Matti Leskinen

Pest insect immigration warning by an atmospheric dispersion model, weather radars and traps

In an experimental set‐up in and around Helsinki, Finland (60°N, 25°E), we have detected pest insect immigration using weather radars and insect traps in the field. This study was part of a project to develop a system to give warning of a possible arrival of long‐range migrant insect pests. Bird‐cherry aphid, Rhopalosiphum padi, and diamondback moth, Plutella xylostella, were found on the ground following migrations in warm airstreams at the end of May 2007. This migration episode was successfully forecast by the meteorologists in the project team. For the summer 2008, we developed a pest insect immigration alarm system based on SILAM, a Finnish Meteorological Institute atmospheric dispersion model. The first important pest insect immigration occurred in late June, bringing bird‐cherry aphids. Our alarm system correctly produced a warning of this immigration. We studied the migration path in the observed events in 2007 and 2008 with the help of the atmospheric dispersion model. Weather radars frequently showed rain echo over the area, but there was also a lot of echoes originating from the migrating insects. Using the polarimetric weather radar in Helsinki, we could differentiate insects from other sources of echoes. Insects were common in layers below 1 km, and were observed up to height of about 2.5 km. Using Doppler weather radars we were able to observe the speed and direction of the migration. The experiment showed that an atmospheric dispersion model is an effective tool for predicting the movement of airborne migrants. The alarm system would work still better, if the sources of the immigrants were known in more detail. In addition, the very simple modelling of airborne migration should be refined. Weather radars, and especially polarimetric systems, are able to detect insect migrations and reveal details of the phenomenon not obtainable by other means.

The influence of synoptic scale flow on sea breeze induced surface winds and calm zones

Abstract High-latitude sea breezes and the related daytime calm zones were studied through fine-scale two-dimensional idealized simulations by varying the direction and speed of the ambient large-scale geostrophic flow in small steps. Strongest coastal afternoon breezes were obtained for moderate large-scale flows 45◦–90◦ left from the pure offshore direction (as seen from the sea), while calm zones appeared near the coast for weak to moderate large-scale flows about 30◦–90◦ right from offshore, and at the seaward edge of the breeze cell for weak ambient flows 45◦–90◦ right from offshore. The complex daytime evolution of the wind field for ambient winds roughly opposing the sea breeze is illustrated via a few key cases.

Doppler Velocities at Orthogonal Polarizations in Radar Echoes From Insects and Birds

The differential Doppler velocities (DDVs) measured with weather radar at horizontal and vertical polarizations in echoes from insects and birds are considered. In weather echoes, DDV is usually less than 0.5 ms- 1, whereas in echoes from flying birds and insects, it can reach 5-7 ms- 1. Such large difference can be used as an additional parameter in distinguishing between weather and nonmeteorological echoes. It is shown that large values of DDV pertain to multipeaked Doppler spectra with different spectral differential reflectivity values in the peaks.