Convective-scale Downdrafts in the Principal Rainband of Hurricane Katrina (2005) - PowerPoint PPT Presentation

Loading...

PPT – Convective-scale Downdrafts in the Principal Rainband of Hurricane Katrina (2005) PowerPoint presentation | free to download - id: 67ad03-NzY3M



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Convective-scale Downdrafts in the Principal Rainband of Hurricane Katrina (2005)

Description:

Convective-scale Downdrafts in the Principal Rainband of Hurricane Katrina (2005) Anthony C. Didlake, Jr. COGS Seminar UW, Dept. Atmos Sci., Seattle, November 6, 2008 – PowerPoint PPT presentation

Number of Views:15
Avg rating:3.0/5.0
Slides: 53
Provided by: didlake
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Convective-scale Downdrafts in the Principal Rainband of Hurricane Katrina (2005)


1
Convective-scale Downdrafts in the Principal
Rainband of Hurricane Katrina (2005)
  • Anthony C. Didlake, Jr.

COGS Seminar UW, Dept. Atmos Sci., Seattle,
November 6, 2008
2
Idealized structure of a tropical cyclone
downwind
  • Inner and Outer eyewalls
  • Stationary Band Complex (SBC)
  • principal band
  • secondary bands

upwind
Willoughby 1988
3
Overview
  • Background and Motivation
  • Description of RAINEX and dataset
  • Methodology Convective separation and cross
    sections
  • Characteristics of downdrafts within principal
    rainband
  • Forcing mechanisms and immediate effects of
    downdrafts
  • Possible impacts on larger tropical cyclone
  • Summary and conclusions

4
Background and Motivation
  • Dynamic role of principal rainband in the larger
    storm remains uncertain
  • Several modeling studies suggest the principal
    rainband impacts the storm intensity
  • PV generation and inward advection
  • Inhibiting inflow of warm, moist air
  • Formation of secondary eyewall via vortex-Rossby
    wave dynamics
  • Important to understand structure and dynamics of
    principal rainband, so that we may better address
    the difficulties in forecasting tropical cyclone
    intensity

5
Houze et al. 2006, 2007
6
Model of Principal Rainband
  • Convective cells embedded in stratiform rain
  • Overturning updraft, two downdrafts

Hence and Houze 2008
7
Downdrafts in the Principal Rainband
IED
LLD
  • Low-level downdraft (LLD)
  • Inner-edge downdraft (IED)
  • Forcing mechanisms, immediate effects, possible
    impacts on larger storm?

8
Downdrafts in ordinary convection
Zipser 1977
  • Convective-scale saturated downdraft forced by
    precipitation drag
  • Mesoscale downdraft due to evaporative cooling

Palmén and Newton 1969, Biggerstaff and Houze
1993, Yuter and Houze 1995
  • Convective-scale downdraft forced by buoyancy
    pressure gradient force (BPGF) field

9
Hurricane Katrina (2005)
10
ELDORA data
Reflectivity at 2 km
11
Convective/stratiform separation
  • Based on local gradients in reflectivity
  • Similar to Steiner et al. 1995, TRMM satellite
    data classification

Convective Stratiform Weak echo No
echo
12
2D frequency distributions
Convective pixels
Stratiform pixels
Reflectivity data
in of height total
13
2D frequency distributions
Convective pixels
Stratiform pixels
Vertical velocity data
in of height total
14
Rainband cross sections
  • Radial cross sections at regular angular
    intervals
  • 0.375 ? 109 cross sections
  • Cross section coordinates based on classification

15
Average vertical velocity
Updrafts (m s-1) Downdrafts (m
s-1)
LLD
Reflectivity (dBZ) as black contours
IED
16
Vertical velocity (m s-1) at 42.2
LLD analysis
  • Located in lower levels
  • Embedded in heavy precipitation

Reflectivity (dBZ) as black contours
17
Average downdrafts (m s-1)
LLD forcing mechanism
  • Located in lower levels
  • Embedded in heavy precipitation
  • Zipsers Convective-scale saturated downdraft
  • Forced down by precipitation drag
  • Attains negative buoyancy from continuous
    evaporative cooling

Reflectivity (dBZ) as black contours
18
Average downdrafts (m s-1)
IED analysis
  • IED investigation area 8.5 km ? 12.5 km

19
Downward vertical mass flux 2D distribution
IED analysis
in kg s-1
20
Conditional probability distribution of IED
speeds at 1 km
IED analysis
  • Condition 4.5 km-IED 3 m s-1 or gt 3 m s-1
  • Weak mid-level IED comes with weaker low-level
    IED, while strong mid-level IED comes with
    stronger low-level IED

21
Vertical velocity at 4 km
IED analysis
  • Intermittent pattern of convective-scale updraft
    and downdraft cores

22
Autocorrelation of vertical velocity, Lag 4 (
4.5 km)
IED analysis
  • Physical relationship between IEDs and updrafts

23
Reflectivity (dBZ) at 35.8
IED forcing mechanism
  • Originates above the melting level, outside of
    heavy precipitation
  • Occurs on the convective scale, rather than
    mesoscale

Overlaid by in-plane wind vectors
24
Vertical velocity (m s-1) at 35.8
IED forcing mechanism
  • Originates above the melting level, outside of
    heavy precipitation
  • Occurs on the convective scale, rather than
    mesoscale
  • Initially forced by the BPGF created by the
    updraft

Reflectivity (dBZ) as black contours
25
Reflectivity (dBZ) at 35.8
IED forcing mechanisms
  • Initially forced by the BPGF created by the
    updraft

2-step process!
26
Reflectivity (dBZ) at 35.8
IED forcing mechanisms
  • Initially forced by the BPGF created by the
    updraft
  • Attains negative buoyancy by evaporating heavy
    precipitation of rainband

2-step process!
27
Sharp inner-edge reflectivity gradient
IED effects
28
Sharp inner-edge reflectivity gradient
Reflectivity (dBZ) at 35.8
IED effects
29
Low-level wind maximum (LLWM)
Tangential wind speed (m s-1) at 35.8
IED effects
30
Tangential wind speed
Vertical velocity
Vertical vorticity
Divergence
31
Increased inward flux of angular momentum
Composite tangential wind speed from Hurricane
Floyd (1981)
Tangential wind speed (m s-1) at 35.8
Possible impacts
Barnes et al. 1983
  • LLWM lies in radial inflow
  • Increased angular momentum results in stronger
    vortex

32
Conceptual model of rainband cross section
33
Commonly observed features of principal rainband
Barnes et al. 1983
Hence and Houze 2008
  • Upwind end consists of newer, robust convective
    cells
  • Downwind end consists of older cells collapsing
    into stratiform precipitation
  • Principal rainband is often stationary relative
    to the storm center

34
Growth and sustenance of principal rainband
Vertical velocity at 42.2
Possible impacts
  • Area of divergence near surface under LLD
  • Preferred region of convergence on upwind side of
    LLD core
  • Growth of updraft on upwind end of rainband

Divergence
Convergence
LLD
Background flow
Plan view
35
Growth and sustenance of principal rainband
Possible impacts
  • Tropical storm Ophelia (2005)
  • Operational radar from Melbourne, FL
  • Discrete propagation of vertical velocity cores,
    rainband cells
  • Stationary rainband relative to storm center

Radar loop
36
Conceptual model of rainband at 2 km
37
Conceptual model of rainband at 2 km
38
Conceptual model of rainband at 2 km
39
Conclusions
  • Principal rainband contains two repeatable
    convective-scale downdrafts
  • Low-level downdraft is forced by precipitation
    drag beneath heavy precipitation
  • Inner-edge downdraft is initially forced by
    pressure perturbations created by nearby buoyant
    updrafts, then evaporative cooling
  • Vorticity dynamics of updraft and IED create a
    low-level wind maximum that leads to increased
    angular momentum of storm
  • Interaction between updraft and two downdrafts
    leads to growing and sustaining convection of
    principal rainband
  • Convective-scale features allow principal
    rainband to continue its impact on the overall
    storm

40
Future Work
  • Analyze convective-scale structures in
    high-resolution model output from RAINEX
  • Investigate outer rainbands and compare to inner
    core of storm

41
Acknowledgments
  • Bob Houze
  • Deanna Hence, Stacy Brodzik
  • Brad Smull, Tomislav Maric, Jian Yuan, Mesoscale
    Group
  • Michael Bell, Sandra Yuter
  • Beth Tully
  • Atmos Grad 2006

42
Extra Slides
43
Convective/stratiform classification
  • Technique used in Steiner et al. 1995, Yuter and
    Houze 1997, Yuter et al. 2005
  • Algorithm separates convective regions from
    stratiform regions by comparing local
    reflectivity to background reflectivity
  • Tuning of algorithm required to recognize
    convective regions the rest is designated as
    stratiform

44
Convective/stratiform classification
  • Convective center if
  • Z ? Zti
  • Z-Zbg ? Zcc(Zbg)
  • Classified convective within R(Zbg) from
    convective center, remaining is classified
    stratiform (unless Z lt Zwe)

Zti 45 dbZ Zwe 20 dbZ R 0.5.23(Zbg-20)
Rbg 11 km a9 b45
45
4 km reflectivity
46
2 km reflectivity
47
2 km vertical velocity
48
6 km vertical velocity
49
Average downdrafts for upwind half
50
Statistical significance testing
  • Two-sided Students t statistic
  • Significance level of 95
  • Null hypothesis that true autocorrelation is zero
  • Number of independent samples determined by
    formula of Bretherton et al. (1999)

51
Figure 16
52
Frequency of strong vertical velocities
Updrafts 3.0 m s-1 Downdrafts 1.5
m s-1
LLD
Reflectivity (dBZ) as colored contours
IED
About PowerShow.com