Yes, Rain is Becoming Heavier

Always worthwhile to pass along an explainer from someone like Kevin Trenberth.
More understanding on why a warming climate makes precipitation more intense, with longer dry spells, and related fires, also more likely.
Above, Energy Secretary Chris Wright insists you not believe your lying eyes.
Former NOAA expert Jim Kossin pushes firmly back.

Kevin Trenberth in the Climate Brink:

Not only have temperatures already risen by 1.5°C, mainly since the 1970s, increases in heat waves have also occurred and caused substantial damages. Many other extremes have also increased and are related to global warming, but in much less obvious ways. As temperatures rise, it seems fairly reasonable that there will be more high temperatures. But it is more than the overall rise in temperatures that is in play. Changes also relate to location, especially land versus ocean, and weather and weather patterns.

As temperatures rise, the water holding of the atmosphere increases by about 7% per °C (a physical law called the Clausius-Clapeyron equation), and this is observed to be happening over the oceans (where the supply of water is unlimited). The relative humidity tends to remain about the same on average. The reason is that rain events occur when the relative humidity exceeds about 85%. So if there is too much moisture, it rains out the excess. If it is too dry, then it doesn’t rain and evaporation occurring at the surface increases relative humidity.

Moreover, because the amount of moisture depends on temperatures, the values differ enormously with latitude and height. In mid-latitudes, typical column amounts are about 2.5 cm (an inch), but values can easily be double that in the tropics and subtropics, or less than half at high latitudes. The near-global average atmospheric water vapor has increased by 7% since the 1990s (Figure below) but these numbers are dominated by the tropics and vary with phenomena like El Niño.

Weather systems reach out and gather in moisture typically from over a distance about four times (3 to 5) the diameter of the precipitating area. The biggest rainfalls occur then when an atmospheric river or winds bring in moisture from lower latitudes and especially tropical regions. Tropical storms and hurricanes are accompanied by copious rainfalls. In this way, rainfall rates can greatly exceed that expected simply from emptying the atmosphere of moisture in a spot.

Moreover, as the moisture condenses to form rain, it gives back to the atmosphere the latent heat that was used to evaporate the moisture in the first place. This adds buoyancy and rising motions to the storms, and further enhances rainfalls.

Because land temperatures are rising faster than ocean temperatures (Figure above), as air moves from the ocean to the land as part of the hydrological cycle (ocean evaporation, wind moisture transport to land, rain on land, runoff and flow of water in rivers back to the ocean), the relative humidity of air drops. This is now clearly observed on land even as the total moisture has increased. Because onset of rainfall depends on relative humidity, rain events on land become less frequent. But when rain events are triggered by weather systems, they become larger, more intense and more likely to cause flooding, as is also observed to be happening.

The more technical details matter and are briefly outlined here. Heat from the Sun occurs mainly at the surface, and nearly all weather systems (clouds, thunderstorms, hurricanes, cyclones) systematically move heat upwards to where it can be carried around by winds and ultimately radiated back to space. 

Such events cannot happen if the atmosphere is stable, as in strong anticyclones. Thus, the vertical temperature structure of the atmosphere matters. Meteorologists have a metric for how stable the atmosphere is: Convective Inhibition (CIN). Meanwhile the tendency for near-surface warm moist air to destabilize and start to rain is measured by another metric: Convective Available Potential Energy (CAPE). Both CIN and CAPE vary spatially, especially with monsoons, the Hadley Circulation and the Walker Circulation, throughout the tropics and subtropics. 

With climate change, CIN tends to increase over continents because of lower relative humidity, while CAPE tends to increase nearly everywhere, especially because of increased surface evaporation and low level moisture amounts. Where it is not raining, the result is longer dry spells, increased droughts, heatwaves and wildfires. Where it is raining, there is a greater risk of heavy rains, or snow, and greater risk of floods. Water is the great air conditioner! All of these phenomena increase with global heating and cause huge disruption and costs, as well as loss of life.