Analysis of frontogenesis and frontolysis by means of Q-vector diagnostics combined with satellite images

Summary

The correlation between the normal component of theQ-vector(Q _n ) and the pixel values in IR satellite images has been investigated on a six-months data set. The average linear correlation coefficients are negative in all three selected isobaric levels, but not of significant magnitude (the correlation is best for the 850 hPa level, with a correlation coefficient of about 0.3), indicating an only slight tendency for negative values ofQ _n to be associated with high IR grey shade values or, in other words, with cold cloud tops. It is explained why a better correlation should be expected for frontal cases, followed by an evaluation of 14 IR images containing cloudiness of cold fronts that occurred in February 1995. The correspondence betweenQ _n at the 850 hPa level and IR pixel values in those cases is compared to the statistics for the whole month of February 1995, including also nonfrontal cases. The more pronounced connection is found for frontal cases, as expected. Finally, examples of frontogenesis and frontolysis demonstrate the possibility of using the prognostic quantityQ _n in combination with IR satellite imagery as a tool for operational nowcasting and/or forecasting of the development or decay of frontal cloudiness.