Case study 1: Precipitation and convection

An ERS-2 SAR image (orbit 2388, frame 1053) was acquired during a cold front passage over the Netherlands, on 4 October 1995. The interferometric combination with an ERS-1 SAR image (orbit 22061), acquired 24 hours earlier, in the absence of significant weather in our target area, enabled the construction of a radar interferogram, see Figure 1. The figure shows coherent phase information over all land areas. Over the water area the intensity image of the SAR acquisition of 4 October is shown. The diagonal line in the lower right corner of the image is associated with the location of the cold front, and the curved anomaly in the center of the image can be associated with a gust front. Weather radar observations show strong precipitation in this region [Hanssen et al.(1999)].

Figure 2 shows a small section of this interferogram, centered over Veluwe Lake. The footprint of the storm is moving over the area from left to right. Over the land area, the increased humidity associated with the convective clouds and rain is measured as an increase in signal delay of approximately 30 mm. At this position, P4, weather radar observations indicated rain rates up to 10 mm/hr. In front of the storm (P5) the interferogram shows approximately uniform color, indicating not much variation in moisture. Behind the storm front (P3), variations in moisture are in the order of 8-12 mm, which can be associated with moisture variations due to, e.g., convective cells. The small circular anomalies near P5 are related to processing errors over single pixels.

The roughness of the water surface is expressed by the normalized radar cross section, $\sigma ^0$, and is well correlated with the near-surface wind conditions [Stoffelen(1998)]. It is likely that the signatures on the water surface are mostly due to wind effects, as the signature of the rain would cause fine scale structures. Moreover, the rain rate is probably too low and the wind speed likely too high for there to be a rain effect by impact on the water surface. A cross-section of panel (a) is shown in panel (b). Collocated with the rain band, an increase in $\sigma ^0$ of approximately 18 dB can be observed, at position P4. To the east of the storm front (P5) the water surface is relatively smooth. Moving towards the west, from P4 to P2, the roughness first decreases approximately 8 dB, (P3), to increase approximately 4 dB at P2. This nearly symmetrical wind shear appears to be closely associated with the storm front. Assuming a wind direction somewhere between upwind and crosswind, which are the horizontal or vertical directions in panel (a) the wind velocities following from CMOD4 are $\sim$0 m s^-1 in front of the storm, P5, increasing up to $\sim$15 m s^-1 in the storm, P4, within a spatial distance of 300 m.

At P4, there is a surplus of water vapor with respect to the surroundings, resulting in an excess delay of 3.6 cm, which corresponds to $5.3~\pm~0.5$ mm integrated precipitable water. The width of the main rain band is 2 km.

From wind observations and common circulation patterns in high-latitude convective systems, one can infer that the radar look direction is crosswind to the flow at location P5 in panel (a), and parallel to the rain band. Behind the rain band, P1-P3, the wind is more along the radar look direction and probably veering. More to the northeast of the rain band surface measurements indicate that the wind has also a component along the radar look direction. This means that besides wind speed changes, there are also wind direction changes that may be relevant for the signature in the radar image (P1-P5). The radar is most sensitive to wind direction changes for wind directions at 45, 135, 225, and 315 with respect to the radar look direction, whereas there is no sensitivity at 0, 90, 180, and 270 degrees.

From these observations, we find a circulation pattern connected to this system termed 'optimal shear' by [Parsons(1992)], depicted in panel (c), where in our case the complete system moves to the east, such that the vertical shear in front of the storm consists of an area with no wind at the surface and probably SW-winds aloft.