Fixed-box method

Overview

The fixed-box tracker (tracking_method: "fixed_box") defines a time-independent analysis center. At all time steps, FrameIt uses the same grid point as the cyclone center, computed once from a user-defined geographic location.

This option is useful when:

the region of interest is fixed (site-focused diagnostics),
tracking variables (MSLP, winds) are not available or not reliable.

Inputs and outputs

Input dataset

The model dataset must provide:

a time coordinate named time,
latitude and longitude coordinates, either: - 1D latitude and 1D longitude (typical AROME-like rectilinear grid), or - 2D latitude and 2D longitude (typical Meso-NH curvilinear grid).

Configuration

Minimal configuration

tracking_method: "fixed_box"

# Fixed center location, [lat, lon] in degrees
fix_subdomain_center: [-17.5, 60.0]

Optional (recommended) parameters

# Atmospheric model identifier used to interpret the latitude/longitude geometry
atm_model: "AROME"     # or "MNH"

Key definitions

tracking_method: Must be set to "fixed_box".
fix_subdomain_center: Fixed center location given as [lat, lon] (degrees).

Algorithm

Step 1, read the fixed target location

The tracker reads the target point from the configuration:

(lat0, lon0) = fix_subdomain_center

Step 2, locate the closest model grid point

FrameIt selects the nearest grid point by minimizing a squared distance in latitude/longitude space.

Rectilinear grid (latitude 1D, longitude 1D)

If latitude and longitude are 1D arrays:

cy = argmin_j (lat[j] - lat0)^2
cx = argmin_i (lon[i] - lon0)^2

Curvilinear grid (latitude 2D, longitude 2D)

If latitude and longitude are 2D arrays:

dist2(j, i) = (lat(j, i) - lat0)^2 + (lon(j, i) - lon0)^2
(cy, cx) = argmin_{j,i} dist2(j, i)

Step 3, build a time-dependent output

The index pair (cy, cx) is repeated for each model timestamp, producing:

cy(time) = constant
cx(time) = constant

Practical considerations and limitations

Longitude convention: ensure that the fixed longitude and the dataset longitudes use the same convention (for example [-180, 180] or [0, 360]). A mismatch can produce an incorrect nearest-point selection.
Distance metric: the method uses a squared Euclidean distance in (lat, lon) space, not a true geodesic distance. This is generally adequate for small domains.
Masked coordinates: if latitude/longitude contain NaNs (masked areas), the minimization must avoid all-NaN distance fields. If all candidate distances are NaN, the selection will fail.
Fixed-box versus extraction box: the fixed-box tracker defines the center used by FrameIt. The extraction domain size is controlled separately (for example x_boxsize_km and y_boxsize_km).

Example

atm_model: "AROME"
tracking_method: "fixed_box"

fix_subdomain_center: [-18, 58.0]

x_boxsize_km: 500.0
y_boxsize_km: 500.0

Illustration

Here is an illustration for the tropical cyclone Batsirai, using the fixed box method.