# reduce-v2 v3.2.3

**Mercurial revision 5786:5c8b45563631 | 2026-08-04**

---

## Description

`reduce-v2` is a tool used to **filter Mog2D archives** using a **Lanczos time filter**. It is developed and used by **CTOH** (Centre de Topographie des Océans et de l'Hydrosphère). The tool computes and writes **clx-formatted filtered Mog2D archives**.

---

## Usage

```bash
reduce-v2 [OPTIONS]
```

---

## Options

> **Note:** If a default is not specified, the option is **mandatory**.

### **Input/Output Options**


| Option                        | Description                                                |
| ----------------------------- | ---------------------------------------------------------- |
| `-h <name>`                   | Root name for **elevation archives**. Default: `analysis`. |
| `-p <name>`                   | Root name for **pressure archives**. Default: `forcing`.   |
| `-iF <format>`, `-f <format>` | Input archive format: `clx` or `cdf`.                      |
| `-oF <format>`                | Output archive format: `clx` or `cdf`.                     |
| `-i <path>`                   | Path for input archives. Default: `./`.                    |
| `-o <path>`                   | Path for output filtered archives. Default: `./`.          |


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### **Filtering Options**


| Option             | Description                                                                                     |
| ------------------ | ----------------------------------------------------------------------------------------------- |
| `-w <days>`        | Filtering window in days. Default: `20.0`.                                                      |
| `-start <MM/YYYY>` | Starting month for filtering request.                                                           |
| `-end <MM/YYYY>`   | Ending month for filtering request.                                                             |
| `-r <int>`         | Ratio factor for archive reduction. Default: `1`.                                               |
| `-m <method>`      | Detiding method to remove S2 signal: `analysis`, `climatology`, or `none`. Default: `analysis`. |
| `--pressure-only`  | Skip elevation processing.                                                                      |
| `--ssh-only`       | Skip pressure processing.                                                                       |
| `-hf`              | Also remove high-frequency signals with a **12.5-hour filter**.                                 |


---

### **Advanced Options**


| Option                      | Description                                         |
| --------------------------- | --------------------------------------------------- |
| `-no_default`               | Disable default values for `-h` and `-p`.           |
| `-pressure_factor <factor>` | Pressure factor. Default: `1.0`.                    |
| `-mesh <file>`              | Path to the mesh file.                              |
| `-ssh <varname>`            | Variable name for elevation (default: `elevation`). |
| `-pressure <varname>`       | Variable name for pressure (default: `p`).          |


---

### **Deprecated Options**


| Option | Replacement       |
| ------ | ----------------- |
| `-d`   | Use `-m` instead. |


---

## Examples

### **Basic Filtering**

Filter elevation and pressure archives from January 2000 to December 2000:

```bash
reduce-v2 -start 01/2000 -end 12/2000 \
  -i ./input_data \
  -o ./output_data \
  -iF cdf -oF cdf
```

### **Filter with Custom Root Names**

Use custom root names for elevation and pressure archives:

```bash
reduce-v2 -h custom_elevation -p custom_pressure \
  -start 01/2000 -end 12/2000 \
  -i ./input_data \
  -o ./output_data
```

### **Detide with Climatology Method**

Remove S2 signal using the `climatology` method:

```bash
reduce-v2 -m climatology \
  -start 01/2000 -end 12/2000 \
  -i ./input_data \
  -o ./output_data
```

### **High-Frequency Filtering**

Remove high-frequency signals (12.5-hour filter):

```bash
reduce-v2 -hf \
  -start 01/2000 -end 12/2000 \
  -i ./input_data \
  -o ./output_data
```

### **Pressure-Only Filtering**

Skip elevation and filter only pressure archives:

```bash
reduce-v2 --pressure-only \
  -p custom_pressure \
  -start 01/2000 -end 12/2000 \
  -i ./input_data \
  -o ./output_data
```

### **Custom Filtering Window**

Use a custom filtering window of 30 days:

```bash
reduce-v2 -w 30.0 \
  -start 01/2000 -end 12/2000 \
  -i ./input_data \
  -o ./output_data
```

### **Resampling**

Reduce archive size with a resampling factor of 2:

```bash
reduce-v2 -r 2 \
  -start 01/2000 -end 12/2000 \
  -i ./input_data \
  -o ./output_data
```

---

## Environment Variables

`reduce-v2` uses **OpenMP** (version 201511) for parallelization. Key environment variables:


| Variable          | Description                               |
| ----------------- | ----------------------------------------- |
| `OMP_NUM_THREADS` | Number of threads to use.                 |
| `OMP_SCHEDULE`    | Runtime schedule type and chunk size.     |
| `OMP_DYNAMIC`     | Enable/disable dynamic thread adjustment. |
| `OMP_NESTED`      | Enable/disable nested parallelism.        |


**Example:**

```bash
OMP_NUM_THREADS=6 reduce-v2 -start 01/2000 -end 12/2000 -i ./input_data -o ./output_data
```

> **Note:** If running on a busy machine, limit threads to the number of **free CPUs** to avoid performance degradation.

---

## Notes

1. **Input Formats:**
  - Supported formats: `clx` (binary, deprecated) and `cdf` (NetCDF).
  - **NetCDF (`cdf`)** is the recommended format.
2. **Detiding Methods:**
  - `analysis`: Uses harmonic analysis to remove S2 signal.
  - `climatology`: Uses precomputed climatology (requires `S2-elevation-atlas.nc` or `S2.ele.s2c`).
  - `none`: No detiding is applied.
3. **High-Frequency Filtering:**
  - The `-hf` option applies an additional **12.5-hour filter** to remove high-frequency signals.
4. **Resampling:**
  - The `-r` option reduces the number of frames in the output archives by the specified factor.
5. **Output:**
  - Filtered archives are saved in the specified output directory (`-o`).
  - Output filenames include the root name, year, and month (e.g., `analysis-filtered-2000.01.nc`).
6. **Performance:**
  - For large datasets, ensure the output directory (`-o`) is on a **fast storage device** (e.g., SSD).
  - Adjust `OMP_NUM_THREADS` based on available CPU resources.

---

## See Also

- [CTOH (Centre de Topographie des Océans et de l'Hydrosphère)](https://ctoh.legos.obs-mip.fr/)
- [OpenMP Documentation](https://www.openmp.org/)