• 近日，下载了ERA5-hourly-reanalysis的降水资料，发现数据单位是m，这与一般观测卫星例如GPCP直接显示的mm/day不同，所以在绘图分析前最好进行单位换算，一般网上的解决办法大都人云亦云，这里简单记录一下正确的换算方式
  首先声明一下我下载的降水资料是单气压层的小时数据total precipitation，如下所示：
  

官网对于该降水资料的描述是，单位为：m (of water equivalent per hour)

• This parameter is the accumulated liquid and frozen water, comprising rain and snow, that falls to the Earth’s surface. It is the sum of large-scale precipitation and convective precipitation. This parameter is accumulated over a particular time period which depends on the data extracted. For the reanalysis, the accumulation period is over the 1 hour ending at the validity date and time. For the ensemble members, ensemble mean and ensemble spread, the accumulation period is over the 3 hours ending at the validity date and time. The units of this parameter are depth in metres of water equivalent. It is the depth the water would have if it were spread evenly over the grid box. Care should be taken when comparing model parameters with observations, because observations are often local to a particular point in space and time, rather than representing averages over a model grid box.

反正一大堆描述，感觉主要的就是这句话：For the reanalysis, the accumulation period is over the 1 hour ending at the validity date and time

• 翻译过来是超过1小时内的累积量，理解过来就是每小时的降水量，每个小时的降水量是分开的，不是持续累积的，第6小时的数据就代表第6小时时间内的降水量，而不是从00时刻持续到06时刻一共的降水量。

因此，对于单位换算来说，原本给的是m，意思就是：m/hour，如果要换算为mm/hour，那么只需要乘以1000即可，：

如果要换算为mm/day，将所有时刻的降水加起来。
The total precipitation over 24 hours is the sum of the individual total precipitation values for each hour.

https://confluence.ecmwf.int/pages/viewpage.action?pageId=197702790