Characterising recent drought events using current reanalysis and remote-sensing soil moisture products with a standardised anomalies-based framework

Drought events have multiple adverse impacts on environment, society, and economy. Monitoring and characterising such events is thus crucial. Here we test the ability of selected current reanalysis and merged remote-sensing products to represent major seasonal and multi-year drought events of the last two decades globally. We consider the ERA5, and the related ERA5-Land, as well as the MERRA-2 reanalysis products, and the ESA CCI, and the corresponding near-real time C3S remote-sensing soil moisture products (both encompassing an ACTIVE, a PASSIVE and a COMBINED product). The considered products offer opportunities for drought monitoring since they are available in near-real time.

We focus on soil moisture (or agricultural) drought and analyse events within pre-defined spatial and temporal bounds derived from scientific literature. Based on standardised daily anomalies of surface and root-zone soil moisture, the drought events are characterised by their magnitude, duration, spatial extent, and severity (i.e., the combination of duration and standardised anomalies below -1.5).

All investigated products are able to indicate the investigated drought events. Overall, responses of surface soil moisture are often strongest for the reanalysis products ERA5 and ERA5-Land and weakest for the remote-sensing products (in particular for the ACTIVE satellite products). The weaker drought severities in the remote-sensing products are related to shorter event durations as well as partially less pronounced negative standardised soil moisture anomalies. The magnitudes (i.e., the minimum of the standardised anomalies over time) are reduced in MERRA-2 and in the ACTIVE satellite products. Diverse global distributions of long-term trends in dry-season soil moisture may explain some differences in the drought responses of the products. Also, the lower penetration depth of microwave remote sensing compared to the top layer of the involved land surface models, as well as sensing issues of active microwave remote sensing under very dry conditions could explain the partly weaker drought responses of the remote-sensing products during the investigated events. In the root zone (based on the reanalysis products), the drought events often show prolonged durations, but weaker magnitudes and smaller spatial extents.