关于ERA5数据同化问题？

mannix

各位大神好，菜鸡遇到了一个问题。我下载了ERA5的Total Precipitation数据，但发现每天凌晨一点（UTC）都有一个突变，猜测应该是同化方式导致的，如同化窗口的设置，但我看ERA5: data documentation（https://confluence.ecmwf.int/display/CKB/ERA5%3A+data+documentation），上面写The 4D-Var data assimilation uses 12 hour windows from 09 UTC to 21 UTC and 21 UTC to 09 UTC (the following day)。所以想请教大家，有没有知道咋回事儿的，能帮解释解释，或者有相关资料的，万分感谢！附件是一个月的Total Precipitation的gif。

mannix

感谢各位大神，在各位帮助下，我找到了这个（https://confluence.ecmwf.int/pages/viewpage.action?pageId=197702790），应该能解释遇到的这个问题了，这个total precipitation变量是每天01UTC-24UTC的累积降水，00UTC时刻是前一天的累积降水，01UTC是新一天累计降水的开始。

大理段戬

帮顶一下，确实值得研究

任性小王子

来看看帖子，顺便顶一下

ljchen1989

utc01时在window里面吧~

龍_彳亍_天_下

帮顶，如果每天都出现这个问题的话确实应该研究的呀！楼主加油

astiny

我怎么感觉你画的是累计降水。。。因为好像24小时里面的值是逐渐变大的。。。你不会是没有减去前一个时次的降水值吧。。。

werewolf

ERA5不止降水，其他变量也有这个问题

mannix

astiny 发表于 2021-3-3 11:21
我怎么感觉你画的是累计降水。。。因为好像24小时里面的值是逐渐变大的。。。你不会是没有减去前一个时次的 ...

感谢回复！好像是24小时内一直累积！这个量确实说是累积量，因为是hourly数据，所以我一直以为是一小时内的累积降水。era5-land的变量说明如下（cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-land?tab=overview）：Accumulated liquid and frozen water, including rain and snow, that falls to the Earth's surface. It is the sum of large-scale precipitation (that precipitation which is generated by large-scale weather patterns, such as troughs and cold fronts) and convective precipitation (generated by convection which occurs when air at lower levels in the atmosphere is warmer and less dense than the air above, so it rises). Precipitation variables do not include fog, dew or the precipitation that evaporates in the atmosphere before it lands at the surface of the Earth. This variable is accumulated from the beginning of the forecast time to the end of the forecast step. The units of precipitation are depth in metres. It is the depth the water would have if it were spread evenly over the grid box. Care should be taken when comparing model variables with observations, because observations are often local to a particular point in space and time, rather than representing averages over a model grid box and model time step.
这边说是from the beginning of the forecast time to the end of the forecast step，也没有明确说什么时间段，我现在还在找相应的文档，但还没找着。

mannix

ljchen1989 发表于 2021-3-3 10:23
utc01时在window里面吧~

感谢回复！请问是因为在窗口里面所以会引起突变吗？为啥是这个时间点呢？抱歉我对同化了解不是很深入，我直觉上觉得如果是在窗口开始结束的那个点出现突变是比较好理解的。希望您能帮忙解答一下