Repeating Patterns of Precipitationand Surface Pressure Evolution

in Midlatitude Mesoscale Convective Vortices

Eric JamesColorado State University

17 August 2009

Previous Studies

23-24 June 1985 OK PRE-STORM MCS

“Onion-shaped” sounding at rear of MCS due to subsidence; heat bursts as subsidence locally penetrates shallow surface inversion

MCV forms within dissipating stratiform region

As stratiform precipitation dissipates, surface mesohigh rapidly transforms into strong mesolow, and vortex becomes visible in satellite imagery

Johnson et al., 1989

Previous Studies

6-7 May 1985 OK PRE-STORM MCS

Cyclonic circulation forms within stratiform region, with westerly rear inflow jet to the south

“Onion-shaped” soundings within rear inflow, with low-level warm and dry air

Significant notch in reflectivity field, with strong mesolow at notch apex, associated with warming within rear inflow

Brandes, 1990

Objectives

Motivated by existence of dense surface observations by the Oklahoma Mesonet, we aim to: Identify numerous MCV cases in this region during 2002-05 Classify cases according to precipitation and surface pressure

evolution Propose some mechanisms for frequently-observed mesolows

Case Selection

We run algorithm of Davis et al. (2002) on hourly RUC analyses in Oklahoma

Algorithm classifies gridpoints as vortices if all criteria are met

Can be run for each analysis time to derive vortex tracks; detected vortices are treated as one if track is continuous

Classification

45 vortices arise from MCSs; these MCV cases are examined in detail

MCV events grouped based on evolution of parent MCS, and surface pressure signatures

Five recurring types of MCVs are found: Three types produce distinct surface mesolows:

Rear Inflow Jet MCV (19 of 45) Collapsing Stratiform Region MCV (8 of 45) Vertically Coherent MCV (1 of 45)

Two types do not produce surface mesolows: Remnant Circulation MCV (14 of 45) Cold Pool Dominated MCV (3 of 45)

Examples of mesolow-producing MCVs are presented, with hypotheses for their surface pressure effects

Rear Inflow Jet MCV:24 May 2003

Oklahoma Mesonet (1400-1700 UTC)

Bow echo moves SE with stratiform rain to N

Small mesohigh on SW end of convective line

Rear inflow notch begins to develop at back edge of stratiform region, S of developing MCV

Intense mesolow forms near apex of rear inflow notch

Rear Inflow Jet MCV:24 May 2003

Both analyses show low-level virtual warming of ~2.5 C, centered near 825 hPa, west of cool anomaly at similar height, and above ~700 hPa

Warming occurs near base of strong rear inflow according to profiler

BAMEX Aircraft (~1930 UTC)Haskell Wind Profiler (0700-2000 UTC)

Collapsing Stratiform Region MCV:9 August 2004

Small dissipating MCS moves into OK from N

Significant mesohigh on S edge of stratiform region

Large mesolow develops within dissipating stratiform region, collocated with developing MCV

Oklahoma Mesonet (0800-1100 UTC)

Collapsing Stratiform Region MCV:9 August 2004

RASS and radiosonde observations show deep layer of warming collocated with mesolow, evidence of dry-adiabatic subsidence at low- to mid levels

Purcell Wind Profiler (0800-1900 UTC)ARM SGP Radiosonde (1136 UTC)

Vertically Coherent MCV:29 July 2004

Oklahoma Mesonet (1300-1600 UTC)

Large stratiform precipitation region moves NE into E OK

Cyclonic circulation of radar echoes evident in animated radar imagery

Well-defined meso-alpha scale low pressure center exists at center of circulation

Cyclonic circulation in mesonet winds

Vertically Coherent MCV:29 July 2004

RUC cross-section of MCV shows deep, vertically coherent tower of PV directly over surface mesolow

PV tower resembles that documented in BAMEX MCV of 11 Jun 2003

Both cases also have weak virtual temperature perturbations

RUC analysis (1200 UTC)

BAMEXAircraft(~1730 UTC11 Jun 2003)

Rear Inflow Jet MCV:Conceptual Model

At MCS maturity (left), mesohigh centred behind strong convective line At second stage (middle), right-hand portion of stratiform region begins to

erode from rear, mesolow begins to form at back edge of precipitation, and midlevel vortex begins to develop

At final stage (right), MCS has become strongly asymmetric, with intense wake low at apex of rear inflow notch on right-hand side of system, and MCV is intensifying

Collapsing Stratiform Region MCV:Conceptual Model

At MCS maturity (left), mesohigh centred in precipitation region At second stage (middle), mesohigh shifts ahead of precipitation,

mesolow begins to form due to subsidence warming in dissipating stratiform region, and midlevel vortex begins to develop

At final stage (right), mesohigh is weakening, mesolow has broadened and deepened within dissipating stratiform region, and MCV is intensifying

Discussion Rear Inflow Jet MCVs produce mesolows due to intense low-level

subsidence warming within a rear inflow jet (i.e., the “wake low”) This mechanism has been documented by Johnson and Hamilton

(1988) and Stumpf et al. (1991) The Brandes (1990) case appears to be in this category

Collapsing Stratiform Region MCVs produce surface mesolows due to broad-scale low- to midlevel subsidence warming within a dissipating stratiform region The Johnson et al. (1989) case appears to be in this category

Vertically Coherent MCVs produce surface mesolows due to a deep warm core and a relatively weak surface-based cold pool The 11 Jun 2003 BAMEX MCV appears to be in this category

Remnant Circulation MCVs have no precipitation in their vicinity and produce no surface pressure effects

Cold Pool Dominated MCVs have extensive precipitation but no mesolow

Conclusions

The Vertically Coherent MCV documented here strongly resembles an incipient tropical cyclone; further study of these systems could help us understand tropical cyclogenesis

Identification of these distinct MCV types has implications for forecasting: The formation of an MCV within an intense, asymmetric MCS could

be a sign that wake low formation is likely, with associated high winds and low-level wind shear hazards

The appearance of a broad mesolow within a collapsing stratiform region could suggest that MCV formation is likely, which plays a role in subsequent convective initiation

Vertically Coherent MCVs moving off the coast over warm water should be monitored for possible tropical cyclogenesis (Bosart and Sanders 1981)

Future work will involve a composite analysis of the synoptic-scale environment of the MCVs, and modeling of some cases

Hydrostatic pressure change associated with 1500-1700 UTC virtual warming in Haskell profile: -0.87 hPa

This agrees well with observed pressure drop at closest OM station (Okmulgee): -0.79 hPa

Tahlequah pressure drop: -3.62 hPa

This suggests core of warming is not sampled by Haskell profilerTahlequah mesonet station

(0700-2100 UTC 24 May 2003)

Hydrostatic pressure change associated with 1100-1300 UTC virtual warming in Purcell profile: -1.75 hPa

This agrees well with observed pressure drop at closest OM station (Washington): -1.52 hPa

Breckinridge mesonet station(0800-2000 UTC 9 Aug 2004)

McAlester mesonet station(0600-2200 UTC 29 Jul 2004)

Purcell Wind Profiler(0600-2100 UTC 29 Jul 2004)

Type # % Longevity (h) Radius (km)Vorticity (s-1)All 45 100 17 225

9.98x10-5

RIJ 19 42 15 2311.32x10-4***

CSR 8 18 14 206*6.75x10-5**

VC 1 2 10 2628.83x10-5

RC 14 31 24* 2258.21x10-5

CPD 3 7 13 2277.92x10-5

*Significantly different from the mean of all the MCVs at the 90% level**Significantly different from the mean of all the MCVs at the 95% level***Significantly different from the mean of all the MCVs at the 99% level