Well, I don't know which Archie tape you watched but he does go into the gain thing quite heavily.
Most of the time you should leave the gain set to the preset position. Asdsuming you have a color scope, what this will do is preset the contours as follows:
Green - 10-19 dbz
Yellow - 20-29 dbz
Red - 30 or more dbz
These return levels are correlated by Trammel in terms of the danger they represent. Basically, he says stay out of the red because that has some legitimate potential to be a thunderstorm.
He points out that according to his research, all the accidents that have been partially attributed to thunderstorms and/or windshear have resulted from storms that were returning 50 dbz or more. This brings up the reason for the gain adjustment to be taken out of the pre-set position. The manufacturer should be abel to provide you with an index of how many dbz each hashmark represents when you hand-tune the gain. By using this you can shift the dbz thresholds for each color contour by redefining the gain values.
So here's how this works. Let's say you have a big return on your scope. It's got some yellow and of course some green around its fringes but red is the attention getter. All you know about the red is that it's 30 dbz or more. By adjusting the gain according to the calibration schedule you can shift the red return so that it appears at 40 dbz and then perhaps 50 dbz. What it will allow you to do is see where the storm is most intense. It will also let you see if the red threshold is just a 30 dbz return and nothing more than that.
Now, the big word of caution - if you take the gain control out of pre-set keep your mind on what you're doing. If you forget and leave it out of the pre-set mode you will have made the radar less sensitive making its indications misleading as to rainfall intensity and thus storm danger. The best rule to follow is whenyou're done playing with the gain put it back to pre-set.
Does that help?
