Caveats

The Virga-Sniffer is a column based detection scheme for virga events. The detection is strongly tuned and manually evaluated for best performance with the EUREC4A datasets LIMRAD94 and Ceilometer. It relies on threshold based tests which might not work in other environments or with different input data setup. Some major caveats to have an eye on while using the Virga-Sniffer are outlined below:

cloud_max_gap to small

Virga sniffer output at a situation to reveal some caveats. At 03:45 UTC - fractured radar signal; at 05:00 UTC - cloud detection; at 05:45 UTC - multi layer cloud transition.

Fractured radar reflectivity signal

Virga events are associated with a certain cloud-base height (if require_cbh = True). Precipitation below a cloud-base height layer, which does not reach the surface is considered virga. In case of a fractured radar signal (see demonstration figure at around 03:45 UTC) the virga associated with the cloud-base will be far below the cloud if the gaps are small enough (cloud_max_gap threshold). This should be kept in mind when using output values of virga_base and virga_top as they mark the maximum extend of virga plus the gaps, which is virga_depth_maximum_extent. The output value virga_depth is calculated by excluding these gaps and therefore should be used when calculating volumetric characteristics. Anyway, this caveat can be circumvented by not ignoring gaps in virga setting the ignore_virga_gaps flag to False, although this would mean to cut some virga (especially in fall streaks).

Cloud - virga differentiation (in upper layer clouds)

If clouds are present in multiple levels (heights), virga detection is challenging, as only the cloud-base is known a priori and the vertical extend of the precipitating cloud is not. The Virga-Sniffer includes a cloud and cloud-top detection which is heavily sensitive to the cloud_max_gap threshold and by essence defines the cloud top where large enough gaps in radar reflectivity occur. This raises two issues if upper layer clouds are present:

  1. cloud_max_gap is too small: Due to uncertainties in observational cloud-base height or radar reflectivity data, misalignment of both data or coarse resolution of radar range-gates, ceilometer detected cloud bases might not connect directly to a valid radar signal. Assume, the cloud-base height value is below the first range-gate with valid radar signal and the gap is larger than cloud_max_gap, the cloud will not be detected and no cloud-top will be assigned. In turn, these range-gates which are not marked as cloud due to that will be potentially marked as virga from higher level clouds if ignore_virga_gaps is True (default) and the precipitation of the upper layer is close to the lower layer cloud. See the demonstration figure at around 05:00 UTC.

  2. cloud_max_gap is too large: Similarly, if the gap allowance is too large, clouds will expand over the precipitation from upper layer clouds when close to lower layer cloud top height.

Multi-layer cloud transition

The data points of radar reflectivity might connect (without gaps or gaps smaller cloud_max_gap threshold) through multiple layers of clouds defined by the ceilometer observed cloud base heights. This is the case for example in the demonstration figure at around 05:00 and 05:45 UTC. During processing with the Virga-Sniffer, these cases are assumed to be connected and lower layer cloud-base height values are dropped. But this might result in sudden jumps of virga extend, if gaps in upper layers of cloud-base height occur. These gaps might occur when the ceilometer beam is attenuated by the lower level cloud to a large extend. Gaps in ceilometer data can be filled (cbh_layer_fill = True) by increasing the layer_fill_limit threshold.