# Presentation: 799777

## Diabatic Digital Filter Initialization For Tropical Cyclone Model Forecasting

- Chi-Sann Liou
- Naval Research Laboratory
- (JHT sponsored project)

## Unbalanced Initial Conditions

**Unbalanced Initial Conditions**

## Unbalanced Initial Conditions

_**Unbalanced Initial Conditions**_

## Static Initialization: (nonlinear normal initialization)

- Dynamic Initialization:
- Balance condition: high frequency tendency=0 at initial time
- Method: damp or filter out high frequency components through back and  forth time integrations
- Advantage: diabatic forcing is included in getting balance conditions
- Disadvantage: cost more

## Digital Filter

- A very selective low pass filter
- Using truncated inverse Fourier transform to remove high frequency components from input signals

- In Frequency Domain:

## Dynamic Initialization with Digital Filtering

**Dynamic Initialization with Digital Filtering**

- Adiabatic:

- Diabatic:

## Issues related to Diabatic Digital Filtering

-  Asymmetry in back and forth integrations
-  Lateral boundary conditions
-  Surface boundary conditions
-  Diffusion
-  Moving grids
-  Cost of the extra time integration
- ===> Shorten the back and forth time integrations

- Use a efficient window in the inverse Fourier transform
- Cutoff Period = 2 hours

## Response Functions with Windows

**Response Functions with Windows**

## Initialization with Digital Filtering

_**Initialization with Digital Filtering**_

## Slide 10

## DDF Impact on COAMPS® Track Forecast:

- depend on how unbalanced initial conditions are
- larger improvements shown in later forecast periods
-   (15 72h forecasts with OI analysis)

## Implement Diabatic Digital Filter Initialization To HWRF

-  Routines to compute weights of digital filtering
-  Routines to apply the weights to prognostic variables
-  Routines to control the initialization time integration

- prepare a FORTRAN-90 module that includes all new routines for DDF initialization
- add new arrays and namelist variables for DDF to the NMM registry file
- write a driver routine to control time integration in different phases of the initialization integration
- integrate the DDF time integration controller into HWRF forecast model.

- Goal: Minimize HWRF code changes ==> single point interface
-           Minimize resource requirement ==> all local work arrays

## Implement Diabatic Digital Filter Initialization To HWRF

-   ddf_init.F:
- allocate and initialize inner meshes
- allocate work arrays
- compute weights
- use ESMF clock utilities to control DDF integrations
- call ddf_integrate and ddf_interface (recursive calls in handling nest grid integration)
- deallocate work arrays

## Summary

- With the Dolph-Chebyshev window and 2-h cutoff period, diabatic digital filtering (DDF) can effectively remove unbalanced initial conditions of a tropical cyclone
- Adiabatic digital filtering only marginally improves initial conditions for tropical cyclone forecast
- Modification to initial conditions by DDF depends upon the degree of unbalance in the initial conditions
- DDF improves track forecast of COAMPS®
- DDF has been implemented to a test version of HWRF and is currently under test