# Presentation: 502363

## New Results on the “Weekend Effect” for Summer Storms over Eastern U.S.

**Thomas L. Bell****NASA / Goddard Space Flight Center**

- Daniel Rosenfeld, Kyu-Myong Kim, Jung-Moon Yoo, Myong-In Lee, and Maura Hahnenberger

**Pollution varies with day of week.**
**Aerosol pollution affects rainfall.**
**Does storm behavior vary with the day of the week?**

**New Results on the “Weekend Effect” for ****Summer Storms over Eastern U.S.**

## Background (see Bell et al. 2008, JGR)

- Weekly cycle of U.S. aerosol concentration is well studied and documented (lower emissions on weekends).
- Both data and modeling studies show that aerosols can affect precipitation (suppression or invigoration).
- We used Tropical Rainfall Measuring Mission (TRMM) _microwave_ and _radar_ data to look for weekly cycle in rain over U.S. region (summers, 1998–2005).  We saw:
-  Midweek increase in afternoon rain over SE U.S.
-  Midweek increase in storm heights.
- NCEP reanalysis data and rain-gauge data consistent with TRMM, and suggest that
- Weekly cycle may have strengthened during past few decades.

## Weekly Cycle of Large-Scale Averages

## Weekly Cycle of Large-Scale TMI Averages

## Weekly Cycle of Large-Scale TMI Averages

- _Data:_	*r*(*t*)  [*t*  =  time in days].
- _Hypothesis:_	*r(t)**	*~	*r*0 + *r**7* cos[ ***7** *( *t – ****7** *) ] ,
- 				***7* = 2*** / *(*7 *d)

- Looking for cycles with sinusoidal fits:

## Weekly Cycle of Large-Scale Averages

**Weekly Cycle of Large-Scale Averages**

- Southeast U.S. (Area B) Summer Afternoon Rain

## Weekly Cycle of Large-Scale Averages

- Southeast U.S. (Area B) Summer Afternoon Rain

## Weekly Cycle of Large-Scale Averages

- Southeast U.S. (Area B) Summer Afternoon Rain

## Weekly Cycle of Large-Scale Averages

- Southeast U.S. (Area B) Summer Afternoon Rain

## Weekly Cycle of Large-Scale Averages

- Southeast U.S. (Area B) Summer Afternoon Rain

## Weekly Cycle of Large-Scale Averages

- Southeast U.S. (Area B) Summer Afternoon Rain

## Weekly Cycle of Large-Scale Averages

- Southeast U.S. (Area B) Summer Afternoon Rain

## Weekly Cycle of Large-Scale Averages

- Southeast U.S. (Area B) Summer Afternoon Rain

## Weekly Cycle of Large-Scale Averages

- Southeast U.S. (Area B) Summer Afternoon Rain

## Weekly Cycle of Large-Scale Averages

- For 8 of 10 summers, fits of weekly cycle to _afternoon_ TRMM rainfall over southeast U.S. peak on Tuesday or Wednesday.

## Weekly Cycle of Large-Scale Averages

- Comparison with “TRMM Multsatellite Precipitation Analysis” (TMPA = “3B42”) 3-hourly product for 1500–2400 LT

- TMI

- TMPA

## TMI vs. TMPA Weekly Cycle (SE U.S.)

- (Note: Areas, times averaged over are
- not precisely the same.)

## TRMM Radar Storm-Height ( ~ 18 dBZ)Evidence of Energized Convection

- _Per storm_, 40% more midweek storms > 9 km than on weekend.

**Notes:**
Fractional rainy area for different periods:
SSM		TWT		SSMmorn		TWTmorn		SSMaft	TWTaft
0.0355088	0.0409762	0.0297210	0.0244783	0.0414749	0.0578555

## Weekly Cycle in Lightning

- U.S. National Lightning Detection Network (Cloud-to-ground strikes)
- Daily flash rates gridded to 8 km x 8 km by Global Hydrology Resource Center (GHRC)*
- Daily product used: 0000–2400 UT.(Local time 6 h earlier than nominal date!)
- Averaged over 100–80W, 32.5–40N (~ Area B).

- *NLDN data provided by the NASA Lightning Imaging Sensor (LIS) instrument team and the LIS data center via the Global Hydrology Resource Center (GHRC) located at the Global Hydrology and Climate Center GHCC), Huntsville, Alabama through a license agreement with Global Atmospherics, Inc (GAI).

- DATA:

## Weekly Cycle in Lightning

- Weekly cycle significance level *p* = 0.084
*r**7** */ *r**0* = 13%

## Weekly Cycle in Terra MODIS Data (“Coll. 5”)

- Data for summers 2000–2005 over SE U.S. (no coasts)

## What Is Causing the Weekly Cycle?

- _Hypothesis:_
- Aerosols reduce droplet size at start of convection ==>
- 	No immediate rain-out ==>
- 		Lighter droplets rise higher ==>
- 			More freezing ==>
- 				Energized convection.
- In moist environment (SE U.S. summer), ample moisture supply fuels further convection.

## “Weekend effect” is strong enough to see in TRMM data. 
 Afternoon changes dominant (over land).
Only obvious in summer months.
Strongest over inland SE U.S. (no TRMM data > 40N).
Cycle reverses over nearby waters (dynamical suppression?).
Storms reach higher altitudes Tue–Thu than Sat–Mon.
More clouds, and higher (colder) clouds, Tue–Thu (MODIS).
More midweek lightning.
Data are consistent with theory and model studies, which suggest that pollution intensifies storms in moist, unstable environments.  (Smaller drops       more ice aloft        more latent heat release.)

-  Afternoon changes dominant (over land).
- Only obvious in summer months.
- Strongest over inland SE U.S. (no TRMM data > 40N).
- Cycle reverses over nearby waters (dynamical suppression?).
- Storms reach higher altitudes Tue–Thu than Sat–Mon.
- More clouds, and higher (colder) clouds, Tue–Thu (MODIS).
- More midweek lightning.
- Data are consistent with theory and model studies, which suggest that pollution intensifies storms in moist, unstable environments.  (Smaller drops       more ice aloft        more latent heat release.)

**Summary**

## The End

**Acknowledgements**

*Many thanks to P. Kundu, David Atlas, Allen Chu, George Huffman, Eugenia Kalnay, Zev Levin, ** Bob Meneghini, Richard Orville, Owen Phillips, Siegfried Schubert, Yogesh Sud, and the TRMM program and Science Team.*
- This research was supported by NASA’s Office of Earth Science as part of the Tropical Rainfall Measuring Mission.  Special thanks to the Data Mining Effort at the Goddard Earth Sciences DAAC, especially James M. McManus, William L. Teng, and Long Pham, who helped us assemble and process the data used in this study.

## Backup Slides

## TRMM Radar Evidence of Energized Convection

- Ratio of midweek (Tue–Thu) to weekend (Sat–Mon) storms

**Notes:**
The TMI data show that the ratios TWT/SSM for frac are, for the morning, 0.81, and for the afternoon, 1.25 (so not as big as for the PR).

## Rain Gauge and NCEP Reanalysis Data

- Rain gauge data: Global Historical Climatology Network, daily, v. 1.0, courtesy of the National Climatic Data Center.
- Reanalysis data: National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), v. 1 (Kalnay et al., 1996) and v. R-2 (Kanamitsu et al., 2002).
- 24-HOUR AVERAGES!

## Historical Behavior of Weekly Cycle

- Area-B Rain-Gauge Avg.
*r**7** / ****7*

- Area-B NCEP1 Avg.
*r**7** / ****7*

- Weekly cycle stronger in last few decades?