Observational Techniques In Situ

Weusually refer to in situmeasurements as thosemade from some platform anchored to the surface, or in the case of the deep ocean from a ship or buoy. Instruments located on the surface but sensing the atmospheric boundary layer (ABL) remotely are not included. Over land the ABL reaches a height of between 1 and 3 kmduring the day, collapsing to a few hundred meters at night; the marine boundary layer is typically 500 to 1000m day and night. The region sampled by in situ techniques is mainly in the atmospheric surface layer (ASL). This corresponds roughly to the lowest 10% of the ABL, and its characteristics are influenced strongly by the nature of the surface. In situ measurements provide data for operational weather forecasting, environmental monitoring, validation for models and remotely sensed observations, and various research topics. The latter include studies of atmospheric structure and transport processes, agricultural and forest meteorology, pollution and climate change. Most applications require measurements of the state variables (temperature, humidity, andpressure),wind speed and direction, incoming and outgoing radiant energy, rainfall, and evaporation. Determination of the fluxes of sensible and latent heat, and of momentum, is also a critical objective of many in situ measurement programs. This article first outlines the theoretical framework of the ASL, which often influences the observations required, and then the sensors commonly used tomake the measurements. This is followed by descriptions of a few different types of field site and some practical aspects of deploying instruments and recording the data.