Trough Identification

 Thomas Schartner$*$ Institut für Meteorologie, Freie Universität Berlin

Version from June 9, 2016

$*$thomas.schartner@met.fu-berlin.de

This is only a brief documentation about the Trough Identification Plugin and is currently still under construction. Comments or any kind of feedback is highly appreciated. Please send an e-mail to the authors.

### 1 Introduction

Precipitation in the the equatorward part of the midlatitudes and the subtropics are connected with minima of geopotential height in the mid- and upper troposphere [Knippertz2003]. The mechanism is caused by the advection of positive vorticity in front of a moving upper-level trough. The advection of positive vorticity leads to divergence and dynamical lifting while the equatorward transport of cold air back of the trough axis leads to a destabilsation of the atmosphere. Upper Level troughs may but need not be induce surface cyclogenesis. In section 2, the methods of the calculation procedure are described [Knippertz2004] Sections 3 and 4 explain the input respectively the output of the TroughIdentification. In the last section (4) an example is given.

### 2 Methods

#### 2.1 Throug Identification

The trough identification scheme assumes a basically north-south-oriented axis of minimum geopotential height, which measures up the mostly wavy structures often found on the anticyclonic side of the polar jet, where troughs tend to stretch out meridionally and thin. The scheme should be applied on the 500-hPa geopotential height field. The following figures shows the most important steps for the identification:

First, 500-hPa geopotential height averages are calculated over a 3x4 gridpoint box ${Z}_{2}$ and the 3x5 gridpoint boxes ${Z}_{1}$ and ${Z}_{3}$ to the west and east of ${Z}_{2}$. The so-called trough parameter (TP) at the center point P of ${Z}_{2}$ is then defined as the difference between the mean over ${Z}_{1}$ and ${Z}_{3}$ minus the mean over ${Z}_{2}$. This reflecting the zonal geopotential height gradient assuming a longitudinal extension of the trough of about 2000 km (at 35$\circ$N).

If TP is greater than 25 gpm (lightly shaded area in figure 2), P is denoted as trough point. The point P with the maximum value TP along a longitudinal range of trough points is termed a trough axis point (TAP).

Several of those are aligned to a trough axis, provided that the TAPs on neighboring lines of latitude are not more than two grid point(5$\circ$) apart in the longitudinal direction (black lines in the conceptional sketch of figure 3). Upper-Level cyclones are characterized by ther nort-south axis and not by the location of their center.

A detailed discussion about the scheme and an application can be found in [Knippertz2004].

### 3 Input

The calculation of the trough identification is based on 6 hourly geopotential height field in 500 hPa.

 Outputdir Output directory mandatory default: /scratch/user/evaluation_system/output/troughidentification Cachedir Cache directory mandatory default: /scratch/user/evaluation_system/cache/troughidentification Cacheclear Option switch to NOT clear the cache. mandatory default: True Variabel geopotential height (zg) mandatory default: zg Project Choose project, e.g. reanalysis, cmip5, baseline1, baseline0 mandatory Product Choose product, e.g. reanalysis, output mandatory Institute Choose institute of experiment, e.g. MPI-M, ECMWF mandatory Model Choose model of experiment, e.g. MPI-ESM-LR, IFS mandatory Experiment Choose experiment name, e.g. decadal1971, ERAINT mandatory Ensemble Choose ensemble, e.g. r1i1p1, r2i1p1 or ”*” for all members mandatory default: * Firstyear Choose first year to be processed. Lastyear Choose last year to be processed. Level Choose level [in Pa], e.g. 50000 only reasonable for zg mandatory default: 50000 Ntask Number of tasks. mandatory default: 24 Makepic Set ”True” for make picture with tool movieplotter mandatory default: False Dryrun Set ”True” for just showing the result of find_files and set ”False” to process data. mandatory default: True Caption An additional caption to be displayed with the results

Table 1: Input options for Trough Identification

At first, you have to specify your output (Outputdir) and cache (Cachedir) directories. The data paths of input files can be selected via the typical MiKlip data structure. Choose the Project, Product, Institute, Model and Experiment you want to process. Further, select ensemble member(s) in the Ensemble field and specify the variable (Variable) you want to analyze. In Firstyear and Lastyear you can choose the range of years which will be processed. The upper level (Level up) and bottom level (Level down) can be chosen. Finally, you have the option to visualize some results (Makepic), to remove the cache directories (Cacheclear) and to show the found input file(s) from your input parameters based on freva - -databrowser (Dryrun).

### 4 Output

The processed files can be found in the selected Outputdir. The ti file contain the trough parameter, the trough points, the trough axis points, the trough axis, the number of through points and the number of trough axis points. If selected, the trough axis is visualized in a time loop for the first ten time steps.

### References

Peter Knippertz. Tropical-extratropical interactions causing precipitation in northwest africa: statistical analysis and seasonal variations. Monthly weather review, 131(12):3069–3076, 2003.

Peter Knippertz. A simple identification scheme for upper-level troughs and its application to winter precipitation variability in northwest africa. Journal of climate, 17(6):1411–1418, 2004.