D2 Vertical Profiles In Situ

Topic

In situ vertical soundings are the standard method for determining the thermodynamic and dynamic structure of the troposphere and lower stratosphere—that is, how temperature, humidity, pressure, and wind change with altitude. The standard instrument for these soundings is the radiosonde: a small transmitter carried aloft by a helium balloon at a speed of approximately five meters per second. As it ascends, the radiosonde measures temperature, relative humidity, pressure, and wind—the latter by precisely tracking the balloon's position—at every altitude it passes through. The radiosonde continues to rise with the balloon to altitudes between twenty and thirty-five kilometers, at which point the balloon bursts and the radiosonde ceases transmission.

These radiosondes are not launched in isolation but as part of an international sounding network that coordinates the launch of some 1,200 probes twice daily. Launches occur simultaneously worldwide—at midnight and noon, based on a shared universal reference time—ensuring that atmospheric measurements taken in different countries can be compared as if they were simultaneous.

Each variable measured by the radiosonde has its own margin of error: temperature is measured with a precision of two to five-tenths of a degree; relative humidity with a precision of five to ten percentage points; pressure with a precision of half a unit to one unit of atmospheric pressure; and wind with a precision of five-tenths to one meter per second. The greater the precision, the more reliable the vertical structure revealed by the radiosonde becomes.

The temperature profile generated by the radiosonde during its ascent allows for the calculation—based on how temperature changes with altitude—of the near-surface mixing layer, atmospheric stability at various levels, condensation levels, and available convective energy, as well as the convective inhibition that this energy must overcome to be released. The wind profile generated by the radiosonde itself allows for the calculation of wind shear between different levels, the thermal wind resulting from horizontal temperature changes, and the momentum available to fuel storms.

The same principle behind the radiosonde—a small instrument that ascends or falls through the atmosphere while measuring its properties—is adapted for more specialized instruments. Ozone sondes replace standard sensors with one that measures the vertical ozone profile via a chemical reaction electrically triggered by the passing air. Dropsondes, conversely, reverse the direction of the sounding: they are radiosondes released by an aircraft in flight that descend by parachute rather than ascending via balloon. This ability to profile the atmosphere from the top down in a matter of minutes makes them particularly useful for profiling tropical cyclones, where waiting for a balloon to rise from sea level would be impractical.

No single sounding, however precise, describes the entire atmosphere on its own, because radiosondes and their variants are launched only from fixed locations and at set times. Therefore, a complete reconstruction of the atmospheric state combines radiosonde data with the state calculated at that same moment by a numerical prediction model using a data assimilation method: the profile observed by the radiosonde corrects the numerical prediction model where the two differ, while the model fills the space between actual sounding points with its own structural data, together producing an optimal analysis of the atmosphere's actual state.

The result of this entire process—from the radiosonde's ascent to the optimal analysis yielded by data assimilation—is the tropospheric structure that *in situ* vertical soundings are designed to reveal: layers of varying stability, temperature inversions (where temperature rises rather than falls with altitude), clouds, and zones of relative humidity. Together, these elements describe how the atmosphere is organized at that specific time and location. Thus, in situ vertical soundings remain the reference method used to obtain—and against which to validate—the thermodynamic and dynamic structure of the troposphere and lower stratosphere with which this description begins.

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