Wednesday, April 23, 2008

and V-notch on radar

The same day, 22 April, few hours later, the same airmass that produced cells with V-shape on satellite image, also produced a High Precipitation tornadic supercell, detected by the WSR-98D S band radar in SE Romania. Doppler radial velocity image, at the bottom, has an intense mesocyclone and in the reflectivities, we can see the supercell structure, with a very pronounced "hook echo" to the S, and V-notch structure, produced by the flow splitting around a very strong updraft. Very high reflectivites, up to 70 dBZ, due to hail presence. Tornadoes were reported in Bulgaria and Romania

Tuesday, April 22, 2008

V-shape on satellite

An intense updraft lifts moisture high into the troposphere and strong upper level divergent winds move the extremely cold moisture (ice crystals) downwind. The moisture fans out as it moves downwind generating a V-shape of the cloudiness on a satellite image (see the cell situated in the South of Romania). The image (Eumetsat) was taken at at 16 UTC. The movement was from south to north so soon Bucharest was affected by the storm. The radar image shows the intense reflectivity (over the Bulgarian teritory). In this case, the satellite image provided more information about the severity of the system (strong updraft, V-shape).

Monday, April 21, 2008

Doppler shift

The Doppler effect was first applied to the apparent shifting frequencies of a train whistle as it approaches and then passes. When the sound is approaching, the waves are pushed together and the wavelength is shorter, and the frequency is higher, and the whistle is heard as a higher pitch. Then, where the waves are stretched, the wavelength is longer, and the frequency is less. So as a train departs, its whistle will sound at a lower pitch. That's the Doppler shift.
The Doppler shift works with light waves as well as sound waves. For example, astronomers have concluded that the universe is expanding because of the shift to longer wavelengths, or red shift, by distant galaxies. That shift indicates that these galaxies are moving away from the earth.

Doppler radar detects precipitation and measures the speed of falling precipitation.
When microwaves are used, the Doppler shift will indicate whether raindrops are moving away from the radar or toward it. That motion is called radial velocity.
These outgoing or incoming motions are color-coded on the Doppler radar so when a sharp change in direction takes place, the colors contrast sharply. That contrast indicates that a severe storm is present, even a tornado. That wind difference is called wind shear. You can demonstrate this easily by putting a pen between your two hands. Move your left hand in one direction and your right hand in the other. What happens to the pen? It will rotate. That rotation can be detected on the radar screen by looking at the color shift of the radial velocity. A strong shift is a tip-off that some severe weather is happening. The warning can be issued as much as 20 minutes before a tornado touchdown in a distance of less than 70 km from the radar; because of the great uncertainty of this forecast and due to the high impact of such a warning the Romanian Met service (and European met services in general) do not regulary issue these warnings.
(Doppler information from infoplease - web site)

Electromagnetic Interference

The space is overcrowded with EM signals and the radar receives them and very often can not distiguish them from other weather signals, so displays them. The radar image (right) shows some radial echoes detected by the Bucharest C-band radar . They are generated by the electromagnetic interference with some other antennas at south of the radar site. How do we know that they are not clouds besides this "artificial" radial aspect? Because in the satellite image (IR) (left), we do not see similar structures. The only meteo feature is the line of clouds oriented W-E over Bulgaria. The other echoes around the radar sites in Timisoara (western Romania) and Medgidia (south-eastern Romania) are ground clutter around S-band sites. Images are from the NMA site, Sunday morning.

Sunday, April 20, 2008

Tornadic Mezocyclone

Yesterday a tornado has been observed in the western Romanian town of Curtici. Here we present the radar image, seen by the S band Doppler radar from Timisoara (situated south of the tornado place) operated by the Romanian met Service. The radar echo was that of a supercell. A mesocylone was detected in the radial velocity field (left side of the image), highlighted with the white circle. The meso was identified due to the relative maximum in the winds, one for the inbound velocities, in green colors, one for the outbout velocities, in yellow, as a consequence of rotational movement. The meso overlaied the southern part of the supercell, in the "inflow" region of it, characterized by the low echo zone, situated right near the "hook" in the reflectivity field. The tornado video was posted by Youtube: