The solar_radiation function computes modern daily total extraterrestrial solar radiation received at the top of Earth's atmosphere.

This function is quite similar to the daily_insolation function, and one may suit your needs better than the other. The daily_insolation function is best suited for investigations involving orbital changes over tousands to millions of years, whereas solar_radiation may be easier to use for applications such as present-day precipitation/drought research.

## Syntax

`Ra = solar_radiation(t,lat)`

## Description

Ra = solar_radiation(t,lat) calculates the extraterrestrial radiation (MJ m^(-2) day^(-1)) based on the dates t and latitude lat. Dates t can be in datetime, datenum, or datestr format, but must be a 1D array or a scalar. lat can be scalar, vector, or grid. If lat is a vector or array with size nrows and ncols then Ra has the size [nrows,ncols,length(t)].

## Example 1: A solar panel in Berlin

Let's assume you have a 1 square meter solar panel in Berlin, Germany (52.5N,13.4E). If there were no atmosphere, how many joules of energy would it receive per day from March 1, 2017 to March 1, 2019?

```% define an array of dates:
t = datetime('march 1, 2017'):datetime('march 1, 2019');

% define the latitude of Berlin:
lat = 52.5;

% Plot the time series:
figure
plot(t,Ra)
axis tight
box off % removes frame
```

## Example 2: Solar energy in the world's oceans

Before we begin this example, it's important to note that the solar_radiation function estimates the top-of-atmosphere radiation, so the following discussion ignores all atmospheric effects. With that caveat in mind let's consider how much solar energy hits the Earth's oceans on a given day.

To begin, pick a day. How about St. Patrick's Day, March 17. Estimating how much energy hit the Earth's oceans on St. Patrick's Day means we have to make a global grid and calculate the solar radiation at each point in that grid. A very dense grid will be more accurate, but whereas a very coarse grid will require less memory. Let's use cdtgrid to create a half-degree global grid, and calculate the radiation at each point on that grid:

```% Make a half-degree grid:
[Lat,Lon] = cdtgrid(1/2);

% Calculate Ra on St. Patrick's Day:

% Plot
figure
imagescn(Lon,Lat,Ra)
hold on
earthimage('watercolor','none')
cmocean solar
cb = colorbar;
title 'St. Patricks Day'
```

Above, the land surface was plotted with earthimage and the colormap was set with cmocean.

Now you may be wondering, how much energy does the ocean receive compared to land? Answering that question requires us to know the area of each grid cell to compute the total energy received at each grid cell. Use cdtarea to get the area of each grid cell, and multiply Ra by area to get the total energy received per grid cell on St. Patrick's Day:

```% Get area of each grid cell:
A = cdtarea(Lat,Lon);  % (m^2)

% Calculate total energy received per grid cell:
E = Ra.*A;
```

So which receives more total solar energy--the land or the ocean? use island to determine which grid cells correspond, then sum up the solar energy that hits land and ocean:

```% Get a mask of grid cells that are land:
land = island(Lat,Lon);

% Total solar energy that hits land on St Patrick's Day:
sum(E(land)) % MJ
```
```ans =
4.0392e+15
```

That's 4*10^15 MJ of solar energy that hit the land surface on St. Patrick's Day. Versus ocean:

```sum(E(~land)) % MJ
```
```ans =
1.1155e+16
```

That's 1*10^16 MJ that hit the ocean. Not surprisingly, the ocean is has a larger surface area than land, so it receives more solar energy.

We can expand this analysis to changes in time by entering t as an array of times, like this, where we calculate daily solar radiation for every day in the year 2019:

```t = datetime('jan 1, 2019'):datetime('dec 31, 2019');

```

Above, we entered 1x365 datetime array t and a 360x720 grid of latitudes Lat. The resulting Ra is then 360x720x365, which corresponds to a gridded solution for each day of the year.

Multiply Ra by grid cell area A to get the gridded time series of energy received at each grid cell, and use the local function to get time series of energy incident on land versus ocean:

```% Daily energy is Ra times grid cell area:
E = Ra.*A;

% Sums of land and ocean energy:
E_land = local(E,land,@sum);
E_ocean = local(E,~land,@sum);

figure
plot(t,E_land)
hold on
plot(t,E_ocean)
plot(t,E_land+E_ocean,'k','linewidth',2)
axis tight
box off
legend('land','ocean','total')
ylabel 'Earth''s daily energy received (MJ)'
```

The plot above shows that at all times of the year, the ocean receives more solar energy than the land surface. That is unsurprising, because the ocean's surface is larger than Earth's land surface. But there are some other interesting things that are happening there too.

The curve for land may not surprise the majority of Earth's population, who live north of the equator. If you live in the northern hemisphere you may see that and think, "yes, the Earth is warmer in the summer." But of course when it's summer in the nothern hemisphere, it's winter in the southern hemisphere, so shouldn't things equal out?

No. Because most's of Earth's land area is in the northern hemisphere, so in terms of sums, the Earth's land surface receives more energy in June than in December.

When we look at how solar energy at the ocean's surface changes throughout the year, we see just the opposite of what we see on land. That's partly because the ocean has more surface area in the southern hemisphere than the northern hemisphere, but that doesn't fully explain the dip in the ocean's energy in June and July.

Looking at the total solar energy received, we see that it's not constant throughout the year. In fact, the Earth is farthest away from the Sun around July 4th of every year, so on the whole, that's when the Earth receives the least amount of solar energy.

## References

The solar_radiation function computes the equation described in:

• Allen, R.G., Pereira, L.S., Raes, D., and Smith, M.: Crop evapotranspiration Guidelines for computing crop water requirements FAO Irrigation and Drainage Paper 56, Food and Agriculture Organization of the United Nations, 1998
• McCullough (1968) McCullough, E.C.: Total daily radiant energy available extraterrestrially as a harmonic series in the day of the year, Arch. Met. Geoph. Biokl., Ser. B, 16, 129-143, 1968.
• McCullough and Porter (1971) McCullough, E.C. and Porter, W.P.: Computing clear day solar radiation spectra for the terrestrial ecological environment, Ecology, 52, 1008-1015, 1971.
• McMahon, T. A., et al. "Estimating actual, potential, reference crop and pan evaporation using standard meteorological data: a pragmatic synthesis." Hydrology and Earth System Sciences 17.4 (2013): 1331-1363 https://doi.org/10.5194/hess-17-1331-2013.