Plot explanations CARIBIC

Some explanations for the use of the CARIBIC meteorological plots.

Trajectories For every 3 minutes of flight a 5-day backward trajectory has been calculated using the horizontal and vertical wind components from the ECMWF model. The wind fields were given on the hybrid sigma-pressure ECMWF model levels at a 1x1 degree horizontal resolution at time intervals of 6 hours. We use linear interpolation in longitude, latitude and log-pressure and cubic interpolation in time to obtain the wind at the trajectory positions. The trajectory integration scheme is the iterative scheme proposed by Pettersen with a time step of 1 hour. For every flight one plot shows all trajectories. Colouring indicates the instantaneous trajectory pressure. For flights where whole air samples (WAS) were taken subsets of trajectories corresponding to the time interval air was let in for each air sample have been made. There are trajectory plots for all 12 individual air samples as well as all samples together.
The trajectories have been calculated with the KNMI trajectory model TRAJKS (Scheele, et al., Meteorol. Appl. 3, 267-273 (1996).
There is a description of the trajectory file format for CARIBIC.

"Surface" maps

CC=cloud cover=fraction of area covered by clouds (between 0 and 1)
High clouds: above 6 km
Medium clouds: between 2.5 and 6 km
Low clouds: below 2.5 km
Total CC: fraction of area covered by clouds (at any height)
H2O column: The total amount of vertically integrated water vapour. If this is large, clouds and precipitation are likely.
6 hr LS Prec: Large scale precipitation accumulated over the last 6 hours.
6 hr Con Prec: Convective precipitation accumulated over the last 6 hours. In the ECMWF model convective and large scale precipitation are separate physical modules. Convective precipitation is a sub-grid scale process calculated in the parametrisation of convective clouds. Large scale precipitation depends only on the grid cell averaged meteorological properties.

Pressure level maps

PV: Potential vorticity calculated from the wind and temperature fields provided by ECMWF. Units are 0.1 PVU. Contouring starts at 1 PVU. Shading at 2 PVU. The tropopause corresponds to a value of about 3.5 PVU at mid-latitudes. The PV of the tropopause at mid-latitudes is usually between 2 and 3.5 PVU.
Wind: shows the wind vectors and contours of the wind speed. Shading occurs above wind speeds of 40 m/s, which allows identification of jet streaks.
W=the vertical wind speed dp/dt in Pa/s. Upward winds are negative and shaded from yellow to red. Downward winds are positive and shaded from green to blue. In general the maximum values of the upward wind are larger than those of the downward wind. The reason may be that upward motions can be intensified by latent heat release (condensation). The vertical wind has its largest values in the mid-troposphere (500 hPa). The reason is that at the surface the vertical wind should approach zero (no flow through the surface) and above the tropopause the increased vertical stability prohibits strong vertical motions.
Spec hum or q in kg/kg is the amount of water vapour per kg air. Shading goes from dry (green) to blue (moist). D and M indicate local dry and moist extremes. When plotted at 250 hPa the dry regions correspond to the stratosphere and the moist regions to the troposphere. The dry regions will coincide with regions with higher PV.
RH: relative humidity in %. This is the relative humidity relative to water when the temperature is above 0 C, and relative to ice when the temperature is below 0 C. No shading or green shading indicates dry areas and blue and purple shading moist areas. At 250 hPa the dry areas will correspond the the stratosphere and the moist areas to the troposphere (compare the PV map). At 500 hPa the moist areas will correspond to cloudy regions, often with upward vertical winds. Note that the relative humidity will decrease (increase) during downward (upward) adiabatic motions. During such motions the specific humidity is conserved.
EpotT: Equivalent potential temperature (in K). Gradients in EpotT are a very good indicator for fronts, since fronts are usually associated with gradients in humidity or temperature or both. The shading is from yellow (cool and dry) to red (warm and moist). Only a small range of EpotT contours is shaded for CARIBIC. The shaded region is typical for subtropical air. The polar air (cold, north of yellow) and tropical air (warm, south of red) are not shaded. The map at 700 hPa will closely correspond to surface analyses of fronts. It also allows identification of warn conveyor belts (tongues of warm moist air, often found moving northward and upward on the eastern edge of extratropical cyclones).

Vertical X-sections These profiles have been interpolated linearly in latitude, longitude and time to the aircraft location. The ECMWF data were available at 6 hour intervals and with a horizontal resolution of 1 degree in longitude and latitude for CARIBIC. I have chosen a time step of 3 minutes between subsequent profiles (aircraft locations). The reason not to have a higher time resolution is that only 360 profiles can be plotted with our current software. A typical CARIBIC flight lasts more than 10 hours.

PV: shows the potential vorticity (red contours) with values between 1 and 5 PVU shaded (from yellow to red). Isotachs (wind speed contours) are shown as blue dashed contours. (Dry) isentropes (potential temperature) are shown as quasi-horizontal blue dashed-dotted contours. Moist isentropes (equivalent potential temperature) are shown as quasi-horizontal light-green dashed-dotted contours. In the stratosphere (which moisture is almost absent) the moist isentropes and dry isentropes coincide. In the troposphere, in moist regions, moist isentropes can get strongly vertically inclined. Closed or buckled dry isentropes hardly ever occur as dry instability is very rare. However, closed or buckled moist isentropes, typical of moist instability do occur in the lower troposphere. These are regions where convection is likely. The 400 K isentrope is plotted in purple (not blue or green). This isentrope more or less coincides with the top of the lowermost stratosphere (middle world) and the tropical tropopause.
Eq pot T: Equivalent potential temperature isocontours (moist isentropes) in red. The (dry) isentropes are shown as green dashed-dotted isolines and coincide with the moist isentropes in the dry stratosphere. Buckled or closed equivalent potential temperature contours indicate (moist) vertical instability, hence likelihood of convection. The 400 K isentrope is plotted in purple (not blue or green). This isentrope more or less coincides with the top of the lowermost stratosphere (middle world) and the tropical tropopause. One can also identify the location of fronts by closely packed isentropes (strong gradients). Other quanties shown are the isotachs (wind speed) as blue dashed isocontours and selected PV isocontours (1.5, 2, 3, 3.5 PVU) as purple dotted isolines indicative of the tropopause region.
Spec hum: isocontours of specific humidity in red. Shading is from green (dry) to blue (moist). In some plots the troposphere in yellow due to an error in the plot software, which should be remedied in later software releases. This X-section allows easy identification of tropopause folds as tongues of dry air (often even local dry extremes) extending downward into the troposphere. Also shown are wind (blue dashed), potential temperature (quasi-horizontal blue dot-dashed), equivalent potential temperature (green dot-dashed) and selected PV-values typical of the tropopause (purple dotted). The 400 K isentrope (near the top of the plot at 100 hPa) is also purple.
RH: shows relative humidity in % with red isocontours. RH is relative to water vapour at temperatures above 0 C and relative to ice below 0 C. Shading starts at 10 % in yellow and ends in blue at 100 %. This plot allows identification of very moist (blue) regions where clouds are likely. Also shown are wind (blue dashed), potential temperature (quasi-horizontal blue dot-dashed), equivalent potential temperature (green dot-dashed) and selected PV-values typical of the tropopause (purple dotted). The 400 K isentrope (near the top of the plot at 100 hPa) is also purple.
Wetb potT: Shows Wet bulb potential temperature (C) as red isocontours. Is useful as an indicator of instability and thunderstorms when is isocontours are buckled or closed over a deep layer near the surface. Its use is very similar to equivalent potential temperature. Near fronts the isocontours become closely packed (large gradient). Also shown are wind (blue dashed), potential temperature (quasi-horizontal blue dot-dashed) and selected PV-values typical of the tropopause (purple dotted). The 400 K isentrope (near the top of the plot at 100 hPa) is also purple.
U and V: eastward and northward wind components. These plots allow easy identification of jets, which most often occur at tropopause height (note the behaviour of the tropopause near the upper level jets). They may also occur a few km above the surface (low level jets). Low level jets are usually accompanied by strong surface fronts, and in the extratropics may be indicative of conveyor belts. Also shown are potential temperature (quasi-horizontal blue dot-dashed), equivalent potential temperature (green dot-dashed) and selected PV-values typical of the tropopause (purple dotted). The 400 K isentrope (near the top of the plot at 100 hPa) is also purple.
W: vertical velocity dp/dt in 0.1 Pa/s (micro-bar/s). Green to blue shading indicates ever stronger downward motions. Yellow to red shading indicates ever stronger upward motions. In regions with strong upward motion cells clouds will in general be present. Also shown are potential temperature (quasi-horizontal blue dot-dashed), equivalent potential temperature (green dot-dashed) and selected PV-values typical of the tropopause (purple dotted). The 400 K isentrope (near the top of the plot at 100 hPa) is also purple.
Cloud water and cloud ice contents in kg/m3: Show the presence (and pronouncedness) of clouds, Also shown are potential temperature (quasi-horizontal blue dot-dashed), equivalent potential temperature (green dot-dashed) and selected PV-values typical of the tropopause (purple dotted). The 400 K isentrope (near the top of the plot at 100 hPa) is also purple.
Cloud cover: fraction of the area of the model grid cells covered by clouds (0 to 1). Also shown are potential temperature (quasi-horizontal blue dot-dashed), equivalent potential temperature (green dot-dashed) and selected PV-values typical of the tropopause (purple dotted). The 400 K isentrope (near the top of the plot at 100 hPa) is also purple.

Please note that the ECMWF cloud products have not been as extensively validated as traditional model parameters like wind and temperatures.

Interpolated data The ECMWF data have been interpolated linearly in latitude, longitude, log-pressure and time to the aircraft location. The ECMWF data were available at 6 hour intervals and with a horizontal resolution of 1 degree in longitude and latitude on hybrid sigma-pressure ECMWF model levels for CARIBIC. I have chosen a time step of 1 minutes between subsequent points (aircraft locations). For most flights measured and model simulated temperatures are within a few K. The measured and model simulated specific humidities do not agree as well as the temperatures. Best agreement is obtained in the middle and lower troposphere where moisture is relatively abundant. In the very dry upper troposphere and lower stratosphere the measured specific humidity is frequently too high. There are well-known problems both with modeling and measuring such low humidities.

CARIBIC participants are welcome to contact Peter van Velthoven if they have any further questions.