“How often does it really rain?”

By Kevin E. Trenberth* and Yongxin Zhang**, Published Online: 2 August 2017

Weather image courtesy NCAR
Weather image courtesy NCAR

*National Center for Atmospheric Research1 (NCAR) Boulder, CO, USA



Hourly precipitation observations at 0.25° allow much improved estimates of the frequency of rain or snow. Precipitation occurs 11% of the time but only 8% of the time over land.

The perception about whether a place is a nice place to live often depends on how often it rains (or snows). The frequency relates to how dreary the weather appears, and it is the duration much more than the amount that clouds perceptions.

Yet information about the frequency of rainfall, or precipitation in general, is spotty at best. Here, we analyze a new near-global (60°N-60°S) dataset at hourly time scales and 0.25° resolution.

The dataset, the newly calibrated CMORPH, enables comparison of results with 3-hourly and daily data, which is what has previously been available, and seasonal aspects are also examined.

The results are quite sensitive to both the spatial scales of the data and its temporal resolution, and it is important to get down to hourly values to gain a proper appreciation of the true frequency.

At 1° resolution values are 35% higher than for 0.25°. At 3-hourly resolution they are about 25% higher than hourly, and at daily resolution they are about 150% higher than hourly on average.

Overall, near-global (60°N-60°S) precipitation occurs 11.0±1.1% (1 sigma) of the time or, alternatively, 89.0% of the time it is not precipitating.

But outside of the ITCZ and SPCZ, where values exceed 30%, and the arid and desert regions, where values are below 4%, the rates are more like 10% or so, and over land where most people live, values are closer to about 8%.

email: trenbert@ucar.edu

1 NCAR is sponsored by the National Science Foundation.

Source: http://journals.ametsoc.org/doi/10.1175/BAMS-D-17-0107.1

About NCAR

The National Center for Atmospheric Research (NCAR) is a federally funded research and development center devoted to service, research and education in the atmospheric and related sciences. NCAR’s mission is to understand the behavior of the atmosphere and related Earth and geospace systems; to support, enhance, and extend the capabilities of the university community and the broader scientific community, nationally and internationally; to foster the transfer of knowledge and technology for the betterment of life on Earth. https://ncar.ucar.edu/.

The Climate Data Guide: CMORPH (CPC MORPHing technique): High resolution precipitation (60S-60N). Retrieved from https://climatedataguide.ucar.edu/climate-data/cmorph-cpc-morphing-technique-high-resolution-precipitation-60s-60n.