April Showers, Bringing May Flowers? …. Global Precipitation Climatology (Photo Credits: Sharon Sullivan, Global Precipitation Climatology Project)

April showers bring May flowers - or so we’ve been told - this phrase can be traced back to the mid-1500s from a collection of writings known as “A Hundred Good Points of Husbandry” - “Sweet April Showers do spring May Flowers”. But, with portions of the Southwestern U.S. still in moderate to severe drought categories, what does that mean for May flowers?

Precipitation is defined by the AMS Glossary as “liquid or solid phase aqueous particles that originate in the atmosphere and fall to the earth’s surface”. Precipitation is important for understanding climate evolution and hydrological applications ranging from agriculture to flooding. Extensive research has been undertaken in recent decades to develop a global precipitation climatology. The main source of precipitation climatology is from the Global Precipitation Climatology Project (GPCP), a joint effort that attempts to merge data from 6000 rain gauge stations, geostationary and polar-orbiting satellites, radar, and sounding observations in order to estimate total monthly rainfall.

Rain gauge observations have been used to estimate precipitation rate, however, the global distribution of gauges is population dependent and assumes there is no potential losses from the melting of frozen precipitation. Radar can be used to estimate rain rate through comparison of hydrometeor size to the radar reflectivity factor, but beam blocking, attenuation, and great distances from the radar can leave gaps in precipitation climatology. While satellites may tend to underestimate precipitation compared to gauges and radar, they can infer precipitation amounts over oceans, complex terrain, and sparsely populated areas by determining cloud types/depths through the visible and infrared channels.

Annual mean precipitation distributions in mm day-1. The global mean is 2.61 mm day-1, or 37.480 in year-1 (Global Precipitation Climatology Project)

On global, regional, and local scales, precipitation is controlled by the availability of water vapor, temperature, aerosols, cloud type, dynamics (latitude), and orography (local). The Northern Hemisphere tropics precipitate on average 50% more than the Southern, due to a greater proportion of land mass. Precipitation tends to follow a diurnal cycle, but this pattern is regionally dependent. During spring, the best of both precipitation dynamics - winter and summer - converge. The jet stream remains strong, holding onto the cold winter chill, as sunlight warms the lower atmosphere. The Rocky Mountain Range in the central United States typically experiences high-frequency precipitation events in the mid-afternoon summers and early fall (suggesting that high surface temperatures drive convective processes), while early morning rainfall may occur in Thailand and the Bay of Bengal during monsoon season. For New Mexico in particular, the summer monsoon months receive almost half the average precipitation for the year. The poles typically have lower precipitation amounts (<1 mm day-1) due to the lower water vapor content, as colder climates aren’t able to hold as much water vapor in the atmosphere above. Another important phenomena associated with global precipitation variability on interannual time scales is El Niño-Southern Oscillation (ENSO), where changes in sea surface temperature can modify the position of storm tracks in the Northern Hemisphere.

Thus, an accurate precipitation climatology is essential to improve model & satellite verification, make short-term forecasts more effective, and even help to predict shifts in global precipitation patterns in the future. Just as a reminder than even the most unpleasant of things (in this case, the heavy rains of April) can spring forth even the most enjoyable of things - the abundance of May flowers.

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©2019 Meteorologist Sharon Sullivan