# Calculate precipitation from WRF model output

From WRF output file,

I find 3 variables related to the precipitation (unit: mm)

1. Precipitation

• variable name: pr
• unit:$kg/(m^{2}s^1)$
• variable: RAINC+RAINNC
• comments: $dt(RAINC+RAINNC)/DT$

2. Max hourly precipitation rate

• variable name: prhmax
• unit:$kg/(m^{2}s^1)$
• variable: RAINC, RAINNC or RAINMAX1Hclwrf
• comments: $MAX(dtsum()/DT)$

3. Convective precipitation

• variable name: prc
• unit:$kg/(m^{2}s^1)$
• variable: RAINC
• comments: $d_t(RAINC)/DT$

My task:

Summarize the amount of days which precipitation > 0.254 mm

What variable should I use for further analysis?

Any advice would be appreciated.

## 1 Answer

Use (1) Precipitation, as it is the sum of convective and non-convective precipitation, integrate it over one day, voilà.

converting [$kg/(m^2 s)$] (precip): $$precip [kg/(m^2 s)] \cdot dt [s] \cdot \frac{1}{ \rho_w [kg/(m^3)]} \cdot 1000 [mm/m] = total\_precip [mm]$$

where $\rho_w$ is the density of water.

Practically the last two factors cancel.

• Thanks for your reply. I have known the meaning of sum RAINS & RAINNS. But you answer let me know why the units change to $mm$. That's very helpful! – Han Zhengzu Mar 26 '16 at 11:09