similar to that over the homogeneous surface. However, when the wind is along the diagonal of the squares, the turbulent characteristics become quite like those in the one-dimensional case with an along-stripe wind direction, and the roll-like circulations can be actively triggered. The wind direction between the above two cases can still trigger the roll-like circulations, which make the CBL height higher than in the homogeneous case. The sensitivity experiments of the secondary circulations in the latter 2-D case are made by varying the wavelength and wind speed. The results reveal that the persistence of the roll-like circulations mainly depends on λ/zi. On the other hand, the circulations are strengthened by larger wind speed as long as the surface heterogeneity wavelength is large enough.(2) WRF-LES is used to model the development of idealized SBC under the condition of diurnal surface heating, and scaling analysis is applied to study their developing characteristics. Multiple cases are carried out by varying the surface heat flux amplitude (HA) and the atmospheric stability (N). The results show that the development of SBC accelerates as HA goes up; the sea breeze depth (Zsb) and the CBL height (h) deepens synchronously; the atmospheric stability determines the vertical expanse of SBC, but hardly affects the onshore distance (l) of the sea breeze front (SBF); the sea breeze strength (Usb) is directly proportional to the speed of SBF; the maximum velocity within the rising current over SBF (wmax) shares the same characteristic scale with Usb in the developing period; the maximum onshore current (Umax) can be characterized by the CBL characteristic velocity w* in the developing period. On the other hand, it is proved that the characteristic scales proposed by Steyn (1998) concerning Zsb and l are inappropriate.The scaling laws proposed by Steyn (1998) are examined. The results show that they fail for the entire development of SBC, but hold only if under the condition of cot-1 roughly. This thesis suggests that the validation can be retrieved by replacing co in the scaling relations with 1/t, since co is unable to represent a time-varying process.The initiating moment of SBC is defined as the time when the position of Umax starts to shift inland from the coastline. Couples of critical phenomena are spotted at this moment such as the formation of a convergence zone at the front of the circulation which results in an increase of the potential temperature and a decrease of pressure at the surface locally, and h is found to be just as high as, or higher than, Zsb. Based on the critical phenomena, this thesis conceptually describes the initiating mechanism as:when h< Zsb, since a certain perturbation around the height of h somewhere in the front of the onshore flow is totally suppressed due to the background horizontal motion, the circulation can develop without changing the current state and maintain its position. However, the same perturbation can be amplified by the rising current of the circulation once h≥ Zsb, and would elicit a velocity increase in the front of the onshore flow, leading to the inland shift of the position of Umax.The convergence zone at the front forms as soon as SBC initiates, and thus contributes to the further development of SBC.(3) Following the same model set-up as in the former chapter, one idealized urban surface is induced onto the land side of the model surface domain in order to investigate the impacts of urban on SBC. It is revealed that the urban at, or at a distance from, the coastline can dramatically influence the development of SBC.In the urban at the coastline:the rough surface can reduce the sea breeze strength (Usb) and enhance the rising current over SBF, while the effect is rather weak and negligible; on the other hand, it can also delay the initiation of SBC. Surface heating first accelerates the development of SBC; Later then the heat island circulation (HIC) couples with SBC as the latter is about to cover the urban area, slowing down the SBF inland penetration and enhancing the rising current in between the two circulations. Higher surface heating results in an earlier initiation of SBC and makes SBC and HIC both develop quickly, leading to an earlier and much stronger coupling. Extending the urban width prolongs the time of individual development for both SBC and HIC before they interact with each other, and also leads to a stronger coupling.The situation of an inland urban is complicated since two of its HIC appear in the way of SBC inland penetration. The time variation of the metrics of SBC indicates that the SBC appears almost unaffected by the urban when approaching to and crossing the urban seaward border, until it merges with the corresponding HIC. However, due to the thermally different origination, the merged SBC has a much asymmetric structure with its onshore current much deeper and stronger especially near the front. This very structure is found to be responsible for the enhancement of the rising current over SBF when, in the following time, the merged SBC interacts with the other HIC over the urban inland border.