## 3 Computation of Climate Normal and Climatology  
In 2017, WMO released its most recent guidelines on the computation of climatological normal ([[https://library.wmo.int/records/item/55797-wmo-guidelines-on-the-calculation-of-climate-normals](https://library.wmo.int/records/item/55797-wmo-guidelines-on-the-calculation-of-climate-normals)]([https://library.wmo.int/records/item/55797-wmo-guidelines-on-the-calculation-of-climate-normals](https://library.wmo.int/records/item/55797-wmo-guidelines-on-the-calculation-of-climate-normals)), accessed on 9 January 2023). It details various aspects of data completeness, homogeneity, data precision, and rounding relevant to computing the climate normal [1]. This report, following the Guide to Climatological Practices, recommends that the individual monthly mean parameters based on the daily means not be calculated if either one of the following is true: (a) the observations are missing for 11 or more days during the month, hereafter referred to as the D11 condition, or (b) observations are missing for a period of 5 or more consecutive days during the month, hereafter referred to as the D5 condition. The report further recommends that, for a normal or average to be calculated for a given month, data should be available for at least 80% of the years in the averaging period. This equates to having data available for that month in 24 or more out of the 30 years for a climatological standard normal or a reference normal.  
In this study, we computed two different versions of climate normal and two different climatologies for the geophysical variables mentioned in Section 2. The rationale and methodology behind these normals and climatologies are explained below.  
CN_WMO: This refers to the \"true climate normal\" computed by adhering to all three WMO recommendations on daily and monthly data completeness mentioned above. In practice, this is carried out as follows. Using the daily means of a geophysical variable in question available at Level 2b, we created flags for each 0.25 degree lat-lon grid that satisfy D5 and/or D11 condition for each month during the 1991-2020 period. We then computed the mean parameter for those grids that satisfy both D5 and D11 conditions and have data available for at least 24 of the 30 years. We refer to this version of climate normal as CN_WMO because it strictly follows all WMO guidelines on data completeness.  
CN_MB: This refers to the climate normal computed only using Level 3 monthly mean data. This is a straightforward and easy-to-provide version of the climate normal. In practice, this means that we apply a basic quality control on the Level 3 data and compute mean parameter if the data are available for at least 24 of the 30 years.  
Clim30: This refers to the 30-year arithmetic climatological average for the 1991-2020 period without applying any condition on data completeness on the daily or the monthly means. Such climatological averages are often used for climate monitoring, climate change studies, to provide climate anomalies, and to study the impact of extreme events. In principle, if the satellite observations are continuous and are of good quality throughout the 30-year period, CN_WMO, CN_MB and Clim30 would be exactly identical.  
Clim40: This refers to the extended 40-year climatological average for the 1981-2020 period. It is possible to calculate this extended climatological average, since the CLARA-A3 climate data record extends back 42 years to 1979. By comparing CN_WMO with Clim30 and Clim40, the readers could obtain an idea about the impact of having 10 extra years of observations.  
In the sections below, the difference between these climate normals and climatologies is illustrated for the month of January as a demonstrative and feasibility case study.