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For an initial state vector X, if
${{M}}$ is used to represent the nonlinear prediction model, the nonlinear equation can be expressed as (Buizza and Palmer, 1995):Here () means
${{M}}$ is a function of${X}$ . Then, a small disturbance${X'}$ is superimposed on the state vector. Over a finite period, the small disturbance can be represented by a linear approximation:where
${{M}_{\rm{l}}}$ is a linear approximation of the nonlinear model${{M}}$ , which can be expressed as:in which
${{L}}(t,{t_0})$ is the forward tangent linear operator,${{X}}'({t_0})$ is the perturbation at the initial time, and${{X}}'(t)$ is the evolved perturbation after linear integration from the initial time${t_0}$ to the evolved time$t$ . The superscript ' is omitted from the following equations, and the state vectors discussed hereafter are all perturbations.The solution of the SV can be achieved with the largest ratio of the evolved perturbed vector to the initial perturbed vector:
where
$\left[, \right]$ denotes the Euler product and${{E}}$ is the transform operator. The state vector${{X}}$ of the physical space needs to be transformed into the dimensionless vector${{\tilde X}}$ in Euler space using a transform operator${{E}}$ :Then, Eq. (4) can be written as:
Equation (5) can be converted into a singular value decomposition problem:
where
$\lambda $ is the eigenvalues of the matrix${{{({{E}}\cdot{{L}}}}\cdot{{{E}}^{ - 1}}{{)}}^{\rm{T}}}\cdot{{({{E}}\cdot{{L}}}}\cdot{{{E}}^{ - 1}}{{)}}$ . The singular value of${{{{E}}\cdot{{L}}}}\cdot{{{E}}^{ - 1}}$ is$\sqrt \lambda $ .Many studies have shown that using the energy norm to calculate SVs can obtain mesoscale baroclinic perturbations (Hoskins and Coutinho, 2005; Diaconescu and Laprise, 2012; Liu et al., 2013).
The input and output variables of the GRAPES TLM and adjoint models in the process of solving the GRAPES-SV are the perturbed wind speeds
$(u',v')$ , the TL vectors of the perturbed Exner pressure ($\prod ' = \prod - {\prod _{\rm{r}}}$ ), and the perturbed potential temperature$(\theta ' = \theta - {\theta _{\rm{r}}})$ . Based on these perturbed variables, the energy norm formula can be expressed as (Liu et al., 2013):where
${\rho _{\rm{r}}}$ is the reference density,${c_p}$ is the constant-pressure specific heat,${T_{\rm{r}}}$ is the reference temperature,${\theta _{\rm{r}}}$ is the reference potential temperature,${\prod _{\rm{r}}}$ is the reference dimensionless air pressure, and$\varphi $ is the latitude of the spherical coordinate.Ehrendorfer et al. (1999) studied the perturbation growth characteristics of SVs under moist physics and proposed that the use of moist energy norms may lead to the unclear growth of disturbances. Hoskins and Coutinho (2005) pointed out that the SV with perturbed water vapor is similar to that without perturbed water vapor, so the dry energy norm is a better choice for calculating SVs. ECMWF uses the total energy norm (i.e., the dry energy norm without water vapor) for calculating SVs. To make a clean comparison between the moist TLM and dry TLM, this paper also uses the energy norm without water vapor.
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The linearized large-scale condensation process developed by Liu et al. (2019) is the large-scale cloud and precipitation scheme developed by Tompkins and Janisková (2004). This scheme describes the relationship between moist processes and clouds. The governing equation for humidity is:
where
$C$ is the condensation or evaporation process of clouds on a large scale,${E_{{\rm{prec}}}}$ is the change in water vapor by the evaporation of precipitation, and${D_{{\rm{conv}}}}$ is the source term of water vapor for the convection outflow.The governing equation for temperature is:
where
$n$ and$m$ signify the freezing of rainwater and the melting of snow, respectively,$L_{\rm{a}}$ is the latent heat coefficient of evaporation/sublimation, and${L_{\rm{a,f}}}$ is the latent heat coefficient of freezing and melting.To avoid the abnormal growth of some false perturbations in the TLM, some constraints and conventions are derived from ECMWF (2017). It can be seen from the above formulae that the calculation of SVs by large-scale condensation is mainly reflected in the temperature and humidity. However, since the SV adopts the dry energy norm, the effect of large-scale condensation is mainly reflected in the temperature term.
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The initial conditions of the control forecast in the ensemble prediction system are derived from the GRAPES 4DVAR method (Zhang et al, 2019) with a horizontal resolution of 0.5° and 60 vertical layers (The parameters of SV is showed in Table 1). After 60 iterations with the Lanczos algorithm, approximately 30 SVs can be produced in GRAPES-GEPS. These 30 SVs can be combined with random (Gaussian) linear combinations to form 30 initial perturbations. In this study, the evolved SVs were not involved in the initial perturbation, and the model perturbation was shut down. The complete test period ranged from 1200 UTC 1 May to 1200 UTC 5 May 2019, and from 1200 UTC 16 May to 1200 UTC 20 May 2019, spanning a total of 10 cases, the evolved time of the initial perturbation was 48 h, the ensemble forecast time was 240 h, and the forecast interval was 24 h. During this period, the atmospheric circulation in the Northern Hemisphere exhibits the characteristics of summer. In addition, the monsoon begins, meaning precipitation is abundant in South China.
Parameter Setting Horizontal resolution 2.5° × 2.5° Vertical levels 60 layers Southern Hemisphere 80°−20°S Northern Hemisphere 20°−80°N Optimal time interval 48 h Norm Energy norm Table 1. SV calculation parameters.
In the original SV calculation process, only the linearized boundary layer scheme, including the terrain parameterization scheme and vertical diffusion, is used. In this study, large-scale condensation was added to calculate the SV, and the result was compared with that of the original SV scheme. The experimental setup is shown in Table 2.
Test name Linearized physical process DRY-SV Linearized PBL scheme MOIST-SV Linearized PBL + large-scale condensation Table 2. Different test sets of linearized physical processes in the SV calculation.