Synoptic, Topographic, and Diurnal Effects
on Summer Convection in South America
Ulrike Romatschke
University of Washington, University of Vienna
Robert A. Houze Jr., Socorro Medina, Kristen Rasmussen
University of Washington
ICAM, Rastatt, Germany, May 15, 2009
Sponsored in part by:
NSF Award# ATM-0505739
NSF Award# ATM-0820586
NASA Award# NNX07AD59G
Objectives
Investigate
 Distribution of convection
 Find regions of
 maximum convective intensity
 maximum precipitation
 Forcing of convection by
 Synoptic conditions
 Topography
 Diurnal heating
 Connection with precipitation climatology
Data
 Tropical Rainfall Measuring Mission
Precipitation Radar (TRMM PR)
 3D reflectivity
 10 years (December 1998 – February 2008)
 Summer months: December, January, February
 NCEP reanalysis
 Large-scale environment
Classification of radar echo structures
within convective systems
 Deep Convective Cores
Contiguous convective echo ≥ 40 dBZ and ≥ 10 km in
height
 Wide Convective Cores
Contiguous convective echo ≥ 40 dBZ and ≥ 1000 km²
in area
 Broad Stratiform Regions
Contiguous stratiform echo ≥ 50 000 km² in area
Distribution of radar echo structures
Deep
convective
cores
Broad
stratiform
regions
Wide
convective
cores
Precip.
climatology
Regions of interest
Foothills
North
Foothills
South
La Plata
Basin
Synoptic forcing
Foothills South, La Plata Basin
Composites, wide convective cores, La Plata Basin
500 mb geopotential height anomaly
Surface pressure anomaly
[mb]
Surface winds (~02 LT)
Foothills North
Composites, wide convective cores, Foothills North
500 mb geopotential height anomaly
Surface pressure anomaly
[mb]
Surface winds (02 LT)
Diurnal and Topographic
forcing
Foothills South, La Plata Basin
Deep convective cores
Wide convective cores
Broad
stratiform
regions
Foothills North
Wide
convective
cores
Broad
stratiform
regions
Deep Convective cores
06 UTC
~02 LT
12 UTC
~08 LT
18 UTC
~14 LT
00 UTC
~20 LT
Wide convective cores
06 UTC
~02 LT
12 UTC
~08 LT
18 UTC
~14 LT
00 UTC
~20 LT
Broad stratiform regions
06 UTC
~02 LT
12 UTC
~08 LT
18 UTC
~14 LT
00 UTC
~20 LT
Foothills South, La Plata Basin
Composite winds and divergence, wide convective cores, La Plata Basin
Divergence
06 UTC
~02 LT
12 UTC
~08 LT
18 UTC
~14 LT
00 UTC
~20 LT
10 m s-1
Foothills North
Composite winds and divergence, wide convective cores
Divergence
06 UTC
~02 LT
12 UTC
~08 LT
18 UTC
~14 LT
00 UTC
~20 LT
10 m s-1
Conclusions
 Synoptic low location critical
Moisture transport by low-level jet from tropics to
subtropics
 Diurnal heating
 Daytime deep and wide convective cores in Foothills
South triggered by lifting over lower slopes
 Divergence at the Foothills North prevents convection
 Nocturnal cooling
Downslope flow and convergence at the foothills
leads to maxima in wide convective cores and broad
stratiform regions.