Can Horizontal Radiation Fluxes Contribute to Net Column Heating?

So, About Those Sideways Radiation Fluxes… Do They Actually Heat Things Up?

We all know the sun warms the Earth, right? But when we talk about how much a particular chunk of atmosphere heats up, we usually focus on the radiation coming straight down from above and going straight back up. That’s the vertical radiation flux. But what about the radiation sneaking in from the sides? Those horizontal radiation fluxes – do they actually matter? Turns out, it’s a pretty interesting question, and one that scientists are still actively digging into.

Think of “net column heating” as the atmosphere’s energy bank account. Is a particular column of air gaining energy (heating up) or losing it (cooling down)? This balance is mostly about what comes in from the sun and what radiates back out into space. Simple enough, except it’s not just about up and down.

Horizontal radiation fluxes are all about energy moving sideways into or out of our atmospheric column. Why would energy move sideways? Well, imagine a hot air mass bumping up against a cold one. Or a big, fluffy cloud sitting next to a patch of clear blue sky. These differences in temperature and cloudiness create what we call “horizontal gradients,” and these gradients can drive radiation sideways.

Now, you might be thinking, “Sideways radiation? Seems kinda minor.” And you’d be right… usually. Most of the time, the vertical fluxes are the big players. But there are definitely situations where those horizontal fluxes can punch above their weight.

Here’s when sideways radiation can be a bigger deal:

• Sunrise and Sunset: Ever notice how long the sun seems to linger on the horizon during sunrise or sunset? That longer path means more opportunity for sideways radiation to do its thing.
• Cloud Shenanigans: Picture a single, lonely cloud floating in an otherwise clear sky. That cloud is going to be radiating energy outwards in all directions, including sideways. That’s a net loss of energy for the air column containing the cloud.
• Stuff in the Air: Different gases in the atmosphere absorb and emit radiation differently. So, if you have a pocket of air with a lot of water vapor next to a drier pocket, you’ll get horizontal radiative fluxes.
• Zooming In: Think big picture, and sideways fluxes tend to average out. But zoom in on a smaller scale – like a single thunderstorm – and those horizontal fluxes can become much more important.

Okay, so how much do these sideways fluxes matter? Some studies suggest they can change the heating rate by a noticeable amount. But pinning down an exact number is tricky.

So, why don’t we always include horizontal radiation in our calculations? A few reasons:

• Brain-Melting Math: Calculating horizontal radiative transfer is seriously complicated. It requires a super-detailed understanding of what’s happening in the atmosphere in three dimensions.
• Big Picture Thinking: When you’re looking at the planet as a whole, over long periods, those sideways fluxes tend to even out.
• Data, Data, Where’s the Data?: Getting good measurements of radiation moving in all directions is tough.

The bottom line? While vertical radiation fluxes are the heavy hitters, horizontal radiation fluxes aren’t always negligible. As our climate models get more and more sophisticated, it’s important to consider these sideways effects, especially in areas with funky cloud patterns or big changes in atmospheric composition. There’s still plenty to learn about how these sideways fluxes contribute to the overall energy balance of our atmosphere. It’s a complex puzzle, but piecing it together will help us better understand and predict our climate.