Climate dynamics

Abstract:

Strong rivers of westerly winds, known as jet streams, are driven primarily by temperature differences between low and high latitudes as well as the rotation of the Earth. The jet streams create and impact weather systems and steer them in the midlatitudes of both hemispheres. Often, these jet streams do not flow directly from west to east, but rather meander north and south in a wave pattern of alternating high- and low-pressure regions. These meanders are Rossby waves, which influence the jet streams via baroclinic instability caused by temperature gradients. Depending on their wavelength, latitude, and the background wind speed, these waves can move to the east or to the west and under certain conditions also be (quasi)stationary. Jet streams can locally increase the gradient of vorticity (atmospheric spin), so that atmospheric wave guides may be formed. These waveguides affect the propagation pathways of Rossby waves, often leading to more zonal propagation, and potentially amplification of waves. Rossby waves, jets, and waveguides affect atmospheric eddies, such as anticyclonic blocks, and can create prolonged weather conditions that lead to extreme weather impacts.

Abstract:

Abstract:

This study diagnoses the degree of gradient wind balance (GWB) in dropsonde observations of 30 tropical cyclones (TCs) divided into 91 intense observation periods. The diagnosed GWB in these observation periods are composited to investigate which characteristics of a TC are significantly related to departures from GWB. This analysis confirms that on average the flow above the boundary layer is approximately in GWB. Supergradient flow is more common near the radius of maximum wind (RMW) in the upper boundary layer than above in the free troposphere or outside the RMW and is also more common in strong storms than in weak storms. In contrast, the degree of GWB does not differ between intensifying, steady‐state and weakening storms. Storms with a peaked wind profile have a higher probability of showing supergradient winds than those with a flat wind profile. The comparison of two commonly used functions to fit observations shows that the diagnosing GWB from dropsonde observations is highly dependent on the analysis technique. The agradient wind magnitude and even sign is shown to depend on which of these functions is used to fit the observations. The use of a polynomial fit consistently diagnoses the presence of supergradient winds far more frequently than a piece‐wise function, and also shows a marked degree of imbalance above the boundary layer. Therefore, caution is warranted when determining the degree of GWB with a polynomial fit.