improver.nowcasting.lightning module

Module for NowcastLightning class and associated functions.

class improver.nowcasting.lightning.NowcastLightning(radius=10000.0)[source]

Bases: improver.BasePlugin

Produce Nowcast of lightning probability.

This Plugin selects a first-guess lightning probability field from MOGREPS-UK data matching the nowcast validity-time, and modifies this based on information from the nowcast.

For each forecast time, the closest-in-time first-guess lightning probability slice is copied and modified thus:

1: Increase lightning probability where lightning is observed or is

nearby.

2: Increase lightning probability where heavy or intense precipitation

is observed.

3: Reduce lightning probability where precipitation is light or

absent.

4: Increase lightning probability where ice is likely in the observed

radar column.

In this doc-string, LR is an abbreviation for the Lightning Risk index output by the CDP (Convection Diagnosis Procedure) and the Met Office nowcast. LR can take five values. 5 is the lowest risk and 1 is highest.

The default behaviour makes these adjustments to the upper and lower limits of lightning probability: lightning mapping (lightning rate in “min^-1”):

upper: lightning rate >= <function> => lightning prob = 1.0 (LR1)

The <function> returns a linear value from 0.5 to 2.5 over a 6-hour forecast_period.

lower: lightning rate == 0.0 => min lightning prob 0.25 (LR2)

precipitation mapping (for prob(precip > 0.5 mm/hr)):

upper: precip probability >= 0.1 => max lightning prob 1.0 (LR1)

middle: precip probability >= 0.05 => max lightning prob 0.25 (LR2)

lower: precip probability >= 0.0 => max lightning prob 0.0067 (LR3)

heavy: prob(precip > 7mm/hr) >= 0.4 => min lightning prob 0.25 (LR2)

equiv radar refl 37dBZ

intense:prob(precip > 35mm/hr) >= 0.2 => min lightning prob 1.0 (LR1)

equiv radar refl 48dBZ

VII (vertically-integrated ice) mapping (kg/m2):

upper: VII 2.0 => max lightning prob 0.9

middle: VII 1.0 => max lightning prob 0.5

lower: VII 0.5 => max lightning prob 0.1

__init__(radius=10000.0)[source]

Initialise class for Nowcast of lightning probability.

Parameters

radius (float) – Radius (metres) over which to neighbourhood process the output lightning probability. The value supplied applies at T+0 and increases to 2*radius at T+6 hours. The radius is applied in “process” using the circular neighbourhood plugin.

_abc_cache = <_weakrefset.WeakSet object>
_abc_negative_cache = <_weakrefset.WeakSet object>
_abc_negative_cache_version = 213
_abc_registry = <_weakrefset.WeakSet object>
_modify_first_guess(cube, first_guess_lightning_cube, lightning_rate_cube, prob_precip_cube, prob_vii_cube=None)[source]

Modify first-guess lightning probability with nowcast data.

Parameters

  • cube (iris.cube.Cube) – Provides the meta-data for the Nowcast lightning probability output cube.

  • first_guess_lightning_cube (iris.cube.Cube) – First-guess lightning probability. Must have same x & y dimensions as cube. Time dimension should overlap that of cube (closest slice in time is used with a maximum time mismatch of 2 hours). This is included to allow this cube to come from a different modelling system, such as the UM.

  • lightning_rate_cube (iris.cube.Cube) – Nowcast lightning rate. Must have same dimensions as cube.

  • prob_precip_cube (iris.cube.Cube) – Nowcast precipitation probability (threshold > 0.5, 7, 35). Must have same other dimensions as cube.

  • prob_vii_cube (iris.cube.Cube) – Radar-derived vertically integrated ice content (VII). Must have same x and y dimensions as cube. Time should be a scalar coordinate. Must have a threshold coordinate with points matching. self.vii_thresholds. Can be <No cube> or None or anything that evaluates to False.

Returns

Output cube containing Nowcast lightning probability.

Return type

iris.cube.Cube

Raises

iris.exceptions.ConstraintMismatchError – If lightning_rate_cube or first_guess_lightning_cube do not contain the expected times.

static _update_metadata(cube)[source]

Modify the meta data of input cube to resemble a Nowcast of lightning probability.

  1. Rename to “probability_of_rate_of_lightning_above_threshold”

2. Remove “threshold” coord (or causes iris.exceptions.CoordinateNotFoundError)

  1. Discard all cell_methods

Parameters

cube (iris.cube.Cube) – An input cube

Returns

Output cube - a copy of input cube with meta-data relating to a Nowcast of lightning probability. The data array will be a copy of the input cube.data

Return type

iris.cube.Cube

apply_ice(prob_lightning_cube, ice_cube)[source]

Modify Nowcast of lightning probability with ice data from a radar composite (VII; Vertically Integrated Ice)

Parameters

  • prob_lightning_cube (iris.cube.Cube) – First-guess lightning probability. The forecast_period coord is modified in-place to “minutes”.

  • ice_cube (iris.cube.Cube) – Analysis of vertically integrated ice (VII) from radar thresholded at self.ice_thresholds. Units of threshold coord modified in-place to kg m^-2

Returns

Output cube containing updated nowcast lightning probability. This cube will have the same dimensions and meta-data as prob_lightning_cube. The influence of the data in ice_cube reduces linearly to zero as forecast_period increases to 2H30M.

Return type

iris.cube.Cube

Raises

iris.exceptions.ConstraintMismatchError – If ice_cube does not contain the expected thresholds.

apply_precip(prob_lightning_cube, prob_precip_cube)[source]

Modify Nowcast of lightning probability with precipitation rate probabilities at thresholds of 0.5, 7 and 35 mm/h.

Parameters

  • prob_lightning_cube (iris.cube.Cube) – First-guess lightning probability.

  • prob_precip_cube (iris.cube.Cube) – Nowcast precipitation probability (threshold > 0.5, 7., 35. mm hr-1) Units of threshold coord modified in-place to mm hr-1

Returns

Output cube containing updated nowcast lightning probability. This cube will have the same dimensions and meta-data as prob_lightning_cube.

Return type

iris.cube.Cube

Raises

iris.exceptions.ConstraintMismatchError – If prob_precip_cube does not contain the expected thresholds.

ice_thresholds = (0.5, 1.0, 2.0)

Expected thresholds for vertically-integrated-ice (VII) data. These are used for increasing prob(lightning) with column-ice data. Units are kg/m2.

Type

(tuple)

process(cubelist)[source]

Produce Nowcast of lightning probability.

Parameters

cubelist (iris.cube.CubeList) –

Where thresholds are listed, only these threshold values will

be used.

Contains cubes of

  • First-guess lightning probability

  • Nowcast precipitation probability

    (required thresholds: > 0.5, 7., 35. mm hr-1)

  • Nowcast lightning rate

  • (optional) Analysis of vertically integrated ice (VII) from radar thresholded into probability slices at self.ice_thresholds.

Returns

Output cube containing Nowcast lightning probability. This cube will have the same dimensions as the input Nowcast precipitation probability after the threshold coord has been removed.

Return type

iris.cube.Cube

Raises

iris.exceptions.ConstraintMismatchError – If cubelist does not contain the expected cubes.