谈哲敏
论文题目：边界层锋面结构动力学
作者简介：谈哲敏，男，1965年01月出生，1997年09月师从于南京大学伍荣生教授，于2000年12月获博士学位。
摘 要
大气锋面系统是大气中一种重要的灾害性天气系统，地面锋系统往往造成局地的大风、强降水等灾害性天气，锋面系统成为业务天气分析预报中人们最关注的系统之一。关于大气低层锋面的锋生过程、锋面结构与环流特征的研究是当前中尺度动力学研究中一个十分重要的课题，其研究不仅对可以加深理解锋面形成的动力学过程，而且可以为锋面系统的数值模拟、数值预报提供理论基础，同时可对中尺度天气的业务预报提供参考。本论文主要采用动力学方法在理论上重点研究了大气边界层锋面的锋生机制、锋面环流、锋面平衡结构以及大气边界层摩擦、地形对边界层锋面的动力学影响作用。通过这些动力学研究，取得了一些新的结果，具体如下：
为了便于边界层锋面动力学研究，在Ekman动量近似的基础上，发展了一个新的非线性多层简化边界层动力学模式。与现有其他简化边界层动力学模式相比，该模式比较完整地考虑了惯性项、湍流粘性项以及湍流粘性系数随高度变化对边界层风场结构特征的影响作用。在边界层动力学特征的描述上，该简化模式是介于原始边界层模式与现有的其他简化边界层模式之间的一类中等复杂边界层动力学模式。模式的理论结果进一步修正了经典Ekman理论以及其他简化边界层动力学模式，为边界层动力学以及锋面动力学的进一步研究提供了理论基础。
利用Ekman动量近似简化边界层动力学模式，详细地讨论了边界层摩擦对地面锋锋生、环流的动力学影响作用。提出了一个新的包含湍流摩擦作用的广义锋生函数方程。对于地面锋的锋生过程，边界层除了直接的湍流扩散作用外，边界层与自由大气相互作用可产生一种新的锋生强迫，这种锋生强迫主要包括边界层摩擦对地转锋生强迫场的减弱以及旋转作用而产生。此外，由边界层摩擦而诱导出的Ekman抽吸的非均匀水平分布也是影响地面锋锋生的一个重要因素，对于这两种锋生强迫，在传统的锋生方程无法描述，这些结果为地面锋锋生动力学提供了新的认识。在粘性热成风动力学平衡的条件下，推导出新的包含边界层摩擦作用的地面锋次级环流控制方程：广义Sawyer-Eliassen锋面次级环流方程。边界层摩擦对地面锋次级环流的影响主要通过直接湍流扩散及与地转流场相互作用两方面的影响而作用，它为地面锋锋面次级环流的形成提供了新的动力学机制。
利用一个包含边界层摩擦和Ekman动量近似下的惯性项作用的二层边界层-锋面简化模型解析地研究了定常地面冷锋的结构和环流动力学特征。地面锋的低层结构主要决定于冷锋暖区的地转流分布、锋面移动速度、跨锋面的温差和边界层摩擦。锋面冷侧的质量通量平衡决定了地面锋平衡结构。冷锋的倾斜随着沿锋面地转风、锋面移动速度的增加、跨锋面温差的减少而增加。与均匀地转流情况相比，冷锋锋面坡度在反气旋性地转流增加，而在气旋性地转流中减少。边界层摩擦作用导致了冷锋在大气低层具有较大锋面坡度，地面锋两侧的强水平辐合，从而导致在地面冷锋线处出现强垂直运动，且在冷锋前缘处最大的水平辐合与垂直涡度相耦合。气旋性地转涡度、正的锋面曲率、地面锋线处流向锋面冷侧的地转流以及大的锋面坡度都有利于锋面附近大的垂直运动形成，这些理论结果可以较好地解释近年来一些关于地面锋高精度观测结果。
利用第四章发展的二层边界层-锋面简化模型，讨论了地形、边界层共同作用下对冷、暖锋锋面结构、环流的动力学影响作用问题。在有地形存在时，冷锋的倾斜不仅与冷锋暖区的地转流分布、锋面移动速度有关、还与锋面与地形的相对位置有关。当冷锋位于迎风坡时，其坡度减小，位于背风坡时，其坡度增大。在静止冷锋的冷区中存在两类不同的环流系，即位于大气低层、地面锋附近的反时针环流系（C1）和远离地面锋的顺时针环流系(C2)。当静止冷锋位于背风坡时，其冷域中C1环流增强，范围增大，而位于冷锋界面上的环流转向点沿锋面上移，暖域中沿锋面的暖空气运动范围变大。当静止冷锋位于迎风坡时，结果相反。冷锋移动较慢时，其冷域C2环流主体可被地形完全阻塞在迎风坡。当静止冷锋移离地形时，由于地形作用可在锋面暖域、地形下游形成一个背风槽。地形对冷锋锋区的垂直运动影响主要通过地形与边界层共同作用，改变锋区流场的水平辐合辐散的分布及地形强迫抬升两条途径实现。由于边界层摩擦的辐合作用，在地面冷锋的前缘可形成一支范围较窄、强度较大的上升运动带，当冷锋位于迎风坡时，其强度增强，当冷锋位于背风坡时,其强度减弱。当冷锋位于背风坡时，在暖区沿锋面上升的暖空气运动范围增大，可以出现沿锋面相间隔的多个上升区。
对于地形边界层流中的暖锋，其坡度主要取决于其暖域地转流、锋面移速，它随锋面移速增大而减小，这与冷锋特征相反。地形对暖锋坡度的影响作用较小。与无地形作用时相比，静止性暖锋冷域中，位于锋面界面附近的闭合正环流系，当暖锋位于地形上游，其伸展范围增大；当暖锋位于迎风坡时，其伸展范围缩小，中心位置上抬；锋面移至背风坡时，其伸展范围重新增大。对于冷域中远离地面暖锋的另一支正环流系来说，当暖锋位于地形上游或迎风坡时，它可被地形完全阻塞于背风侧，地形高度越高，地形阻塞作用越大。在暖锋锋区附近主要存在三支垂直上升运动带：(a)由于边界层摩擦辐合作用，导致在地面暖锋后缘暖域区中形成一支水平尺度较小、强度较大的垂直运动带，它随着暖锋移速增大而减弱。当暖锋位于地形迎风侧，该垂直运动带强度增加；而当暖锋位于地形背风侧，其强度减弱 ；(b)在锋区暖域沿锋面存在均匀的暖空气上升运动；(c)在冷域远离地面暖锋处，存在一支水平范围较宽，其中心位于边界层顶部附近的垂直运动带，当暖锋位于迎风坡时，这支垂直运动带可被地形阻塞于地形背风侧。
地转适应过程是大气低层中另一类锋面锋生的重要机制之一。初始非平衡的风场与气压场之间，通过地转适应过程，可以重新调整，从而可形成一定的锋面结构。本文中进一步研究了不同的非平衡的初始条件，在地转适应以及锋生过程的作用。不同的非平衡的初始条件，可以导致形成不同的锋面结构。对于初始位温扰动占优情形中，在低层存在较强的水平辐合，导致不连续结构的出现，其形成的锋面具有类重力流结构，具有较明显的移动性，而当初始风场扰动占优时，通过地转适应调整而形成的锋面具有与一般天气尺度锋面相类似结构，具有较好的定常性。
Structure
and Dynamics of Fronts in the Planetary Boundary Layer
ABSTRACT
Frontal systems are one of the most important server
weather systems in the atmosphere. Surface fronts are always accompanied by the
characteristic phenomena: sudden changes of temperature, wind intensity and
direction. Therefore, the research on surface frontal dynamics is a important
research subject in the mesoscale weather analysis and dynamics, and will deep
the understanding of the dynamics of frontogenesis, frontal structure in the
low-level, and also provides a theoretical base for the mesoscale numerical
simulation and predication of frontal systems. In this dissertation, from the
theoretical view we extremely investigate the frontogenesis mechanism, balanced
frontal structure and circulation in the boundary layer. Also the dynamic
effects of boundary-layer friction, orography on the surface frontal structure
and circulation have been studied. These results provided some new sights for
surface frontal dynamics.
In Chapter two, a simplified nonlinear multi-layer
intermediate planetary boundary layer (PBL) dynamic model, based on the Ekman momentum
approximation (EMA) theory, was proposed. The effect of inertial terms with the
EMA and height-dependent eddy viscosity are included in this model that
improves some present simplified boundary-layer dynamic models. In comparison
with the primitive boundary layer model (PE), the model could be keeping more
dynamic behaviors than other simplified PBL models, however it more convenient
to the dynamic analysis in the boundary layer than that in PE. The results of
model’s simulation improve the classical Ekman theory and dynamics of other simplified
PBL models.
In Chapter three, with the simplified boundary layer
model based on the EMA, the frontogenesis and frontal secondary circulation in
the frontal boundary layer are discussed. A new viscous frontogenesis equation (VFE)
was proposed. Since the effect of boundary-layer friction was included in VFE,
the interaction between the boundary layer friction and geostrophic flow forcing
provided a new frontogenesis forcing that included the dissipative and
rotational role of boundary layer friction in the geostrophic forcing, and the non-uniform
horizontal distribution of Ekman pumping.
With a new viscous thermal wind balance relation, a new viscous Sawyer-Eliassen
equation (VSE) for the secondary frontal circulation in the boundary-layer
fronts was developed. The VSE will be reduced to the conventional SE equation (CSE)
when the turbulent diffusion is neglected. The difference between the VSE and
other extended SE was discussed. Boundary layer friction affects the frontal secondary
circulation by three different dynamic processes. There include the interaction
between the boundary layer friction and geostrophic flow, the directly
turbulent diffusion and that the Ekman flow structure-dependent coefficient of VSE
equation, which can directly impact on the secondary circulation of surface
fronts.
In Chapter four, a two-layer
frontal-boundary layer model represents the Ekman boundary layer dynamics by
the inclusion of inertial acceleration with the EMA and constant eddy viscosity
is developed to examine analytically the structure and dynamics of surface cold
front (SCF). The existence of
boundary-layer friction allows a force balance among the Coriolis force,
pressure gradient and boundary layer friction in the low-level for the SCF. An
analytical expression of the balanced boundary layer motion in the frontal zone
is obtained. The dynamic behavior of the SCF is controlled by the geostrophic
wind in the warm side, frontal-moving speed, and temperature contrast across
frontal interface and boundary layer friction.
The equilibrium of mass flux
in the cold side of frontal interface leads to a balanced frontal shape. The
inclination of frontal interface is increased as the along-frontal geostrophic
wind component is increased, frontal moving speed is enlarged and density
difference between the two air masses is weaker. In comparison with the
solution of uniform geostrophic flow, the cold frontal slope is increased in an
anticyclonic and decreased in a cyclonic system. A relative larger inclination
of frontal interface in the lowest-levels near the SCF that results in an
enhanced vertical motion at its leading edge, and becomes a marked similarity
of frontal structure between in kata- and ana-cold front. The maximum
convergence and vertical cyclonic vorticity are collocated at the leading edge
of surface front. There is an upward contribution to vertical motion of
boundary layer in the frontal zone arises from (a) cyclonic geostrophic
vorticity, (b) positive curvature of frontal interface, (b) the geostrophic
wind directed cold-ward or to the right of gradient of frontal isobath in the
vicinity of surface front, (d) a large frontal slope in the low-level.
In Chapter five and six, a simplified two-layer
frontal boundary layer model is incorporated to investigate the effect of
orography and boundary layer friction on the structure and circulation of the
SCF, and surface warm front (SWF), respectively. The inclination of the SCF not
only depends on the geostrophic wind in the warm sector and frontal moving
speed, but also the position of the SCF relative to orography. The slope of SCF is increased in the upward-side，but decreased in
the leeward-side. There are two different circulation regimes in the cold
sector for the steady SCF，i.e. a closed
anti-clockwise circulation in the low-level near the SCF and a clockwise one
away from the SCF. In the upward-side the intensity and scope of the closed
circulation in cold sector are enlarged and the turning point of frontal
circulation at the frontal interface is lifted which leads to an enhancing
upward motion of warm air ahead of the SCF. There is a contrary result when the
cold front moves to the leeward side. The clockwise circulation in the cold air
can be obstructed wholly in the upward side by orography for the moving slowly
steady cold front. A lee trough over the leeward side is induced by the
interaction of orography with the SCF. The orography dynamic lifting and the
distribution of convergence, which related to the interaction between the
boundary layer friction, frontal structure and orography, would induce the
vertical motion in frontal zone. An intense upward motion ahead of the SCF may
produce by the boundary-layer friction that can be inflated in the upward side
and reduced in the leeward side. There are multiple vertical motion zones in
the warm side when the cold front passages in the leeward
side.
For the SWF, the inclination of SWF depends mainly
on the frontal moving speed and geostrophic flow in the warm sector that is
increased with the frontal speed increasing. The feature is opposite to that of
the SCF. The effect of orography on the slope of SWF is relative smaller than
that on the SCF. There is a uniform upward motion in the warm air of SWF that
is decreased with the frontal moving speed increasing. However, there are three
different circulation regimes in the cold air for the stationary SWF. They are
a closed clockwise circulation in vicinity of the SWF, an anticlockwise
circulation in the upper-level part of cold sector along the frontal interface
and a relative intense clockwise one away from SWF. Therefore the motion near
the surface in the cold sector is directed to the SWF. Moreover the closed
circulation regime in the low-level near the SWF is vanished in the case of
moving SWF. Comparing with the features of flat case shows that the closed
circulation near the SWF in the cold sector for the stationary SWF is enlarged
when the SWF locates the upstream of orography and contracted on the upslope.
However, it will be expanded again when the SWF moving to the leeside. The
clockwise circulation away from the SWF in the cold side may be blocked in the
leeside whose blocking will be increased when the orography height increasing.
There are different vertical zones in the SWF, (a) a
small horizontal scale and strong vertical motion in the vicinity of the SWF
due to the convergence induced by boundary layer friction that decreased with
the increasing of frontal moving speed. Moreover the vertical motion related to
the SWF will be increased in the upside and decreased in the leeside that is
associated with the orography dynamic effect. (b) a uniform upward motion along
the frontal interface in the warm sector that is related to the lifting effect
of frontal inclination. (c) a wide vertical motion zone is away from the SWF in
the cold side whose center is placed the top of boundary layer and will be
blocked in the leeside when the SWF located the upslope.
In Chapter seven, the effects
of different initial unbalance between the air masses and wind on the
geostrophic adjustment, which may lead to the frontogenesis, were studied in
detail. The adjustment between the non-uniform potential temperature gradients
and vertical shears of horizontal wind was considered one possible mechanism of
frontogenesis when the large-scale deformation filed or shear forcing is vanished.
In the most of past studies only including the role of initial potential
temperature unbalanced on the geostrophic adjustment frontogenesis process. The
present results show that the different initial unbalanced conditions play an important
role in the time-scale of adjustment frontogenesis and frontal structure. The
front formed in case of initial potential temperature perturbation adjustment
has a gravity-current-like feature at the leading edge of front and unsteady. However,
the front developed in initial velocity perturbation adjustment has a
synoptic-scale front feature and steady.
A summary and some further
research directions associated with the present subject are given in Chapter
eight.
