In section 3, the deformation frontogenesis function [Eq. (13)] was derived based on the resultant deformation equation [Eq. (10)] of (Gao et al., 2008). However, both equations include too many terms, which increase their complexity. At least in form, the advantage of the above deformation frontogenesis function [Eq. (13)] is not too evident compared with the traditional frontogenesis function. Therefore, simplification, or another derivation, is necessary to cut down the complexity of the deformation frontogenesis function. An alternative derivation of the deformation frontogenesis function is performed as follows:
The horizontal motion equation and its component forms can be written as
\beginsubequations \begin{align} \begin{array}{rcl} \dfrac{\partial \bm{V}_{\rm h}}{\partial t}&=&-(\bm{V}\cdot\nabla_3)\bm{V}_{\rm h}-\nabla\phi-2\Omega\times \bm{V}+\bm{F}\\ &&\qquad\bm{J}_1\qquad\quad\ \ \bm{J}_2\qquad \bm{J}_3\qquad\ \bm{J}_4 \end{array} ,\\ \label{eq15} \begin{array}{rcl} \dfrac{\partial u}{\partial t}&=&-(\bm{V}\cdot\nabla_3)u-\dfrac{\partial\phi}{\partial x}+fv+F_x\\ &&\qquad J_{x,1}\qquad\ J_{x,2}\quad J_{x,3}\ \ J_{x,4} \end{array} ,\hspace*{6.8mm}\\ \label{eq16} \begin{array}{rcl} \dfrac{\partial v}{\partial t}&=&-(\bm{V}\cdot\nabla_3)v-\dfrac{\partial\phi}{\partial y}-fu+F_y\\ &&\qquad J_{y,1}\qquad\ J_{y,2}\quad J_{y,3}\ \ J_{y,4} \end{array} ,\hspace*{7.4mm} \end{align} \endsubequations
with \(\bmJ=\sum\limits_m=1^4\bmJ_m\), \(J_x=\sum\limits_m=1^4J_x,m\), \(J_y=\sum\limits_m=1^4J_y,m\), Vh=ui+vj and J=Jxi+Jyj; Jx,m and Jy,m(m=1,2,3,4) are the forcing terms of the local change of horizontal wind components.
From Eq. (8), we have \begin{equation} \label{eq17} \dfrac{\partial E}{\partial t}=\dfrac{1}{E}\left(E_{\rm st}\dfrac{\partial E_{\rm st}}{\partial t}+E_{\rm sh}\dfrac{\partial E_{\rm sh}}{\partial t}\right). \end{equation}
Combining with Eq. (15), ∂ E st/∂ t and ∂ E sh/∂ t in Eq. (18) become
\begin{eqnarray*} \dfrac{\partial E_{\rm st}}{\partial t}&=&\dfrac{\partial}{\partial t}\left(\dfrac{\partial u}{\partial x}-\dfrac{\partial v}{\partial y}\right) =\dfrac{\partial}{\partial x}\left(\dfrac{\partial u}{\partial t}\right)-\dfrac{\partial }{\partial y}\left(\dfrac{\partial v}{\partial t}\right)\\ &=&\dfrac{\partial J_x}{\partial x}-\dfrac{\partial J_y}{\partial y}=J_{\rm st} ,\\ \dfrac{\partial E_{\rm sh}}{\partial t}&=&\dfrac{\partial}{\partial t}\left(\dfrac{\partial v}{\partial x}+\dfrac{\partial u}{\partial y}\right) =\dfrac{\partial}{\partial x}\left(\dfrac{\partial v}{\partial t}\right)+\dfrac{\partial}{\partial y}\left(\dfrac{\partial u}{\partial t}\right)\\ &=&\dfrac{\partial J_y}{\partial x}+\dfrac{\partial J_x}{\partial y}=J_{\rm sh} .\qquad \end{eqnarray*}
Note that, similarly, we have the relations
\begin{eqnarray*} J_{{\rm st},m}&=&\dfrac{\partial J_{x,m}}{\partial x}-\dfrac{\partial J_{y,m}}{\partial y}\ {\rm and}\\ J_{{\rm sh},m}&=&\dfrac{\partial J_{y,m}}{\partial x}+\dfrac{\partial J_{x,m}}{\partial y}\ {\rm with}\ m=1,2,3,4. \end{eqnarray*}
Thus, Eq. (16) becomes
\begin{equation} \label{eq18} \dfrac{\partial E}{\partial t}=\dfrac{1}{E}(E_{\rm st}J_{\rm st}+E_{\rm sh}J_{\rm sh}) . \end{equation}
From Eq. (17), Eq. (11) can be rewritten as
\begin{eqnarray} \label{eq19} F&=&\dfrac{\partial}{\partial t}|\nabla E|=\dfrac{1}{|\nabla E|}\left(\dfrac{\partial E}{\partial x}\dfrac{\partial}{\partial x}+ \dfrac{\partial E}{\partial y}\dfrac{\partial}{\partial y}\right)\left(\dfrac{\partial E}{\partial t}\right)\nonumber\\ &=&\dfrac{1}{|\nabla E|}(\nabla E\cdot\nabla)\left(\dfrac{\partial E}{\partial t}\right)\nonumber\\ &=&\dfrac{1}{|\nabla E|}(\nabla E\cdot\nabla)\left[\dfrac{1}{E}(E_{\rm st}J_{\rm st}+E_{\rm sh}J_{\rm sh})\right] . \end{eqnarray}
Equation (18) can be further simplified to
\begin{equation} \label{eq20} F=\bm{n}\cdot\nabla({\rm tg}\alpha J_{\rm st}+{\rm ctg}\alpha J_{\rm sh}) , \end{equation}
with n=∇ E/|∇ E|, tgα=E st/E and ctgα=E sh/E.
For convenience of calculation and analysis, Eq. (21) is expanded and noted as the sum operator of several forcing factors Xm(m=1,2,3,4):
\beginsubequations \begin{align} \label{eq21} F=\sum_{m=1}^4X_m , \end{align} \endsubequations
where
\begin{equation*} X_m=\bm{n}\cdot\nabla({\rm tg}\alpha J_{{\rm st},m}+{\rm ctg}\alpha J_{{\rm sh},m}),\quad m=1,2,3,4 .\eqno{\rm (20b)} \end{equation*}
In Eq. (20b), Xm denotes the co-action of two kinds of deformation effects. One is the deformation of the wind field itself ( tgα=E st/E, ctgα=E sh/E), and the other is the stretching deformation (J st,m) and shearing deformation (J sh,m) of four forcing terms in the horizontal wind local change tendency equation [Eq. (15)]. Taking m=1 (or 2,3,4) as an example, the second type of deformation effect (J st,m and J sh,m) represents the advection (or pressure gradient force, Coriolis force, friction and/or turbulence mixture) forcing deformation. For brevity, Xm(m=1,2,3,4) is termed advection forcing (X1), pressure gradient forcing (X2), Coriolis forcing (X3), and friction forcing (X4) of the deformation frontogenesis function.
This form of the deformation frontogenesis function [Eq. (20)], recorded as a sum operator, looks simple and offers an easy explanation relative to the expression [Eq. (13)] derived in section 3. Equation (20) includes four terms with a clear physical sense for each term. At least in form, it is not more complicated than the traditional frontogenesis function. Furthermore, it is not harder for it to provide a clear physical explanation for each forcing term in Eq. (20). Also, based on its advantage from the dynamic viewpoint (e.g., the confluence of airflow induced by deformation can dynamically drive the isolines of any one tracer, be it θ or θe, closer together), the deformation frontogenesis function will exert its maximal advantage if its diagnostic analysis for frontogenesis and associated precipitation via a case study is good.
The difference and relationship between Eqs. (13) and (20) should be discussed to develop contextual linkages, since the latter is an alternative form of the former. By rigorous derivation, we have the relations X1=F1+F2+F3+F6, X2=F5, X3=F4, and X4=F7. After careful derivation and repeated validation, it is shown that the above relations are rigorous and without any conditions of assumptions and simplifications. All these analyses indicate that, in addition to being concise in form, Eq. (20) can reflect all of the information contained in Eq. (13). Therefore, the calculation in the case study in the next section based on Eq. (20) can be considered reasonable.
Note that Eq. (13) is retained in the paper for two reasons:
(1) Because the deformation frontogenesis function F is defined as the local change rate of |∇ E|, it is natural to derive F based on the E equation. It is logical to derive the E equation first, and then F based on the E equation. Furthermore, from E equation to its extended application (F), a set of complete and systematic derivation about deformation theory is built up, just as have done in the first author's Ph.D dissertation (Yang, 2007). Because too many terms are included in Eq. (13), Eq. (20) is derived as an alternative form of the deformation frontogenesis function.
(2) The origin of Eq. (13) is important as a "true" value to validate Eq. (20), both in theory and in calculation. By testing, Eq. (20) can reflect all of the information contained in Eq. (13). Therefore, their calculation results in a real case analysis are also consistent.
Certainly, Eq. (20) can be further simplified under certain conditions. For example, if pure stretching deformation is assumed (which can be achieved by rotating the coordinate system), E sh=0 and E=E st, which lead to tgα=1 and ctgα=0. Therefore, Eq. (20b) becomes
\begin{equation} \label{eq22} X_m=\bm{n}\cdot\nabla J_{{\rm st},m} , \end{equation}
which leads to the frontogenesis function, Eqs. (22) and (21), to become
\begin{equation} \label{eq23} F=\sum_{m=1}^4\bm{n}\cdot\nabla J_{{\rm st},m}=n\cdot\nabla J_{\rm st} . \end{equation}
At this moment, the shear deformations of both the wind field itself and the forcing terms of wind change disappear. Only the effects of their stretching deformations work. In the next section, a case study is reported in which we validate the application of the deformation frontogenesis function.