Eight Years of TRMM Data: Understanding Regional Mechanisms Behind the Diurnal Cycle

Transcription

1 Eight Years of TRMM Data: Understanding Regional Mechanisms Behind the Diurnal Cycle Steve Nesbitt, Rob Cifelli, Steve Rutledge Colorado State University Chuntao Liu, Ed Zipser University of Utah Funding provided by NASA PMM Science (Ramesh Kakar) Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 1

2 Introduction Understanding the diurnal cycle of rainfall requires comprehension of the global and local problem Precipitation at a given location often falls from storm systems that were initiated some distance from that location Since cloud systems have a life cycle, the diurnal cycle of their morphology must be understood There are common pathways in space and time of storm initiation, upscale growth, propagation, and decay which lead to the observed diurnal cycle of rainfall (MCSs are an extreme case) This study seeks to use a multi-sensor satellite approach to find these pathways Examine TRMM s high resolution multi-sensor view of the diurnal cycle of clouds and rainfall using its 8 year climatology Use the TRMM precipitation feature database to examine the diurnal cycle by system type Investigate the diurnal cycle modes over coastal and open ocean using a TRMM precipitation and TRMM ocean product combined dataset Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 2

3 From TRMM Tropicswide observations: Over ocean, all types of precipitation features produce the most rainfall at night around 6 AM, mainly controlled by MCSs Over land, the total rainfall peaks in the afternoon when the atmosphere is least stable, however MCS rainfall peaks later at night, around midnight, due to their longer life cycle Nesbitt and Zipser (2003) Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 3

4 Datasets TRMM 0.5 resolution TMI rainfall product (3G68 v.6) TRMM precipitation feature database (v.6, Nesbitt et al. 2000) Individual storms are identified where PR near surface reflectivity is! 20 dbz or TMI 85 GHz PCT " 250 K. TRMM 0.25 gridded VIRS 11.7!m T b product (G1B01, v6) TRMM 0.5 ocean products (from Remote Sensing Systems, Inc., v3) Wind speed (10 GHz) All products are 8 year seasonal composites (Jan 1998-Dec 2005) using climate seasons (DJF, MAM, JJA, SON) - 47 million PFs TRMM is one of few low-earth orbit satellites that is not sunsynchronous (repeat cycle days) Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 4

5 First diurnal harmonic of TMI rain rate (mm/hr) All seasons Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 5

6 IR counts of T b < 235 K June-September m/s Data: NOAA CPC via NASA GSFC DAAC Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 6

7 First diurnal harmonic of TMI rain rate (mm/hr) All seasons Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 7

8 Seasonal variability in TMI f r!jja DJF" Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 8

9 First diurnal harmonic of VIRS f 235K and f 210K f 235 K # f 210 K $ Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 9

10 First diurnal harmonic of VIRS 11.7µm f 235K and f 210K f 235 K # f 210 K $ Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 10

11 First diurnal harmonic of TMI 10 GHz wind speed Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 11

12 Rain rate Wind speed JJA RR - WS Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 12

13 The TRMM domain was seasonally categorized over land and ocean areas based upon the criteria below: Region PR Rain Rate PR PF Mean 20 dbz ETH Latitude Dry < 0.8 mm dy -1 - < 27.5 Intermediate Shallow 0.8 " RR < 5 mm dy -1 < 3 km (ocean) 5 km (land) < 27.5 Intermediate Deep 0.8 " RR < 5 mm dy -1! 3 km (ocean) 5 km (land) < 27.5 Wet > 5 mm dy -1 - < 27.5 Subtropical - -! 27.5 Recall: Total rain = conditional rain rate in PFs * mean PF area * number of PFs (Nesbitt and Zipser 2003) Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 13

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19 Phase of: Total PR rain rate Shallow Small cold MCS Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 19

20 Summary Identified primary source of regional variability in the diurnal cycle: organized convection MCSs are a dominant, yet complicating factor over land and ocean Open ocean and coastally-propagating regimes different: What defines their boundaries, intensity, speed, and extent? It is clear that global models will have to be able to represent many more processes better to capture the diurnal cycle properly Gravity wave propagation MCS life cycle (e.g., cold pools) Topographic convective initiation and gravity wave generation The good news is that GCMs are increasing in resolution and capability, and may be able to resolve some of these effects routinely, and CRM and RGCMs can be run and evaluated now Monday, May 1, th Conference on Hurricanes and Tropical Meteorology Slide 20