Polarimetric radar analysis of convection in northwestern Mexico

 Timothy J. Lang, Angela Rowe, Steve Rutledge, Rob Cifelli

Steve Nesbitt

Three radars in core Monsoon region during summer 2004(NAME Tier I; NW Mexico)

Network covered Gulf of California, Sierra Madre Occidental, Coastal Plain, Baja Peninsula, Pacific Ocean

S-Pol – S-Band, Polarimetric, DopplerSMN – Cabo and Guasave radars (C-Band, Doppler)

Lang et al. (2007) reduced-dimension analyses

Regime ADefined as enhanced rainfall over the Gulf and coastal plain

Regime BDefined as enhanced northward coast-parallel movement of systems

Lang et al. (2007)

Regime AB (both A and B occurring)

More rainfall, with larger, more organized features

Rowe et al. (2008)

Radar rainfall and vertical structure vs. topography

More frequent, less intense rainfall at highest elevations

Gulf ~12h out of phase with land

More intense vertical reflectivity structure over low elevations

Conceptual Model

Lang et al. (2007), Nesbitt et al. (2008), Rowe et al. (2008)

Convection forms over highest terrain, then moves westward

During disturbed regimes, convection organizes upscale, and persists after sunset into morning hours

Question: How does the microphysical structure of convection vary with terrain and meteorological regime?

Storm Microphysics~90 hours total spread over ~35 casesUsually 150-km range2-3 PPIs with 0-1 RHIs in 15 min360s @ rain-map angles (0.8,1.3,1.8°)

ClimatologyUsed most frequently; 200-km rangeFull-volume 360s, complete in 15 minRain-map angles (0.8,1.3,1.8°) & 0.0°

S-Pol 24-h Ops7/8-8/21Two Modes

Only S-Pol data used in this study

● Threshold on HV, (DP) – noise, clutter ● Threshold on ZH, ZDR – insects● Threshold on LDR, DP – second trip● 21-pt (3.15 km) FIR filter on DP, Adaptive linear fit to calculate KDP ● ZH, ZDR rain attenuation correction via DP method (Carey et al. 2000)● ZH corrected for gaseous attenuation (Battan 1973)● Rainfall from CSU blended polarimetric algorithm (Cifelli et al. 2002) (Base Z-R: Z=133R1.5 pol-tuned via Bringi et al. 2004)● Blockage corrected via Cifelli et al. (2002) and Lang et al. (2007; 2008)● Data gridded to 0.02° x 0.02° x 1 km 3-D grid, matched in horizontal to 2-D regional grids of Lang et al. (2007), using SPRINT● 15-min temporal resolution (3801 volumes for entire project)● Grids extend ~1.6° in each direction from S-Pol● Grids include all polarimetric variables plus fuzzy-logic hydrometeor ID via

Tessendorf et al. (2005)● Convective/Stratiform partitioning via Rowe et al. (2008)● Break down gridpoints by terrain (over water, 0-500 m land, 500-1500 m,

and 1500+ m MSL) to investigate possible topographic influences● Using lowest gridpoint containing rainfall, examine reflectivity, ZDR and D0,

KDP, and rainfall rate, following Carey et al. (2001)● D0 via D0 = 1.529*(ZDR)0.467

● Use Cifelli et al. (2002) to examine ice and liquid water mass contents

DATA QC

Sanity check - S-Pol domain rainfall diurnal cycle results match well with previous studies

D0 stratified by topography - Largest D0s occur most frequently over lowest elevations, and least frequently over water

Gulf has smallest D0 and ZH values over full spectrum of rainfall

Gulf - least amount of ice mass, but relatively high water massLand - Decreasing ice and water mass as elevation increases

Gulf - Reduced D0 and ZH during Regime AB

During Regime AB, all terrain bands but 1500m+ have increased ice mass

Ice

Water

Relatively large increases in water mass over Gulf and 0-500m during Regime AB

Polarimetric tuning via Bringi et al. (2004)

Use ZH, ZDR, and KDP to estimate a in Z=aR1.5

Examine distribution over all points

Regime AB

No Regime

Lower a values over Gulf during disturbed regimes, with stratification by elevation over land

In ordinary circumstances, variability in a with terrain is less obvious

Conclusions

Land• Convection has the largest D0s, along with ice and water mass, at the lowest elevations (0-500m)• At highest elevations (1500m+), convection does not change much during disturbed regimes; lowest elevations show some change in terms of ice/water mass, D0, etc.• Consistent with inference from past studies of upscale organization during westward travel

Water• Storm microphysical structure is different over the Gulf of California than over the land• Convection over water produces smaller drops via reduced ice phase microphysical processes compared to land • During disturbed regimes, both warm and cold microphysical processes become more important, and net result is smaller drop sizes• Warm rain processes more important over Gulf