Rain: Fixed Period
Definition: Record of the amount of precipitation within a given "fixed" period, and a daily estimate of the water balance.
This datatype has four choices corresponding to frequencies of "Daily" 0900 reading (metcode=181), "Hourly" (metcode=182), "Synoptic" (three hourly) (metcode=185) and "Hourly Rain_Table" (metcode=1820, see below).
There are two sources of rain data (i.e. rain gauges) at many NIWA EWS (Electronic Weather Station) sites. These are a standard tipping bucket gauge (normally an Ota 0.2mm/tip) and a RIG (Rain Intensity Gauge).
In the RIG gauge, the rain is passed through a tube to produce equi-sized drops and the drops are counted every 6 seconds. The RIG gauges are ideal for analysing the high temporal variability of rainfall. More information on the RIG gauge can be found at The Optimization and Calibration of a Rain Intensity Gauge in the "Journal of Atmospheric and Oceanic Technology" (published by the American Meteorological Society who hold the copyright). Note an AMS subscription may be required to view the document.
Notes
- Synthesis of the hourly data is processor intensive. Periods of more than a year and two or more stations may cause the browser to time-out.
- Hourly rain is a synthesis of data from the "RAIN" (tipping bucket) and the "RAIN_RATE" (RIG) tables. Hourly rain will first look in the RAIN_RATE table. If there is no data in the "RAIN_RATE" table then the "RAIN" table will be used. The data ORIG_OBS_ORIGIN will indicate the "true" source of the hourly data. The "rain_rate" table is considered the more accurate as it uses NIWA's RIG (Rain Intensity Gauge) gauges and data from digitised and scanned pluviographs.
- The "Hourly Rain_Table" datatype gets hourly rain from the RAIN table only i.e. no synthesis with data in the RAIN_RATE (Rig) table. Sometimes, particularly in heavy rain periods where the origin is "O" (Dynes Pluviograph (Digitised)) the two sources may be significantly different.
- As the RIG gauge has a much higher temporal resolution, rounding errors can produce differences in rainfall amounts when compared with the tipping bucket gauge. These differences tend to balance out over a period of several hours.
- "Daily" rain values (at NIWA EWS stations) are measured only from the tipping bucket gauge (RAIN table). As noted above, the daily total from hourly values may be slightly different than the daily stored value.
- If the time standard being used is local, then in the change over from NZST to NZDT, the 0200 local reading will not be present due to the time moving forward one hour at 0200.
- The "Synoptic" choice will produce hourly data if the hourly data exists - use this mainly for searching for three hourly synoptic data.
- WARNING: In the past it has been standard climatological practice to allocate the 0900 local time daily (DLYCLI) reading to the PREVIOUS DAY since 16 hours of the 24 relate to it. The Climate Database stores this rainfall at the ACTUAL date and time of the MEASUREMENT ie the data is NOT not put back a day for normal CliFlo output.
See also: