# Training Module on Upper-Level Lows

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## Chapter I: Introduction

- Introduction

### Introduction

Upper-level lows (ULL) usually have a strong impact on surface weather although they are not connected to fronts. They are commonly associated with cold and rainy weather, bringing snowfall in winter and persistent rainy periods during the warmer seasons. Thunderstorms are observed frequently, especially over warm seas. Their propagation often seems erratic because they are separated from the main belt of westerlies, which would otherwise steer them progressively to the east.

Upper-level lows are, as the name indicates, meteorological phenomena that are found in the pressure field of the upper troposphere. In most cases surface pressure shows no anomalies, although sometimes a weak high-pressure region can be found underneath. Upper-level lows are relatively long-lived phenomena, mostly lasting from 2 to 10 days.

Due to dry, sinking air masses in the area of the pressure anomaly, the ULL can best be seen in water vapor imagery and hence in Airmass RGBs where it takes a reddish color. The cold air drop usually goes hand in hand with a lowering of the tropopause.

Figure 1: Upper-level low propagating northeast over the North Sea. SEVIRI Airmass loop from 2 July 2021 (06:00 UTC) to 3 July 2021 (06:00 UTC).

Upper-level lows always have a cyclonic circulation around a pressure minimum at higher levels. The size and depth of the pressure minimum characterizes the horizontal extent of the upper air phenomenon. Upper-level lows usually show horizontal lengths of several hundred to a few thousand kilometers and form during a so-called cut-off process (most frequently) or are remnants of dissipated cyclones over land (more rarely).

After discussing the life cycle of upper-level lows, we will focus on an example that illustrates the impact of a slow moving ULL on surface weather and especially on flooding and inundation with the example of the Central European Floods in July 2021.