Cold Air Outbreak: Constrain Case Study Kirsty McBeath, Paul Field, Richard Cotton, Humphrey Lean, Emilie Carter

Table of Contents Introduction Overview of radar observations Cluster analysis of data Comparisons of radar and Unified Model data Variations applied to the UM High resolution UM versions for this case

Constrain Overview Constrain campaign took place in January 2010, based in Prestwick Led by Richard Cotton and Paul Field Aimed to Constrain constants used in model parameterisations Variety of cloud-based sorties flown 5 x Cirrus spiral flights 4 x Cumulus cold air outbreak flights 2 x Stratocumulus/Altostratus flights

Introduction MODIS 31st Jan 2010 Looking at cases of cold air outbreak in the Northwest approaches 4 flights during Constrain examined these conditions during January 2010 Radar data from these cases used for comparisons with UKV model This data is from a case on January 31st 2010 which coincides with flight b507 of the BAe-146 research aircraft MODIS 31st Jan 2010

Overview of radar observations

Composite radar data 1.91 ±0.36km Data available every 5 minutes for 24 hour period Scans performed at a range of elevation angles: 0.5°, 1.0°, 1.5°, 2.5° 4 scan angles intercept cells at different distances from radar Data from 4 scan angles combined to produce one dataset which captures all cells 0.5°: 75-150km 1.0°: 54-85km 1.5°: 32-64km 2.5°: 30-42km 150km 1.91 ±0.36km

Reflectivity and rain rate Conversion from radar reflectivity (dBz) in to rain rate (mm/h), is typically approximated by: Z = 200R1.6 This relation includes assumptions about drop size distributions and phase of precipitation This conversion can introduce additional uncertainties into the radar data 30dBz≈66mm/day 20dBz≈16mm/day 10dBz≈4mm/day 5dBz≈1mm/day

Model data

Model data Comparison done between radar data and UKV* (1.5km resolution, stretched to 4km at edges) model output. Region examined is too far North for UKV (edge effects close to boundaries) Nested version of UKV model set up to cover the region without any edge effects Covered region from: 14.35W – 0.66E & 55.10N – 68.50N UKV* UKV

Model Reflectivity Radar Model Reflectivity values for model data computed using model microphysics data, this reduces processing done on radar data and removes assumptions used when converting reflectivity to rain-rate

Cluster analysis

Cluster Analysis 10dBz (~4mm/day) threshold used to select regions of precipitation in both datasets

Cluster Analysis Identified cells tracked in time for both datasets Whole frame advected to find overlap between cells Fractional overlap for calculated for each overlapping pair of cells Overlap threshold used to determine if identified cells are the same Cells excluded from dataset if they moved outside the region of interest, or were only seen to decay 133 cells tracked in radar data 75 cells tracked in model data

Results of radar/model comparison using cluster analysis

Comparison of cell statistics for radar and model Model has more long lived cells than seen in radar And more cells with long decay times than radar Mean cell lifetime Radar: 54±3 minutes Model: 78±6 minutes Mean cell decay time Radar: 28±2 minutes Model: 44±4 minutes

Comparison of cell statistics for radar and model Model shows good agreement with radar for cell diameter values. Examining cell area shows that the model is producing more large cells than are seen in the radar data. Ratio of cell area and enclosing circle area examined. This Fill Fraction parameter used to examine cell shape. This provides a measure of cell circularity Mean fill fraction values Radar: 0.50±0.01 Model: 0.77±0.01 Model produces more circular cells than are seen in the radar data Cell area Cell diameter

Variation in area with lifetime UKV resolution (1.5km) may be too coarse to resolve the cells in this case. Larger cell areas present throughout cell lifetime More pronounced for cells with lifetimes over 1 hour Little change in cell fill fraction seen with lifetime

High resolution work

High Resolution Model Work The difference in cell shape between the model and radar was still seen when model physics was modified This difference may be due to this case exhibiting characteristics at the limits of UKV resolution. To examine the impact of model resolution, as series of nested runs has been run with resolutions of 500m, 200m and 100m. Runs have also been done with increased vertical resolution of 140 levels (UKV has 70 levels) 500m 200m 100m

Initial results 500m 200m 100m 70 level model runs

Initial results 500m 200m 100m 140 level model runs

High Resolution Data Clustering Increasing model resolution appears to produce an improvement in cell area Cell fill fraction values reduce with increasing resolution, a bit too much at 100m First look: cells get smaller and more circular with increasing resolution

Next steps Further analysis of high resolution model data Compare this case to work done by Mesoscale Modelling Group at MetOffice@Reading Comparison to (available) aircraft data for this case Repeat analysis for PikNMix case study (more aircraft data available) Use high resolution model to asses cases during COPE (summer 2013)

Questions