Observed profile types

Initial analysis of a selection of RHIs and profiles suggested that this dataset should not simply be classified as “stratiform” or “convective”, but that three categories would be more appropriate. This contrasts with the majority of previous literature, which bases VPR classification around the two categories of stratiform “bright band” and convective rain (eg Steiner et al., 1995). However, this finding of more than two types of precipitation profile is far from new (eg Fabry and Zawadzki, 1995; Bringi et al., 2009; Delrieu et al., 2009; Matrosov et al., 2016). In this case, the three observed classes in the high resolution dataset align well with three of the five VPR types identified by Fabry and Zawadzki (1995) using a vertically pointing X-band radar:

Low-level rain: shallow, light rainfall developing below the zero degree isotherm in stratiform conditions.

Rain with bright band: cold rain developing above the zero degree isotherm in strat- iform conditions. This profile shows a clear increase in reflectivity with onset of melting and decreasing Z below the melting layer, forming the traditional reflec- tivity bright band.

Rain from compact ice: similar to the “rain with bright band” profile, in that in- creased reflectivity occurs with the onset of melting, but no decrease in Z is ob- served below the melting layer. Fabry and Zawadzki (1995) speculate that this profile shape “is likely caused by the melting of fast-falling snow pellets or dense graupels”. This is supported by later DSD analyses of Matrosov et al. (2016).

Figure 4.2: Example stratiform RHI: reflectivity and LDR from 14:16 UTC, 13th October 2014, truncated at 10 km height and 20 km range. The bright band is clearly visible as a region of enhanced reflectivity and LDR at 2 km altitude, just below the 0oC isotherm (see also figure 4.5, top panel). Note that the bright band is clearly visible in LDR even at 15-20 km range, contrasting with the weaker reflectivity bright band in this region.

Figure 4.3: Example “rain from compact ice” RHI: reflectivity and LDR from 13:46 UTC, 9th October 2014, truncated at 10 km height and 20 km range. The “compact ice” region around 5-7 km range shows no clear bright band in reflectivity, and correspondingly lower LDR than in the surrounding bright band regions. However there is a sharp increase in reflectivity in the melting layer at 2 km altitude (see also figure 4.5, middle panel), which is not consistent with convective updrafts.

Figure 4.4: Example convective RHI: reflectivity and LDR from 22:20 UTC, 3rd July 2015, truncated at 10 km height and 20 km range. A weak bright band in the region of 8-10 km range, marking the 0oC isotherm at 3.5 km altitude (figure 4.5, bottom panel), contrasts sharply with the convection at 10-15 km. High reflectivity in this region extends consistently around 2 km above the melting layer. LDR values in the convective region are lower than in the weak bright band region, and much lower than in the strong bright band case of figure 4.2.

Figure 4.5: Top: vertical profiles of reflectivity and LDR from the stratiform RHI in figure 4.2 (7.5 km range). Middle: vertical profiles of reflectivity and LDR from the compact ice RHI in figure 4.3 (5.5 km range. Bottom: vertical profiles of reflectivity and LDR from the convective RHI in figure 4.4 (12.5 km range). Limits of the LDR-determined melting layer are shown in green, and the wet bulb freezing level by the dashed grey line. Annotated red stars show values at the key levels: reflectivity at the top (Zice) and bottom (Zrain) of the melting layer, peaks (Zpeak) for stratiform and compact ice cases, and the peak melting layer LDR (Lpeak).

Figure 4.6: Average stratiform, compact ice and convective reflectivity profiles with height relative to the model derived (Brown et al., 2012) wet bulb freezing level. Height levels are at six evenly spaced intervals between the lowest usable reflectivity (LUS) and the freezing level (FL), and then in 200 m steps above the freezing level.

Showers: shallow, light rainfall developing below the zero degree isotherm in convective conditions.

Deep convection: the unstratified profiles observed where updrafts are present in thun- derstorms, squall lines and embedded convective cells.

On the basis of Fabry and Zawadzki (1995), the VPR dataset was sorted into “rain with bright band” (hereafter “stratiform”), “compact ice”, and “convective” classes. Examples of each profile type are shown in figures 4.2-4.4. It is entirely possible that both of the other profile classes (“low level rain” and “showers”) were also observed by Wardon Hill during the study period. However, both of these profile types were excluded from this investigation by design, since in these cases precipitation formation and growth occurs in the liquid phase and there is no melting layer to classify.

Using the three observed categories of VPR, the “true” classification for each profile in the dataset was determined based on the shape of the melting layer peak. Figure 4.5 shows how the maximum reflectivity in the melting region (Zpeak) and at the top (Zice)

and base (Zrain) of the LDR-determined melting layer were compared. Classification

rules were applied as follows:

1. If the peak-to-rain reflectivity difference ∆Z = Zpeak−Zrain ≥ 3 dB, the profile

has a bright band and is therefore “stratiform”

2. If there is no bright band but the peak-to-ice reflectivity difference Zpeak−Zice≥

6 dB, the profile is “compact ice” 3. Otherwise, the profile is “convective”

The choice of quantitative ∆Z and peak-to-ice classification thresholds is discussed in section 4.2.4.