The potential for use of crowd‐sourced data in the atmospheric sciences is vastly expanding, including observations from smartphones with barometric sensors. Smartphone pressure observations can potentially help improve numerical weather prediction and aid forecasters. In this contribution a method of collecting data from smartphones is presented, other methods are discussed and guidelines are derived from the experience. Quality control is vital when using crowd‐sourced data. Screening methods aimed at smartphone pressure observations are presented. Results from previous studies, showing a substantial but long‐term stable bias in combination with high relative accuracy, are confirmed. The collection of Danish smartphone pressure observations has been very successful, with over 6 million observations during a 7 week period. Case studies show that distinct weather patterns can be seen in unprocessed data. The screening method developed reduces the observational noise but filters out the majority of observations. Assimilating smartphone pressure observations in a single case study, using the 3D variational data assimilation system of the HARMONIE numerical weather prediction system, proved to decrease the bias of surface pressure in the model without increasing the root mean square error and the skill of accumulated precipitation increased. It is found that the altitude assignment of smartphones needs improvement.

In December 2018, the Danish Meteorological Institute organised an international meeting on the subject of crowdsourced data in numerical weather prediction (NWP) and weather forecasting. The meeting, spanning 2 days, gathered experts on crowdsourced data from both meteorological institutes and universities from Europe and the United States. Scientific presentations highlighted a vast array of possibilities and progress being made globally. Subjects include data from vehicles, smartphones, and private weather stations. Two groups were created to discuss open questions regarding the collection and use of crowdsourced data from different observing platforms. Common challenges were identified and potential solutions were discussed. While most of the work presented was preliminary, the results shared suggested that crowdsourced observations have the potential to enhance NWP. A common platform for sharing expertise, data, and results would help crowdsourced data realise this potential.

Crowd-sourced observations (CSO) offer great potential for numerical weather prediction (NWP). This paper offers a synthesis of progress, challenges and opportunities in this area based on a special session of the EWGLAM Meeting in 2019, concentrating on high-resolution limited-area models (LAMs). Two main application areas of CSO are described: data assimilation and verification. One part of data assimilation developments concentrates on smartphone pressure observations, which represent a large volume of data. However, special care has to be taken about data protection and the quality of observations. In this paper, two examples are presented: the SMAPS experiment from Denmark and the uWx experiment from the United States. Another data assimilation topic is citizen observations with low-cost weather sensors; here an example from Norway is presented using Netatmo stations. The other application area is the use of CSO for model verification. One novel method developed in the United Kingdom is applying social media data to detect severe weather events. This approach is especially important because one future application area of LAM NWP models is impact-oriented warnings.

Crowdsourced data is now seen as a potential source of high-resolution observations in the atmospheric sciences. In this paper we investigate a potential data source, wind observations obtained using anemometers connected to handheld smartphones. The aim of this paper is twofold: to assess the quality of raw and height extrapolated wind measurements from the handheld anemometer against professional-grade SYNOP stations, and to use this data of opportunity to infer a more accurate estimation of terrain roughness lengths. Roughness lengths are essential in numerical weather prediction; however, they are often poorly determined. Roughness lengths are also necessary when correcting near-surface wind observations for height offsets. For the analysis we performed a series of field experiments measuring wind profiles using handheld anemometers at roughly 2 m above ground. These raw measurements were then extrapolated to 10 m height using roughness lengths from three different sources. The extrapolation enabled us to compare the quality of roughness lengths estimated from smartphone measurements to those from traditional sources, as well as assess the quality of these wind measurements against the professional-grade stations. We find the handheld wind measurements are comparable in quality to wind measurements from SYNOP stations at 10 m height, and that for some cases the handheld measurements can be more representative than SYNOP stations only about a kilometre away. To determine the roughness lengths, we examine a method based on the turbulent intensity derived from the high-frequency signal of the smartphone wind measurements. Under certain circumstances, the roughness lengths obtained with the approach presented here are superior to traditional sources.