Radar doesn't differentiate between a cumulonimbus and simply a heavy downpour of rain. Certain discriminating modes such as windshear prediction do look at the movement of the moisture that's reflecting the radar energy and make certain elementary calculations, as does the turbulence mode to a certain degree. However, when all is said and done, what you're seeing is a return on moisture. In general, the larger the moisture droplets, the greater the return. However, the greater the water density or saturation, and the higher the moisture content in the cloud, also the greater the return.
When approaching a return, scanning up and down to examine the return is important, as well as necessary to help discriminate between cloud and ground, or even cloud and abberations caused by an imperfect radome or dish problems. The latter two can cause different features such as lobe projections (streaks going away from the aircraft-center of the return) or simply cause attentuation, reflection, refraction, or dark spots that don't show what's reallyout there. These returns and the character they impose on what you see on the radar changes with temperature and moisture; if you have a poorly sealed radome, for example, and you're flying in moisture, you're going to be accumulating moisture in the radome and the character of what you'll see will change. Especially with changes in the range.
When you do shift from long range to shorter ranges, then you're going to see the instensity and of course size of the return change, as depicted on your screen.
Beyond about 80 miles, you're not seeing much of value anyway; if you've been watching at longer ranges and then come in, suddenly you're going to see appear what may not have appeared before as you look at a narrower band with a greater concentration of energy, and closer reflective ranges. You can work with the intensity squelch feature of your set to change what you're interpreting at longer distances. Suffice it to say that if you're not frequently tilting up and down as well as ranging in and out, you can easily miss important information on radar. Radar is every bit as much about what you're not seeing, as what you are seeing.
So far as rapidly building cells or the turbulence that grows between them...your parents may have told you not to walk between parked cars in a parking lot, but rather to walk around them. That was good advice which also applies to flying between cells in a line of thunderstorms. Very strong shears can exist between them, not simply vertical shears inside the storms. Vertical shears can be far in excess of 12,000 fpm; vertical shears can be created as the pressure changes in the airmass draw air from higher pressure to low.
Last year while flying a thunderstorm penetration in a Lear 35A, I was working at night, adjacent to a very strong return with a very steep gradient. This was indicated by a thin line of green a thin line of yellow, then all red. I was on the upshear, or upwind side of the cell. It was very sizeable. My purpose in being there was to take samples of the atmosphere around the storm, and start working into some of the developing cells on the upshear side. A thundercell isn't simply a system of vertical currents; it's constantly forming on the upshear side, constantly dying and falling apart on the downshear side, even though it doesn't look that way. A steady wind blows through the storm, from upwind to downwind, as the storm is constantly being created and dying. My mission was to find an area of maximum icing in the cells that were being created, and begin firing flares and pyrotechnics into the building cells from the minus ten degree level so they could be tracked and recorded with some specially enhanced ground based radar. Aircraft were flying beneath the storm base taking readings, as well.
I made several passes, getting a little closer each time, through the narrow green edgeof the cell. We were in and out of cloud a lot in the dark, with both radar and visual cues by lightening to guide us. I passed the cell and started a left turn around one side of the cell, with nothing but black on the radar scope. At that point we entered a building cell on the upshear side, apparently. We immediately got the shaker and then the pusher. We fell sharply and the aircraft rolled right past 90 degrees. Everything that wasn't tied down went up. One of the systems operators in back broke his headset (my headset that he was wearing). My laptop computer, in a paded case in the baggage section) was pulled apart, with the internals actually pulled out of the computer case), and it hurt. We had flown through a minor shear.
It became very quiet. I applied maximum thrust, pushing the thrust levers to the stops and then retarding them an inch, applied forward stick pressure, and recovered to wings level as we flew out of a stalled, unusual attitude condition; unusual because we were nose down steeply, rolled past 90 degrees, stalled, without much aerodynamic response to control input. Due to the violence of the encounter, I elected to recover at a nearby field and re-evaluate, before going back out.
Don't underestimate a cell. With up and down shears that can damage an aircraft all by themselves, the sum of their velocities are compounded; a 6,000 fpm downshear against the same value going up (entirely possible) is a 12,000 fpm shear. It can break your airplane and do things to you that you probably can't imagine. The upset we experienced was fairly minor. We were in a small, robust airplane. We were also in somewhat of a remote area, in the dark. It can, and will rattle you. It can, and will disorient you. It can cost you control of the aircraft rapidly and make recovery difficult. It can rapidly place you in an unusual attitude, cause engine stalls or flameouts, tumble instruments. Negative loadings can damage wings and control surfaces, cause oil or fuel starvation, do engine damage, and of course, hurt people inside the airplane.
I always like to compare the thunderstorm to the finger of God. We're the mosquito, and there you have an apt comparison to where you stand in your aircraft (be it a single engine Cessna, four engine B747, or built-like-a-tank F-16) in the face of a real cell. Nothing to fool with, everything to respect, always something to avoid if you can at all do so.