Research Update

10 February 2012Updated 15 February 2012

Background

• What is the reason for increased number of storm reports with the presence of an ALT?– Background

conditions similar, ALT acts as trigger?

– ALTs associated with increased CAPE?

ALT at 18Z NO ALT at 18Z0

2

4

6

8

10

12

14

16

18 17.0

9.3

Average Severe Reports per Severe Period (1800 to 0000 UTC)

Procedure• Calculate MUCAPE at 6 different stations at

1800 UTC over the length of the climatology (n=1530)– Stations: CLT, RDU, LYH, RIC, IAD, PHL• Each is located in a different section of the domain (SW,

SE, WC, EC, NW, NE)– MUCAPE is calculated from most unstable parcel

in the lowest 180 hPa of the atmosphere– Dataset used: CFSR– Partitioned into ALT/Non–ALT based on whether

or not an ALT was present at 1800 UTC

Whiskers: 10th and 90th percentile

NARRCFSR

Whiskers: 10th and 90th percentile

NARRNARRCFSR

All median ALT > all median non-ALT at 99% confidence level

All median ALT > all median non-ALT at 99% confidence level

Whiskers: 10th and 90th percentile

CFSR

NARR

Whiskers: 10th and 90th percentile

CFSR

NARR

CLT RDU LYH RIC IAD PHL0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%% ALT Days MUCAPE > 0% Non Days MUCAPE > 0

Percentage of Days with MUCAPE > 0

• Higher proportion of ALT days are associated with >0 MUCAPE compared to non-ALT days

NARR

Whiskers: 10th and 90th percentile

Whiskers: 10th and 90th percentile

Percentage of Days with MUCAPE > 100

CLT RDU LYH RIC IAD PHL0%

10%

20%

30%

40%

50%

60%

70%% ALT Days MUCAPE > 100

% Non Days MUCAPE > 100

• Higher proportion of ALT days are associated with >100 MUCAPE compared to non-ALT days

• MUCAPE > 0 is more likely on ALT Days• Median MUCAPE is mostly higher on ALT Days, especially

at RIC, IAD and PHL• Median MUCAPE and 75th/90th percentiles are highest at

IAD and PHL• MUCAPE values themselves may be a little low – dataset

issues?• Could this help explain the maximum in storm reports

near IAD?• Why are ALTs associated with higher MUCAPE?– ALTs associated with higher low-level θe?– ALTs associated with steeper low to mid-level lapse rates?

Key Results / Further Questions

Determining CAPE/Shear Phase-Space of First Storm Reports of the Day

• Determine CAPE/shear phase space in which severe thunderstorms in ALT Zone occur– This information could be useful to forecasters in

determining if severe weather is expected

Objective

Procedure• Find location and time of first severe report on

a certain day (0400–0359 UTC)• Calculate MUCAPE and Sfc–500 hPa bulk shear

at location of storm report using nearest NARR analysis time 0.5 to 3.5 hours prior to storm report (n=576)

First report (UTC) Corresponding NARR analysis

time (UTC)1530–1829 1500

1830–2129 1800

2130–0029 2100

• First storm report occurred between 1530 and 0029 UTC on 76.3% of all days in climatology

Problems from last time

Email 13 Jan 2012

• Items (a), (b) and (c) are addressed in the following slides

Additional Procedures• Clustering – attempt to control for

inconsistencies in “reports per storm”– Overlay a 0.5° by 0.5° grid box over the domain– If a storm report occurs within a certain grid box

on a certain day, that grid box is considered “active” for the day• Any subsequent storm reports occurring within the

active box are discarded for the day• The number of active grid boxes for each day are tallied

to measure how widespread the severe weather was on that day

1st Report CAPE/Shear by Active Grid Boxes

• Some evidence for higher MUCAPE/shear on highly convectively active days, but not likely to be a statistically significant difference

Additional Procedures• Subsectioning – attempt to control for

CAPE/shear of first storm report not being representative of environment in which most storm reports occur

CENTER

NORTH

SOUTH

• North sector shows a higher proportion of days with greater areal coverage of convection

• South sector peaks earlier (1800 UTC) than north sector (2000 UTC)

• Center sector has flat peak between 1800–2100 UTC

1st Report CAPE/Shear by Sectors

1st Report CAPE/Shear by Intervals of Active Grid Boxes per Day – North Sector

• Not much to distinguish days with/without large areal coverage of severe weather

1st Report CAPE/Shear by Intervals of Active Grid Boxes per Day – Center Sector

• Not much to distinguish days with/without large areal coverage of severe weather

1st Report CAPE/Shear by Intervals of Active Grid Boxes per Day – South Sector

• Not much to distinguish days with/without large areal coverage of severe weather

Few first storm reports occurred in this phase-space

Few first storm reports occurred in this phase-space

No first storm reports occurred in this phase-space

1st Report CAPE/Shear by Month – South Sector

• Monthly variability exists in CAPE/shear of first storm reports

1st Report CAPE/Shear by Month – Center Sector

• Monthly variability exists in CAPE/shear of first storm reports

1st Report CAPE/Shear by Month – North Sector

• Monthly variability exists in CAPE/shear of first storm reports

1st Report CAPE/Shear by Intervals of Active Grid Boxes per Day – Center Sector, Jun, Jul, Aug

• Even when broken up by month, still not much to distinguish days with/without large areal coverage of severe weather

1st Report CAPE/Shear by Intervals of Active Grid Boxes per Day –

Center Sector, May & Sep

• Some evidence of preference toward high shear/low CAPE during May and Sept., but “n” is small

• Clustering/subsectioning approaches show:– North sector showed highest proportion of days

with greater areal coverage of severe reports– North (south) sector has a peak in time of first

severe report at 2000 (1800) UTC, with center sector showing a flat peak between 1800–2100 UTC

– First storm reports in north (south) sector occur in environments of greater shear (CAPE), lesser CAPE (shear) than the other sectors

– North sector shows an area on the CAPE/shear phase space where severe weather does not happen

Key Results / Further Questions

• Clustering/subsectioning approaches show:– In all sectors, median CAPE is higher in JJA, while

median shear is higher in MS• Highest CAPE/lowest shear occurs in August in

North/Central sectors; July/August in South sector– Boxplots show that CAPE/shear at first storm report

is not a good indicator of how convectively active a particular day will be• Why?

– Need dataset with better temporal resolution?– Background conditions are of less importance than the strength

of forcing for ascent?

Key Results / Further Questions

Key Results / Further Questions• Given the background convective parameters of

the ALT Zone, what is the role of the ALT/PFT in triggering convection in the ALT Zone?– We have seen that ALTs are associated with above-

average MUCAPE, especially in the North Sector– Where does convection/severe weather occur with

respect to the ALT/PFT?– What are the processes by which the ALT/PFT

triggers/sustains/enhances convection?