Advances in Meteorology

Volume 2018, Article ID 4329475, 11 pages

https://doi.org/10.1155/2018/4329475

## 3D Variable Coefficient KdV Equation and Atmospheric Dipole Blocking

^{1}College of Atmospheric Science, School of Electronic and Information Engineering, Nanjing University of Information Science and Technology, Nanjing, Jiangsu 210044, China^{2}School of Physics and Electrical Engineering, Anqing Normal University, Anqing, Anhui 246133, China

Correspondence should be addressed to Juanjuan Ji; moc.621@1270jjj and Yecai Guo; moc.361@iacey-oug

Received 4 January 2018; Revised 5 March 2018; Accepted 14 March 2018; Published 17 May 2018

Academic Editor: Herminia García Mozo

Copyright © 2018 Juanjuan Ji et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A (2 + 1)-dimensional variable coefficient Korteweg-de Vries (3D VCKdV) equation is first derived in this paper by means of introducing 2-dimensional space and time slow-varying variables and the multiple-level approximation method from the well-known barotropic and quasi-geostrophic potential vorticity equation without dissipation. The exact analytical solution of the 3D VCKdV equation is obtained successfully by making use of CK’s direct method and the standard Zakharov–Kuznetsov equation. By some arbitrary functions and the analytical solution, a dipole blocking evolution process with twelve days’ lifetime is described, and the result illustrates that the central axis of the dipole is no longer perpendicular to the vertical direction but has a certain angle to vertical direction. The comparisons with the previous researches and Urals dipole blocking event demonstrate that 3D VCKdV equation is more suitable for describing the complex atmospheric blocking phenomenon.

#### 1. Introduction

Atmospheric blocking is a nonlinear phenomenon with a long lifetime occurring in mid-high latitude regions. Its dynamical study has been an important research topic in the atmospheric science field because of its significant influence on disaster weathers and extreme cold events. In the past decades, many investigators have proposed various nonlinear theories such as Rossby soliton [1–4], envelope Rossby soliton that is described by the Schrodinger equation [5, 6], and others to explain the formation of atmospheric blocking and its life process. Although the eddy-forced envelope soliton model can describe a life cycle of atmospheric dipole blocking [7, 8], the KdV-type soliton cannot represent the time variation or life of atmospheric dipole blocking [2, 3]. However, by considering the time slow- varying basic flow, the KdV-type Rossby solitary wave can represent the life cycle of dipole blocking [9]. Previously derived constant coefficient KdV- and Schrödinger-type equations and variable coefficient KdV- and Schrödinger-type equations are all one-dimensional models on the space [10–15]. However, in the nature, propagation of a solitary wave is usually two-dimensional on the space, and only thinking about one-dimensional model may not be enough.

The purpose of this paper is to extend the solitary Rossby wave model to the 3D case, namely, two-dimensional space and time. We aim to derive a (2 + 1)-dimensional variable coefficient KdV (3D VCKdV) equation from the barotropic and quasi-geostrophic potential vorticity equation without dissipation on a beta-plane.

The structure of this paper is as follows: the 3D VCKdV equation is derived in Section 2; in Section 3, the exact analytical solution of the equation is obtained; Section 4 is devoted to study atmospheric dipole blocking by some arbitrary functions and parameters, and a comparison with the previous model and Urals dipole blocking is made. In the last section, some conclusions are given.

#### 2. Derivation of 3D VCKdV Equation

The barotropic and quasi-geostrophic potential vorticity equation without dissipation on a beta-plane in the atmospheric dynamical system is as follows:which is a highly nonlinear equation. It is very difficult to solve. In (1), is the stream function; , in which is the earth’s radius, is the angular frequency of the earth’s rotation, and is the latitude; , in which is the Coriolis parameter, is the gravitational acceleration, and is the atmospheric average height.

Let us assume that there is a base flow independent of variable in the atmospheric system; thus, the stream function is rewritten aswhere is the perturbation stream function, is a small parameter, and base flow field is a function of variables and ; in the previous studies, it is often taken only as a function of . For simplicity of notation, the prime is dropped out in the remaining of this paper.

Substituting (2) into (1), we obtain

Because of the multiple time-space scale features of the solitary wave in the fluid, we introduce 2-dimensional space and time slow-varying variables:where is an arbitrary constant. From (4), we have

Consequently,

Substituting (5)–(7) into (3) yields

Expand the perturbation stream function in terms of in the form

Substituting (9) into (8), and then requiring all the coefficients of different powers of to be zero, we obtain the following first-order equation of :

We assume that has the following variable separation solution:

Substituting (11) into (10), we obtainwhere is an arbitrary integral function of variable .

In order to obtain solitary wave amplitude equation, we continue solving second-order equation about :

Then, we assume that has the following variable separation solution:

Substituting (15) into (14), we have

The governing equation of solitary wave amplitude still cannot be obtained from (14), and we continue solving the following high-order problem:

Substituting (11), (13), and (15) into (17), the following can be obtained:

In the derivation of many nonlinear equations, the *y*-average method is the traditional method and commonly utilized, but we remove this treatment and introduce higher order aswhere () are arbitrary functions of variables and .

Therefore, we have

Substituting (20) and (21) into (18), the following can be obtained:where ().

When () satisfy the relationships,a (2 + 1)-dimensional variable coefficient KdV (3D VCKdV) equation is derived as follows:where () are arbitrary functions of variable .

#### 3. Exact Solution of 3D VCKdV Equation

It is not easy to obtain the exact analytical solution of 3D VCKdV equation (24) in the case that 5 variable coefficients are kept arbitrary. As we know, quite a few methods for obtaining solitary wave solution of nonlinear systems have been proposed, for instance, the hyperbolic function method [16], generalized Darboux transformation [17], and CK’s direct method [15]. CK’s direct method is a very simple and effective method. In this section, we are going to construct the exact analytical solution of 3D VCKdV equation by CK’s direct method.

The Zakharov–Kuznetsov equation is as follows:which is taken as a two-dimensional form of constant coefficient KdV equation, and it has the following solitary wave solution [18]:where is an arbitrary constant.

According to CK’s direct method, we suppose the exact analytical solution of 3D VCKdV equation in the following form:where satisfy (25).

From (27), we have

Substituting (27)–(39) into (24), the following can be obtained:where

Comparing the coefficients of (25) and (40), we obtainwhere and are constants, and these variable coefficients must satisfy the following equation:

Thus, the exact analytical solution of 3D VCKdV equation is as follows:

#### 4. Atmospheric Dipole Blocking Phenomenon

The analytical solution of 3D VCKdV equation (44) including the three variables , , and and five arbitrary functions can be used to describe some complex atmospheric phenomena such as atmospheric blocking.

If we assume , , , which is an arbitrary constant, returning the variables , , and , the first-order approximation solution of basic system equation (1) can be obtained:

When taking the basic flow aswhere and are arbitrary constants, by (12) and (13), we obtainwhere is an arbitrary function of variable ; here, we supposeand other parameters as follows:

A dipole blocking evolution with a life cycle of twelve days is displayed in Figure 1.