# Surface quasi-geostrophic equation

### From Mwiki

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Fractional diffusion is often added to the equation $$ \theta_t + u \cdot \nabla \theta + (-\Delta)^s \theta = 0.$$ | Fractional diffusion is often added to the equation $$ \theta_t + u \cdot \nabla \theta + (-\Delta)^s \theta = 0.$$ | ||

- | The equation is used as a toy model for the 3D [[Euler equation]] and [[Navier-Stokes]]. The main question is to determine whether the Cauchy problem is well posed in the classical sense. In the inviscid case, it is a major open problem as well as in the supercritical diffusive case when $s<1/2$. It is believed that inviscid SQG equation presents a similar difficulty as 3D Euler equation in spite of being a scalar model in two dimensions | + | The equation is used as a toy model for the 3D [[Euler equation]] and [[Navier-Stokes]]. The main question is to determine whether the Cauchy problem is well posed in the classical sense. In the inviscid case, it is a major open problem as well as in the supercritical diffusive case when $s<1/2$. It is believed that inviscid SQG equation presents a similar difficulty as 3D Euler equation in spite of being a scalar model in two dimensions <ref name="CMT"/>. The same comparison can be made between the supercritical SQG equation and [[Navier-Stokes]]. |

The key feature of the model is that the drift $u$ is a divergence free vector field related to the solution $\theta$ by a zeroth order singular integral operator. | The key feature of the model is that the drift $u$ is a divergence free vector field related to the solution $\theta$ by a zeroth order singular integral operator. | ||

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For the diffusive case, the well posedness of the equation follows from perturbative techniques in the subcritical case ($s>1/2$). In the critical case the proof is more delicate and can be shown using three essentially different methods. In the sueprcritical regime ($s<1/2$) only partial results are known. | For the diffusive case, the well posedness of the equation follows from perturbative techniques in the subcritical case ($s>1/2$). In the critical case the proof is more delicate and can be shown using three essentially different methods. In the sueprcritical regime ($s<1/2$) only partial results are known. | ||

- | Global weak solutions, as well as classical solutions locally in time, are known to exist globally for the full range of $s \in [0,1]$. | + | Global weak solutions, as well as classical solutions locally in time, are known to exist globally for the full range of $s \in [0,1]$ <ref name="R">. |

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The equation is well posed globally. There are three known proofs. | The equation is well posed globally. There are three known proofs. | ||

- | * '''Evolution of a modulus of continuity''': An explicit modulus of continuity which is comparable to Lipschitz in small scales but growth logarithmically in large scales is shown to be preserved by the flow. The method is vaguely comparable to [[Ishii-Lions]]. | + | * '''Evolution of a modulus of continuity''' <ref name="KNV"/>: An explicit modulus of continuity which is comparable to Lipschitz in small scales but growth logarithmically in large scales is shown to be preserved by the flow. The method is vaguely comparable to [[Ishii-Lions]]. |

- | * '''De Giorgi approach''': From the $L^\infty$ modulus of continuity, it is concluded that $u$ stays bounded in $BMO$. A variation to the parabolic [[De Giorgi-Nash-Moser]] can be carried out to obtain Holder continuity of $\theta$. | + | * '''De Giorgi approach''' <ref name="CV"/>: From the $L^\infty$ modulus of continuity, it is concluded that $u$ stays bounded in $BMO$. A variation to the parabolic [[De Giorgi-Nash-Moser]] can be carried out to obtain Holder continuity of $\theta$. |

- | * '''Dual flow method''': Also from the information that $u$ is $BMO$ and divergence free, it can be shown that the solution $\theta$ becomes Holder continuous by studying the dual flow and characterizing Holder functions in terms of how they integrate against simple test functions. | + | * '''Dual flow method''' <ref name="KN"/>: Also from the information that $u$ is $BMO$ and divergence free, it can be shown that the solution $\theta$ becomes Holder continuous by studying the dual flow and characterizing Holder functions in terms of how they integrate against simple test functions. |

=== Supercritical case: $s<1/2$ === | === Supercritical case: $s<1/2$ === | ||

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* Existence of solutions locally in time. | * Existence of solutions locally in time. | ||

- | * Existence of global weak solutions. | + | * Existence of global weak solutions. <ref name="R"/> |

* Global smooth solution if the initial data is sufficiently small. | * Global smooth solution if the initial data is sufficiently small. | ||

- | * Smoothness of weak solutions for sufficiently large time. | + | * Smoothness of weak solutions for sufficiently large time. <ref name="S"/> <ref name="D"/> <ref name="K"/> |

=== Inviscid case === | === Inviscid case === | ||

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* ??? | * ??? | ||

+ | |||

+ | == References == | ||

+ | |||

+ | {{reflist|refs= | ||

+ | <ref name="CMT">{{Citation | last1=Constantin | first1=Peter | last2=Majda | first2=Andrew J. | last3=Tabak | first3=Esteban | title=Formation of strong fronts in the 2-D quasigeostrophic thermal active scalar | url=http://stacks.iop.org/0951-7715/7/1495 | year=1994 | journal=Nonlinearity | issn=0951-7715 | volume=7 | issue=6 | pages=1495–1533}}</ref> | ||

+ | <ref name="CV">{{Citation | last1=Caffarelli | first1=Luis A. | last2=Vasseur | first2=Alexis | title=Drift diffusion equations with fractional diffusion and the quasi-geostrophic equation | url=http://dx.doi.org/10.4007/annals.2010.171.1903 | doi=10.4007/annals.2010.171.1903 | year=2010 | journal=[[Annals of Mathematics|Annals of Mathematics. Second Series]] | issn=0003-486X | volume=171 | issue=3 | pages=1903–1930}}</ref> | ||

+ | <ref name="S">{{Citation | last1=Silvestre | first1=Luis | title=Eventual regularization for the slightly supercritical quasi-geostrophic equation | url=http://dx.doi.org/10.1016/j.anihpc.2009.11.006 | doi=10.1016/j.anihpc.2009.11.006 | year=2010 | journal=Annales de l'Institut Henri Poincaré. Analyse Non Linéaire | issn=0294-1449 | volume=27 | issue=2 | pages=693–704}}</ref> | ||

+ | <ref name="KNV">{{Citation | last1=Kiselev | first1=A. | last2=Nazarov | first2=F. | last3=Volberg | first3=A. | title=Global well-posedness for the critical 2D dissipative quasi-geostrophic equation | url=http://dx.doi.org/10.1007/s00222-006-0020-3 | doi=10.1007/s00222-006-0020-3 | year=2007 | journal=[[Inventiones Mathematicae]] | issn=0020-9910 | volume=167 | issue=3 | pages=445–453}}</ref> | ||

+ | <ref name="KN">{{Citation | last1=Kiselev | first1=A. | last2=Nazarov | first2=F. | title=A variation on a theme of Caffarelli and Vasseur | year=2009 | journal=Rossiĭskaya Akademiya Nauk. Sankt-Peterburgskoe Otdelenie. Matematicheski\u\i Institut im. V. A. Steklova. Zapiski Nauchnykh Seminarov (POMI) | issn=0373-2703 | volume=370 | pages=58–72}}</ref> | ||

+ | <ref name="K">{{Citation | last1=Kiselev | first1=A. | title=Regularity and blow up for active scalars | url=http://dx.doi.org/10.1051/mmnp/20105410 | doi=10.1051/mmnp/20105410 | year=2010 | journal=Mathematical Modelling of Natural Phenomena | issn=0973-5348 | volume=5 | issue=4 | pages=225–255}}</ref> | ||

+ | <ref name="R">{{Citation | last1=Resnick | first1=Serge G. | title=Dynamical problems in non-linear advective partial differential equations | url=http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:9542767 | publisher=ProQuest LLC, Ann Arbor, MI | year=1995}}</ref> | ||

+ | <ref name="D">{{Citation | last1=Dabkowski | first1=M. | title=Eventual Regularity of the Solutions to the Supercritical Dissipative Quasi-Geostrophic Equation | publisher=[[Springer-Verlag]] | location=Berlin, New York | year=2011 | journal=Geometric and Functional Analysis | issn=1016-443X | volume=21 | issue=1 | pages=1–13}}</ref> | ||

+ | }} |

## Revision as of 01:38, 24 May 2011

$ \newcommand{\R}{\mathbb{R}} $

The surface quasi-geostrophic (SQG) equation consists of an evolution equation for a scalar function $\theta: \R^+ \times \R^2 \to \R$. In the inviscid case the equation is $$ \theta_t + u \cdot \nabla \theta = 0,$$ where $u = R^\perp \theta$ and $R$ stands for the Riesz transform.

Fractional diffusion is often added to the equation $$ \theta_t + u \cdot \nabla \theta + (-\Delta)^s \theta = 0.$$

The equation is used as a toy model for the 3D Euler equation and Navier-Stokes. The main question is to determine whether the Cauchy problem is well posed in the classical sense. In the inviscid case, it is a major open problem as well as in the supercritical diffusive case when $s<1/2$. It is believed that inviscid SQG equation presents a similar difficulty as 3D Euler equation in spite of being a scalar model in two dimensions ^{[1]}. The same comparison can be made between the supercritical SQG equation and Navier-Stokes.

The key feature of the model is that the drift $u$ is a divergence free vector field related to the solution $\theta$ by a zeroth order singular integral operator.

For the diffusive case, the well posedness of the equation follows from perturbative techniques in the subcritical case ($s>1/2$). In the critical case the proof is more delicate and can be shown using three essentially different methods. In the sueprcritical regime ($s<1/2$) only partial results are known.

Global weak solutions, as well as classical solutions locally in time, are known to exist globally for the full range of $s \in [0,1]$ **Cite error: Closing </ref> missing for <ref> tag**
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