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A Data Assimilation Algorithm for the Subcritical Surface Quasi-Geostrophic Equation

Michael S. Jolly, Vincent R. Martinez and Edriss S. Titi


In this article, we prove that data assimilation by feedback nudging can be achieved for the three-dimensional quasi-geostrophic equation in a simplified scenario using only large spatial scale observables on the dynamical boundary. On this boundary, a scalar unknown (buoyancy or surface temperature of the fluid) satisfies the surface quasi-geostrophic equation. The feedback nudging is done on this two-dimensional model, yet ultimately synchronizes the streamfunction of the three-dimensional flow. The main analytical difficulties are due to the presence of a nonlocal dissipative operator in the surface quasi-geostrophic equation. This is overcome by exploiting a suitable partition of unity, the modulus of continuity characterization of Sobolev space norms, and the Littlewood–Paley decomposition to ultimately establish various boundedness and approximation-of-identity properties for the observation operators.

Communicated by Paul Rabinowitz

Funding source: National Science Foundation

Award Identifier / Grant number: DMS-1418911

Award Identifier / Grant number: DMS-1109640

Award Identifier / Grant number: DMS-1109645

Funding source: Leverhulme Trust

Award Identifier / Grant number: VP1-2015-036

Funding source: Office of Naval Research

Award Identifier / Grant number: N00014-15-1-2333

Funding statement: Michael S. Jolly was supported by NSF grant DMS-1418911 and the Leverhulme Trust grant VP1-2015-036. The work of Edriss S. Titi was supported in part by the ONR grant N00014-15-1-2333 and the NSF grants DMS-1109640 and DMS-1109645.

A Appendix

In this section, we verify that volume element (see (A.7) and (A.8) below) and modal projection interpolants (see (A.18) and (A.21)) are of Type I and II, respectively. For convenience, we let Ω=𝕋2 denote the 2π-periodic box throughout, where 𝕋2=(/(2π))2, so that 𝕋2=[-π,π]2. Let 𝒵 and Vβ be the spaces defined by (2.1) and (2.4), respectively. The main claim is the following:

Proposition A.1

Suppose that Jh is a linear operator that is defined by either (A.7), (A.8), (A.17), (A.21), below. Then:

  1. (0.1)

    suph>0JhϕLpϕLp, for any p(1,), ϕLperp(𝕋2),

  2. (0.2)

    JhϕLph2/p-2/qϕLq, for any 1qp<, ϕLperq(𝕋2),

  3. (0.3)

    JhϕH˙βh-βϕL2, for any β0, ϕLper2(𝕋2).

If Jh is defined by (A.7), (A.8), then for any β(0,1] we have

  1. (1.1)

    ϕ-JhϕL2hβϕH˙β, for any ϕH˙perβ(𝕋2) (Vβ if Jh is (A.8)),

  2. (1.2)

    ϕ-JhϕH˙-βhβϕL2, for any ϕLper2(𝕋2) (Lper2(𝕋2)𝒵 if Jh is (A.8)).

If Jh is defined by (A.18) or (A.21), then for any ϕH˙perβ(T2) we have

  1. (2.1)

    ϕ-JhϕH˙αhβ-αϕH˙β, for any ϕH˙perβ(𝕋2),β>α,

  2. (2.2)

    ΛβJhϕ=JhΛβϕ, for any ϕH˙perβ(𝕋2),β.

Moreover, when Jh is given by (A.7), (A.8), or (A.21), then property (0.1) and property (0.2) are valid for p=1, and p=, respectively.

We then define Type I operators as any linear operator, Jh, that satisfy properties (0.1)(0.3) and properties (1.1)(1.2), while Type II operators are those that satisfy properties (0.1)(0.3) and properties (1.1)(1.2).

A.1 Local Averages (Type I)

Let us recall the following construction of a partition of unity from [5]. Let N>0 be a perfect square integer and partition Ω into 4N squares of side-length h=π/N. Let 𝒥={0,±1,±2,,±(N-1),-N}2 and for each α𝒥, define the semi-open square

Qα=[ih,(i+1)h)×[jh,(j+1)h),where α=(i,j)𝒥.

Let 𝒬 denote the collection of all Qα, i.e.,


Consider the functions


In particular, ψα is the 2π-periodic extension of the characteristic function, 𝟙Qα, of Qα to 2.

Given ϵ>0 fixed, we mollify ψα as


with the function ρϵ(x):=ϵ-2ρ(xϵ), where ρ is given by


and K0>0 is the absolute constant given by


Now suppose that N9 and ϵ=h10. For each α=(i,j)𝒥, let us also define the augmented squares, Q^α and Qα(ϵ), by


so that QαQα(ϵ)Q^α for each α𝒥, and the “core”, Cα(ϵ), by


Finally, for ϕL1(𝕋2), define


Then we have the following proposition, which follows from the definition of ψ~α. We note that properties (i)–(iii) below can be found in [5], while property (iv) can be proved by using a characterization of the Sobolev space norm (see Remark A.1 below and rescaling Proposition A.5 (below) and using the fact that {ψα} satisfies (i)–(iii) (see Remark A.1 below and the proof of Corollary A.6 for relevant details).

Proposition A.2

Let N9, h:=L/N, and ϵ:=h/10. The collection {ψα}αJ forms a smooth partition of unity satisfying

  1. (i)

    0ψ~α1 and sptψ~α(Qα(ϵ)+(2π)2),

  2. (ii)

    ψ~α=1 for all x(Cα(ϵ)+(2π)2) and α𝒥ψ~α(x)=1 for all x2,

  3. (iii)

    ψ~α=(h/(2π))2 and 4h/5ψ~αL2(Ω)6h/5,

  4. (iv)

    supα𝒥ψ~αH˙βh1-β for all β0.

Remark A.1

When σ0, let [σ] denote the greatest integer such that [σ]σ. Then define


where for 0<β<1, we define


Then H˙~σ is equivalent to (2.2) when σ0 (cf. [1, 9] and Proposition A.5). Indeed, there exists an absolute constant, C>0, such that for all ϕHperσ(𝕋2) with σ0, we have


Therefore, to see (iv), let ψ~α𝐤:=(𝐤ρ)h/10*ψα, for 𝐤2{}. Observe that


and by Young’s convolution inequality we have


where C𝐤 depends on 𝐤ρ, but not h. On the other hand, by (A.2) one can show that


where C depends only on ρ,β, but not h. Thus, from (A.1), (A.3), (A.4), and (A.5) that for β>0, we have


as desired.

For ϕLloc1(Ω), define


where a(Q) denotes the area of Q and


At this point, let us emphasize that ψ~α are non-negative for each α𝒥. Observe that for each α𝒥, there exists an absolute constant c>0, independent of h,α,ϵ, such that


Finally, we define the smooth volume element interpolant by


and the “shifted” smooth volume element interpolant by


Observe that we have the following relation between h and Ih.

Lemma A.3

Let Ih,Ih be given by (A.7), (A.8). Let ϕLloc1(T2). Then:

  1. (i)


  2. (ii)


  3. (iii)

    ΛβIhϕ=Λβhϕ for β>0.

Now let us prove Proposition A.1 when Jh is given by either h or Ih, as defined by (A.7), (A.8), respectively.

Proof of Proposition A.1: Part I.

We will establish properties (0.1)(0.3) for Jh given by either (A.7) or (A.8). It will suffice to consider Jh=h given by (A.7). Indeed, by Lemma A.3, the fact that


and the triangle inequality, we have that the properties (0.1)(0.3) applied to Jh=h easily imply the corresponding properties for Jh=Ih given by (A.8).

Suppose then that Jh=h. Observe that for each x𝕋2 and α𝒥, we have that

nα(x):=#{Qα(ϵ):xQα(ϵ) for some α𝒥 and ψ~α(x)0}

is independent of h. In particular,


for some fixed positive integer n0, independent of N,h. It follows that


for some absolute constant C>0 depending only on n0.

We prove property (0.1). For 1p, it follows from (A.6), (A.10), and Hölder’s inequality that


for some absolute constant C>0 that depends on n0.

Next, we prove property (0.2). Suppose pq. It follows from (A.6), (A.10), and the Cauchy–Schwarz inequality that


for some absolute constant C>0, depending on n0.

To prove property (0.3), it follows from Proposition A.2 (iv), (A.6), and the Cauchy–Schwarz inequality that


as desired. Note that the absolute constant above depends on ψ~α, but is independent of h. ∎

To prove part II of Proposition A.1, we will require the following two results, the first of which is a fractional Poincaré-type inequality. The second provides an alternate characterization of Sobolev norms.

Lemma A.4

Lemma A.4 ([45])

Let QR2 be a closed square. Let 1qp< and δ,ρ(0,1). Then for ϕLp(Q), we have


where the suppressed absolute constant is independent of ϕ.

Proposition A.5

Proposition A.5 ([9])

Let 0<β<1. Then for ϕH˙perβ(T2), we have


We then have the following corollary.

Corollary A.6

Let 0<δ<1 and QR2 a closed square. Then for ϕHperδ(Q), we have



Let Q2 be a closed square and let x0 denote its center. Let Q0 denote the same square, but centered at the origin. Let ϕHperδ(Q) and ρ14 and let ϕx0(x)=ϕ(x+x0). Then observe by translating and rescaling, we have


where ϕ~(x)=ϕ((|Q|1/2/(2π))x). Thus, by setting p=q=2, then applying Lemma A.4 and Proposition A.5, we obtain


as desired. ∎

The next result adapts Corollary A.6 to modified local spatial averages. In particular, given a square Q2 and ϵ>0, define Q(ϵ)=Q+B(0,ϵ). Suppose that ψ~C(2) is an arbitrary non-negative function satisfying 0ψ~1, sptψQ(ϵ), and ψ~|Q>0. Then, given ϕLloc1(2), we define


Corollary A.7

Let 0<δ<1, and QR2 a closed square. Then for ϕHperδ(Q), we have



First observe that


Then by Hölder’s inequality, we have


It follows then from Minkowski’s inequality and convexity that


Therefore, by (A.6) and Corollary A.6, we obtain (A.11). ∎

Finally, we are ready to complete the proof of Proposition A.1 when Jh is given by (A.7) or (A.8).

Proof of Proposition A.1: Part II.

First suppose that Jh=h is given by (A.7). The case β=1 follows from the classical Poincarè inequality, so let β(0,1) and ϕH˙perβ(𝕋2). Thus, by Proposition A.2, (A.10), and Corollary A.7, it follows that


which proves property (1.1). To establish property (1.2), first observe that for g,hLloc1(𝕋2) we have


and by symmetry


It then follows from this and Proposition A.2 (ii) that for gLloc2(𝕋2), we have


Thus, given gH˙perβ(𝕋2), it follows from (A.13), Proposition A.2, and the Cauchy–Schwarz inequality that


Then Corollary A.7 implies


Therefore, by duality we have


which is precisely property (1.2).

Now let Jh=Ih be given by (A.8). To show that property (1.1) holds, first observe that h1=1 and Ih1=0. Given ϕLloc1(𝕋2) such that ϕ=0, it follows from the fact that h,Ih are linear and Lemma A.3 (i) that


Thus, property (1.1) for Jh=Ih and ϕVβ follows from Minkowski’s inequality, (A.9), and (A.12). To see property (1.2) for Jh=Ih, simply observe that if g=0, then (A.15) implies that


Recall that Lemma A.3 (ii) shows that Ihϕ𝒵 for any ϕLloc1(𝕋2). In particular, ϕ-Ihϕ𝒵. Thus, given ϕLper2(𝕋2)𝒵, it follows from duality and (A.16) that


Arguing as we did for (A.14), we have that Jh=Ih satisfies property (1.2), as desired. ∎

A.2 Modal Projection (Type II)

Here we let Jh be given by projection onto Fourier modes. The projection can be given by either rough or smooth cut-offs in the frequency side. The “rough projection” will be given by convolution with the square Dirichlet kernel, while the “smooth projection” will be given by Littlewood–Paley projection. As in the previous subsection, we work with rescaled variables, so that the 2π-periodic box is given by Ω=𝕋2=[-π,π]2.

Rough Modal Projection.

Let N>0. For k2, k=(k1,k2), denote by ϕ^(k) the k-th Fourier wave-number and define the “rough modal projection” by PN by




is the two-dimensional Dirichlet kernel. In particular, we have


Let us now prove Proposition A.1 with


Proof of Proposition A.1.

It is classical that Jh defined by (A.18) this way satisfies property (0.1) with constant independent of h (cf. [42]). One also has the following estimate on the Dirichlet kernel for q(1,) (cf. [42]):


where q and q are Hölder conjugates and the suppressed constant depends on q.

To show that PN satisfies property (0.2), we apply Young’s convolution inequality and (A.19) to obtain


To prove property (0.3), we apply Plancherel and estimate as follows:


Clearly, for β0, ΛβIh=IhΛβ by the Plancherel theorem, which proves property (2.2).

To prove property (2.1), let β>α. Then from (v), the Cauchy–Schwarz inequality, and the Plancherel theorem, it follows that


as desired. ∎

Smooth Modal Projection.

We define this projection by the Littlewood–Paley decomposition. We presently give a brief review of this decomposition. More thorough treatments can be found in [7, 29, 59, 61]. We state the decomposition for 2 for convenience, but point out that it is also valid in the case 𝕋2 for periodic distributions. In particular, the Bernstein inequality (Proposition A.8) stated below also hold in 𝕋2 provided that one redefine the Littlewood–Paley blocks, k, in a suitable manner (cf. [29]).

Let ψ0 be a smooth, radial bump function such that ψ0(ξ)=1 when [|ξ|14]2, and


Define ϕ0(ξ):=ψ0(ξ/2)-ψ0(ξ). Observe that


Now for each integer j0, define


Then, in view of the above definitions, we clearly have


If we let ϕ-1:=ψ0 and ϕj0 for j<-1, observe that

(A.20)jϕj(ξ)=1for ξ2.

One can then define


where ϕk:=ϕˇk is the inverse Fourier transform of ϕk. We call the operators k Littlewood–Paley projections.

One can show that (A.20) implies that

g=j-1jgfor all g𝒮(2),

where 𝒮(2) is the space of tempered distributions over 2.

For N>0, define Jh by


That properties (0.1)(0.3) and (2.1)(2.2) are satisfied by Ih defined in this way follows from the Bernstein inequalities (cf. [7]).

Proposition A.8

Proposition A.8 (Bernstein inequalities)

Let 1pq and gS(R2). Then for βR and j-1 we have


For σ0 and j-1, we have


where the suppressed absolute constants depend only on β,ϕˇ0,ψˇ0.

Indeed, let us prove Proposition A.1 for Jh=SN given by (A.21).

Proof of Proposition A.1.

Property (0.1) and property (0.2) follow immediately from the Bernstein inequalities. For property (0.3), simply observe that for β0, we have


To prove property (2.1), observe that for α,β, with βα, we have


Also, property (2.2) holds simply by applying the Fourier convolution theorem. Therefore, Jh given by (A.21) is of Type II. ∎


The authors would like to thank the Institute of Pure and Applied Mathematics at UCLA, where part of this work was performed. The authors would also like to thank Cecilia Mondaini for insightful discussions in the course of this work.


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Received: 2016-12-10
Accepted: 2016-1-4
Published Online: 2017-1-13
Published in Print: 2017-2-1

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