RADAR OBSERVATIONS DURING NAME 2004

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RADAR OBSERVATIONS DURING NAME 2004 PART II
PRELIMINARY RESULTS Timothy J. Lang, Stephen W.
Nesbitt, Steven A. Rutledge, and Rob
Cifelli Colorado State University, Fort Collins,
Colorado David Ahijevych and Richard E.
Carbone National Center for Atmospheric Research,
Boulder, Colorado
JP3J.6
1. Introduction
4. Defining Regimes
6. Environmental Influences
The enhanced observation period (EOP) of the
North American Monsoon Experiment (NAME Higgins
et al. 2005) took place during July and August of
2004. A major component of the EOP was
observations from a multi-radar network placed in
Tier I (Lang et al. 2005), the core monsoon
region consisting of the Gulf of California (GoC)
and the Sierra Madre Occidental mountain range
(SMO) in northwestern Mexico (Higgins et al.
2005). The primary goal of the radar network
was to characterize and understand convective and
mesoscale processes in the complex terrain of the
core monsoon region. We examine several
outstanding research questions and hypotheses
relevant to the North American Monsoon
(NAM)   a. Diurnal cycle of precipitation Radar-b
ased assessment of the spatial and temporal
variability of precipitation in a tropical region
of significant orography (e.g., the SMO) has
never been attempted before. Thus, we will use
the NAME radar network to observe and describe
statistically the regular daily cycle of
precipitating systems over the high SMO, the
western slopes, the Gulf of California coastal
plain, and the southern Gulf region. b.
Intraseasonal variability of precipitation A
major goal of NAME is to better understand
regimes associated with intra-seasonal
variability of convection during July-August in
the Tier I region and its linkages to
precipitation in the southwestern U.S., including
influences of Gulf surges, jets, easterly waves,
surface fluxes, and topographic blocking.   c.
Relative importance of organized systems for
precipitation It is hypothesized that mesoscale
convective systems and other modes of organized
convection contribute significantly to total
rainfall within the Tier I domain. We will
examine the relative importance of organized
systems as a function of both the diurnal cycle
and meteorological regime (i.e., intraseasonal
variability).   d. Role of terrain in initiating
and organizing convection It is hypothesized that
terrain plays a major role in organizing
convection over the Tier I domain. S-Pol and SMN
radar observations can be used to assess the
morphology and organization of storms relative to
major terrain features.
We observe at least two distinct precipitation
regimes, as explained by Ahijevych et al.
(2005). Figure to left shows an example of
reduced dimension time series from Ahijevych et
al. (2005). LHS is Hovmoller in cross-coast
dimension RHS is along-coast. Regime A is
mainly characterized by a coherent progression of
enhanced rainfall from the SMO to the GoC.
Precipitation over the GoC is mainly nocturnal.
Propagation is 7 m s-1. Regime B is defined as
conditions when a northward progression of
precipitation is prominent. Propagation is 10 m
s-1. Regimes A and B overlap considerably.
Regime-composite skew-T/log-p diagrams at Los
Mochis. Regime AB soundings are the moistest on
average. All Regime-averaged soundings contain
at least 1500 J kg-1 of CAPE. However, there is
a significant cap in the averaged soundings, with
40-70 J kg-1 of CIN present. It is apparent that
thermodynamic profiles are sufficient for deep
convective initiation if the cap could be broken.
Regime-averaged wind profiles composited as
above, and rotated 35 to look at coast-normal
and coast-parallel shear profiles. Steering
winds blow toward the coast at 3-4 m s-1. Values
of 0-4 km shear are larger at Mazatlan, nearly by
a factor of two during disturbed Regimes. This
increased shear may explain the prevalence of the
longer-lived convective systems over the southern
portion of the radar domain, which were observed
to last into the early morning hours.
5. Precipitation Feature Statistics
To examine large-scale flow patterns
corresponding with these Regime periods, North
American Regional Reanalysis data (NARR
Mesinger et al. 2005) are used. During Regime AB,
the radar domain is located in close proximity to
a tropical easterly wave (EW) trough (in a
composite sense).
Time series of PF statistics. During disturbed
Regimes, there are increases in feature rainfall,
max dimension, and organized feature fraction.
Mean convective fraction is approx. constant.
Diurnal cycle of PF stats by Regime. Regime AB
behavior is similar to Regime A or Regime B only.
Bi-modal behavior - early morning and evening
peaks
2. Methodology
7. Conclusions
Terrain played a key role in initiating and
organizing convection in our study domain.
Precipitation along the SMO initiates in the late
afternoon and advects west toward the GoC, where
it peaks overnight. The GoC maximum mainly
occurs in the south, under disturbed
meteorological conditions (Regimes A and B), when
there is more shear. During propagation there is
gradual growth in size of PFs. During the NAME
EOP, there were at least two significant Gulf
surges (Higgins et al. 2005). Gulf surges tended
to overlap with disturbed precipitation Regimes.
But there were many disturbed Regime periods not
associated with a Gulf surge. However, this study
suggests a possible link between tropical EWs and
precipitation regimes.   The development and
propagation of organized systems (e.g., MCSs and
other large precipitation features) are a key
component of the diurnal cycle of precipitation
in this region, particularly during disturbed
Regimes. Organized systems are particularly
important for rainfall in the evening along the
SMO, and during the early morning along the
southern portions of the coast and GoC.
Radar composite domain (Lang et al. 2005) rotated
and cropped to examine along-coast and
cross-coast variability. Exclude Baja and
Pacific Ocean areas due to potential sea clutter
problems with Version 1 composites.
Use Precipitation Feature (PF) analysis technique
based on Nesbitt et al. (2000). Contiguous areas
(including corner pixels) of composite equivalent
radar reflectivity 15 dBZ are considered as a
PF within each composite. An ellipse-fitting
procedure, developed by Nesbitt et al. (2005),
was applied to each PF to objectively estimate
the maximum dimension (i.e. twice the major axis
length of each ellipse) of each feature.
Features were subsetted geographically, by regime
(defined below), and by their characteristics. A
subset of PFs has been identified as organized
features. These features meet the criteria that
their maximum horizontal dimension is 100 km,
and the storm contains at least 16 km2 of
convective area as determined by
convective-stratiform separation.
Diurnal cycle of PF stats by cross-coast
location. SMO Peaks and Foothills peak in
evening, while GoC peaks in morning. Coastal
Plain has combination of signals from afternoon
sea-breeze convection and evening PFs drifting
off SMO.
Diurnal cycle of PF stats by along-coast
location. Morning signal only important in
southern portion of domain.
8. References
Location of all organized features during the
day. Organized features mainly appear over land
during afternoon, with some propagation toward
coast. During undisturbed periods organized
features are mostly confined to land. The
majority of over-Gulf early morning organized
features occur during the disturbed AB Regime,
and in the south.
Ahijevych, D. A., R. E. Carbone, T. J. Lang, and
A. V. Manzanillo, 2005 The diurnal cycle of
rainfall and the identification of rainfall
regimes within the North American monsoon of NW
Mexico. 32nd Conf. on Radar Meteorol., Amer.
Meteorol. Soc., Albuquerque, NM. Higgins, W., et
al., 2005 The North American Monsoon Experiment
(NAME) 2004 field campaign and modeling strategy.
Submitted to Bull. Amer. Meteorol. Soc. Lang, T.
J., R. Cifelli, L. Nelson, S. W. Nesbitt, G.
Pereira, S. A. Rutledge, D. Ahijevych, and R.
Carbone, 2005 Radar observations during NAME
2004. Part I Data products and quality
control. 32nd Conf. on Radar Meteorol., Amer.
Meteorol. Soc., Albuquerque, NM.  Mesinger, F.,
G. DiMego, E. Kalnay, K. Mitchell, P. C. Shafran,
W. Ebisuzaki, D. Jovic, J. Woollen, E. Rogers, E.
H. Berbery, M. B. Ek, Y. Fan, R. Grumbine, W.
Higgins, H. Li, Y. Lin, G. Manikin, D. Parrish,
and W. Shi, 2005 North American regional
reanalysis. Bull. Amer. Meteor. Soc, in
revision. Nesbitt, S. W., E. J. Zipser, and D. J.
Cecil, 2000 A census of precipitation features
in the Tropics using TRMM Radar, ice
scattering, and lightning observations. J.
Climate, 13, 4087-4106. Nesbitt, S. W., R.
Cifelli, and S. A. Rutledge, 2005  Storm
morphology and rainfall characteristics of TRMM
precipitation features.  Submitted to Mon. Wea.
Rev. 
Composite reflectivity from 5 August 2004 at 2315
UTC (1715 LT). The best-fit ellipse of each
feature is plotted over each identified PF within
the composite.
Contact Info Timothy Lang, CSU Atmospheric
Science, Ft Collins, CO 80523 (970)
491-6944, tlang_at_atmos.colostate.edu