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Some ECMWF parameters are described as being valid at the specified time, the validity time. Sometimes they are referred to as instantaneous parameters, and may have "instantaneous" in their name. Some forecast and all analysed parameters fall into this instantaneous category. Such parameters are valid at the specified time, NOT over the model time step.
Although they are valid at the specified time, such parameters cannot represent variability on short time scales. This arises because the model values are a grid box average, without variability on spatial scales smaller than the grid. In contrast, high frequency observations at a fixed point will show temporal variability due to advection of small scale spatial variability. Therefore, when comparing with high frequency observations, the latter should be temporally averaged so as to remove the variability that is not represented in the model.
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