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.TH trace
.SH NAME
.B trace - trace meteorological fields along trajectories
.SH SYNOPSIS
.B trace
.I inpfile
.I outfile
[
.I optional arguments
]
.SH DESCRIPTION
Trace meteorological fields along the trajectories given in the input file 
.I inpfile
and write a new trajectory file
.I outfile
. The meteorological fields to trace are listed in a 
.I
tracing file 
(default: tracevars). Partly they can be computed "online" (see ONLINE CALCULATIONS below), normally they are availbale on the primary and secondary P and S files.
.SH PARAMETERS
.TP 15
.I inpfile
input trajectory file; the appendix determines the format (see
.B reformat 
for details).
.TP 15
.I  outfile
output trajectory file; the appendix determines the format (see
.B reformat 
for details).
.SH TRACING FILE
Normally the meteorological fields for tracing are listed in a file with name 
.B "tracevars". 
The name of the tracing file can be changed with the optional argument "-v" (see below). The format of the tracing file is as follows:
.br
.TP 5
Format
.I field[:shift]
.I scale
.I computation 
.I prefix  
.TP 5
Shifts (optional)
.B - field:+100km[lat] 
- get field at trajectory position + 100 km shifted to north. A shift to south is obtained with field:-100km[lat].
.br
.B - field:+100km[lon] 
- get field at trajectory position + 100 km shifted to east. A shift to west is obtained with field:-100km[lon].
.br
.B - field:+2[dlat] 
- get field at trajectory position + 2 grid spacings dlat shifted to north. A shift to south is obtained with field:-2[dlat].
.br 
.B - field:+2[dlon] 
- get field at trajectory position + 2 grid spacings dlon shifted to east. A shift to west is obtained with field:-2[dlon].
.br 
.B - field:+50hPa 
- get field at trajectory position + 50 hPa shifted in vertical. A shift to lower pressures is obtained with field:-50hPa.
.br 
.B - field:+1dp 
- get field at trajectory position + 1 grid spacing DP shifted in vertical. A shift to lower pressures is obtained with field:-1dp. Note that DP is not fixed but varies with height.
.br 
.B - field:+6h 
- get field at trajectory position, but 6 h in the future. Shifts to the past are poeeible with field:-6h. In addition to hours (h), the time shift can be specified in minutes (min).
.TP 5 
Examples
.B - TH 1. 0 S : 
trace potential temperature (TH), scale it with 1 (no scaling); it is available on the S file (no computation is needed: 0).
.br
.B - Q 1000. 0 P :
trace specific humidity (Q), scale it with 1000 to have g/kg; it is available on the P file (no computation is needed: 0).
.br
.B - RH 1. 1 * :
trace relative humidity (RH), no scaling is needed (1.); relative humidity is not available on either P or S file and must be computed (1). 
.br
.B - TH:100hPa 1. 0 S :
As in the first example, but now the potential temperature is taken 100 hPa below the air parcel position.
.SH OPTIONAL ARGUMENTS
.TP 15
.TP 15
.I -i hours
time increments (in hours) for input P and  S files. If not explicitely specified, this is determined from the P and S files i
n the current directory.
.TP 15
.I -v varfile
Change the name of the tracing file from its default value "tracevars" to "varfile".
.TP 15
.I -f field scale
Trace field (with scaling scale) along the trajectories; the computation flag and the prefix for the data file is automatically set. This options allows the quick tracing of a field, without specifying a tracing file.
.TP 15
.I -changet
flag whether the times of the P and S files should be changed or not before a calculation; the default is that the
times are 
.B not 
changed. 
.TP 15
.I -noclean
flag whether parameter and criterion files should be kept; this is particularly helpful for debugging.
.TP 15
.I -timecheck
enforce a time check on the data file
.SH VECTOR FIELDS
Note for tracing of vector fields, e.g wind (U,V), there are two options:
.TP 5
- the tracing variable in the tracing field can be given as U, for instance, and then the component U will be traced along the trajectory; but this U is the wind component in the rotated coordinate system, i.e. the zonal wind along the rotated latitude circles!
.TP 5
- if the tracing variable is specified as U.V, a vectorial transformation is applied, i.e. the wind components are transformed into true zonal and meridional wind components. This option is physically more meaningful!  
.TP 0
Note that ALL vector fields (A,B) should be traced in their vectorial form A.B; the components A and B in the rotated system are not physically meaningful!
.SH ONLINE CALCULATIONS
If the computation flag in the tracing file is set to 1, a meteorological field is calculated based upon the already traced fields and/or based on the fields on the primary and secondary P and S files. The following fields are implemented for online calculations:
.TP 5
.B - TH
potential temperature (in K).
.TP 5
.B - RHO
density (in kg/m^-3).
.TP 5
.B - RH
relative humidity (in %).
.TP 5
.B - THE
equivalent-potential temperature (in K).
.TP 5
.B - LHR
latent heating rate (K per input time step, typically K/6h). 
.TP 5
.B - D[U,V,T,TH]DX
horizontal derivative d[U,V,T,TH]/dx in west-east direction along pressure surfaces - zonal distance in m. U=zonal wind component (m/s), V=meridional wind component (m/s), T=temperature (deg C or K), TH=potential temperature (K). 
.TP 5
.B - D[U,V,T,TH]DY
horizontal derivative d[U,V,T,TH]/dy in south-north direction along pressure surfaces -meridional distance in m.  
.TP 5
.B - D[U,V,T,TH]DP
vertical derivative d[U,V,T,TH]/dp - pressure p in Pa.
.TP 5
.B - NSQ
squared Brunt-Vaisala frequence (in m^-2).
.TP 5
.B - RELVORT
relative vorticity (in s^-1) - RELVORT = DVDX - DUDY.
.TP 5
.B - ABSVORT
absolute vorticity (in s^-1) - ABSVORT = DVDX - DUDY + F, F being the Coriolis parameter.
.TP 5
.B - DIV
horizontal divergence of the velocity field (in s^-1) - DIV = DUDX + DVDY.
.TP 5
.B - DEF
horizontal deformation of the velocity field (in s^-1) - DEF = SQRT( ( DVDX + DUDY )^2 + (DUDX-DVDY)^2 ).
.TP 5
.B - PV
Ertel potential vorticity (in PVU) - PV = g * ( ABSVORT * DTHDP + DUDP * DTHDY - DVDP * DTHDX ).
.TP 5
.B - RI
Richardson number - RI = NSQ / (DUDP^2 + DVDP^2 ).
.TP 5
.B - TI
tubulence indicator according to Ellrod & Knapp - TI = DEF * SQRT( DUDP^2 + DVDP^2 ) * ( RHO * G).
.TP 5
.B - DIR
wind direction relative to zonal flow: (U,V)=(1,1) -> 45 deg; (U,V)=(1,-1) -> -45 deg; (U,V)=(-1,-1) -> -135 deg; (U,V)=(-1,1) -> 135 deg. A westerly flow has 0 deg, a southerly flow 90 deg, and a northerly one -90 deg. 
.TP 5
.B - DIST0
spherical distance (in km) from starting position.
.TP 5
.B - DIST
length of the trajectory (in km): integrated along great circle sections between the trajectory vertices.
.TP 5
.B - HEAD
heading of the trajectory: (DX,DY)=(1,1) -> 45 deg; (DX,DY)=(1,-1) -> -45 deg; (DX,DY)=(-1,-1) -> -135 deg; (DX,DY)=(-1,1) -> 135 deg. A path increment to east has heading of 0 deg; to the north 90 deg; to the south -90 deg; and to the west -180 deg. 
.SH EXAMPLES
.TP 5
.B [1] trace TRAJECTORY.1 TRAJECTORY.1 -changet
Read the trajectory file TRAJECTORY.1, trace all fields in the file "tracevars" along the trajectories and overwrite the existing trajectory file. In preparation, all times on the P and S files are changed prior to the tracing.
.TP 5
.B [2] trace INPTRA.1 OUTTRA.1 -f PV 1.
Trace PV (with scaling factor 1.) along the trajectories in trajectory file "INPTRA.1" and write a new trajectory file "OUTTRA.1".
.TP 5
.B [3] trace INPTRA.1 OUTTRA.1 -f PV:-100HPA 1.
As in example [2], but the PV is taken at a position 100 hPa higher (lower pressure) than the air parcel's position.
.TP 5
.B [4] trace INPTRA.1 OUTTRA.1 -f DIST0 1.
Get the spherical distance (in km) of the air parcel from its starting position.
.TP 
.B [5] trace INPTRA.1 OUTTRA.1 -f U.V 1.
The wind (U,V) is traced along the trajectory; the two component U and V are treated as part of a vector (U,V) and the components are transformed into true zonal and meridional winds. 
.SH AUTHOR
Written by Michael Sprenger and Heini Wernli (January 2011).