How do the various forms of precipitation map into the IMERG "probabilityLiquidPrecipitation" data field?
IMERG provides a data field that estimates the probability that the retrieved precipitation amount is “liquid”, which is defined to include “mixed” (liquid and solid) precipitation. In retrospect the field name should have been “ice”, but “liquid” had already been set. The rational is that mixed precipitation is very rare and transient, so it should be lumped with either “liquid” or “ice”. Furthermore, the primary effects of “ice” are to 1) prevent the falling precipitation from immediately entering the hydrological system (until it melts), and 2) to create (potentially) dangerous travel conditions. “Mixed” typically ends up not creating either of these effects, so lumping it with “liquid” seems appropriate.
Even given this basic definition, there are numerous forms of precipitation, and it might not be obvious how they end up being classified in IMERG. The key fact is that the phase is computed diagnostically at present, based on work by Guosheng Liu (Florida State University) and students. The Liu scheme uses data from a numerical model or model analysis to compute a “specification”, without reference to the satellite data, including whether or not IMERG estimates that precipitation is occurring, or even possible to estimate. Thus, probabilityLiquidPrecipitation (pLP) is a globally complete field whenever the relevant model data exist. An additional factor is that there is a conceptual difference between how the half-hourly phase is computed and how phase is defined in this probability framework for the monthly data. We will handle the half-hourly first, for which the Liu specification equation is directly relevant.
Liu determined that the primary factor for phase is the surface wet bulb temperature (Tw), a combination of temperature and humidity, with small contributions from the low-altitude Tw lapse rate and the surface pressure, and with systematic differences between ocean and land areas. In practice, the fitted probability as a function of Tw is converted to separate look-up tables for ocean and land.
Typical results for different forms of precipitation are:
- Rain: Ordinary falling liquid typically happens for Tw>0°C, so pLP is high.
- Freezing Rain: Liquid that freezes upon contact with the Earth's surface typically falls in Tw<0°C, so pLP is low.
- Snow, ice pellets, snow pellets: These frozen hydrometeors occur around or below Tw<0°C, so pLP varies from around 50% to very low.
- Sleet: Frozen droplets (U.S. definition) typically fall in Tw<0°C, so pLP is usually below 50%.
- Mixed snow and rain; falling slush: The mixed category is likely to occur around the pLP=50% mark. If one uses 50% as a liquid/solid threshold, that implies that mixed cases will end up in both categories, depending on the details.
- Hail: Hail typically occurs when the surface air temperature is well above freezing (i.e., on summer afternoons). Thus, pLP is very high. But, hail is even rarer than mixed and unlikely to be correctly specified in this scheme, and anyway, in such conditions it rapidly melts and so is properly lumped into "liquid".
- Dew and frost: These phenomena are not forms of precipitation. They are liquid or solid water that condenses directly at the Earth's surface. For this reason, any amount of surface accumulation due to dew or frost is not included in the IMERG precip estimate.
As the time interval for the data values lengthens, it becomes increasingly likely that both liquid and solid might have fallen, at which point the meaning of pLP should change to “what fraction of the estimated precipitation amount fell as liquid or mixed?” This is the definition of pLP for both the monthly IMERG Final Run pLP and the set of GIS IMERG files (TIFF+WorldFile) providing estimated accumulations longer than three hours.