Data analysis and numerical modeling efforts that are related to the 1986 Satellite Precipitation and Cloud Experiment (SPACE) are discussed. Progress during this period includes the following: further testing and development of the RAMS numerical modeling system on the Alabama CRAY X-MP/24; a continuation of the observational analysis of the 13 July 1986 mesoscale convective system (MCS); and an initial investigation of a small MCS that formed over the COHMEX region on 15 July 1986. Details for each of these individual tasks are given

Knupp, Kevin R.

English

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ANALYSIS AND MODELING OF SUMMERTIME CONVECTIVE CLOUD
AND PRECIPITATION STRUCTURE OVER THE
SOUTHEASTERN UNITED STATES
Report for the period 15 September to 14 June I989
NASA Grant NAG8-654
Kevin R. Knupp
Atmospheric Science and Remote Sensing Lab
Johnson Research Center
University of Alabama in Huntsville
Huntsville, AL 35899
(NA_A-CR-I85436) ANALYSIS AND MODELING OF
SUHNERTIN_ CONVECTIVE CLOUO ANO
PRECIDITATION STRUCTURE OVER THE
SOUTHEASTFRN UNTTFD STATES Semiannual
_eport t 1_ Sep. 1988 - 14 Jun. 1989
N90-12996
Unclas
0218048
https://ntrs.nasa.gov/search.jsp?R=19900003680 2020-03-19T23:45:48+00:00Z
1. Introduction
This report describes work completed under NASA Grant NAG8-654 for the
period 15 September 1988 through 14 June 1989. This work involves primarily data
analysis and numerical modeling efforts that are related to the 1986 Satellite
Precipitation and Cloud Experiment (SPACE). Progress during this period includes
the following:
1) further testing and development of the the RAMS numerical modeling
system on the Alabama CRAY X-MP/24;
2) a continuation of the observational analysis of the 13 July 1986 mesoscale
convective system (MCS);
3) an initial investigation of a small MCS that formed over the COHMEX
region on 15 July 1986.
Details under each of these individual tasks are given in the following
sections.
2. RAMS Testing and Development
The Regional Atmospheric Modeling System, developed at Colorado State
University, was previously installed on both the Johnson Research Center VAX
11/785 and the Alabama CRAY X-MP/24 supercomputer in May 1988. Further
testing and code development has been completed since 15 September 1988. Some
effort was required to develop an efficient communication protocol between the
CRAY and VAX, such that model activities can be efficiently submitted, monitored
and returned to the VAX without direct login access to the Cray. All model
computations will be conducted on the CRAY, while limited model development
and analysis of simulations will be completed on the VAX. Since model output can
be voluminous (up to 300 Mbyte per run), an efficient data management and storage
scheme will be required.
2
5i
Because the RAMS does not contain perturbation techniques to efficiently
initiate cloud-scale circulations, some effort was required to develop realistic, yet
economical ways of producing convective clouds. Two options have been developed
so far. The first is a moisture perturbation which is designed to simulate the
t/me-integrated effects of low-level convergence, as well as the initial cloud. In this
method, a cylindrical perturbation of specified radius and height (input by the user)
contains a moisture perturbation whose core contains a constant water vapor mixing
ratio from the surface up to a maximum height. Any supersaturated value above
cloud base is forced to remain at saturation, with the excess converted to cloud
liquid water. From the edge of the core (at half radius) to the maximum radius of
the cylinder, the mixing ratio is assumed to decrease linearly to environmental
values.
The second method utilizes a scheme developed by McNider and Kopp
(1989), in which the horizontal dimension of temperature perturbations within the
ABL are scaled according to the ABL depth. Thus, this technique incorporates
realistic constraints in the size of initial thermal circulations. The technique
requires input of two parameters, surface heat flux and boundary layer depth. This
algorithm has be encoded into RAMS and is currently undergoing final testing.
A number of modeling applications are planned using the RAMS. For the 13
July MCS event, both mesoscale and cloud-scale modeling activities are planned.
Initial work will utilize a 2-D version of the model to examine the growth and
mature structure of the 13 July case. Additional 3-D simulations will be conducted
on the cloud scale in order to further understand specific cloud-scale processes and
the interaction with the mesoscale.
3
3. 13 July 1986 COHMEX Analysis Work
Several goals have been defined for this case, including (1) a general
description of the growth and structure (kinematic and precipitation) of the MCS
observed on this day; (2) a definition of the relationship between the cloud and
mesoscale flows, with a focus on variability of deep convection, and the impact of
deep convection on the MCS. These two objectives require considerable analysis of
both cloud-scale flows (using multiple Doppler radar) and mesoscale flows. The
following subsections summarize work conducted on both scales.
a) Conceptual model
A number of data sources are being analyzed in detail in order to describe
the structure of the 13 July MCS. Data examined to date include RADAP, surface
data, Doppler radar data, GOES data and rawinsonde data. One applied research
goal of this work is to relate the results of a comprehensive analysis to existing and
future satellite measurements of cloud structure and precipitation. Such
measurements require detailed knowledge of precipitation characteristics, including
type, concentration and distribution. A conceptual model illustrating the developing
and mature phase structures of the 13 July MCS presented in Fig. 1 incorporates all
data sets mentioned above. During the developing stage (2.5 h after first echo) a
broad spectrum of deep convective clouds were observed, exhibiting a range of
cloud tops (8-16 kin), associated updraft/downdraft vigor and precipitation
intensity. Only the more vigorous clouds developed significant cold low-level
outflow pools, which independently and interactively triggered secondary deep
convection, and thereby promoted horizontal expansion of the system. Three hours
later during the mature MCS phase, a uniform mesoscale anvil had formed, and
much less vigorous deep convection was confined to the leading (south) edge of the
system. Radar analyses described in more detail below meast_red the vertical and
horizontal flow magnitudes and precipitation distribution shown in Fig lb.
4
It is interesting to note that at the mature stage the highest (coldest) cloud
tops were not generally colocated with the deep convection, but occurred in some
portions of the MCS over the mesoscale anvil -50 km to the rear of the convective
line. Such patterns appear to be common within MCSs entering their mature stage
(e.g., the 10-11 June Pre-STORM squall line; Zipser, personal communication).
Additional analyses of the 13 July MCS should provide further insight on this
relation. At this point, it appears that the cold cloud tops over the mesoscale anvil
represent the location of both previously intense convection and continued
mesoscale ascent within the anvil.
b) VAD analyses
The movement of the MCS directly over the Doppler radar network allowed
for determination of mesoscale flows using VAD analyses with the CP-4 radar. A
number of such analyses have been completed during the mature to dissipating
stages of the MCS. Figure 2 shows results within the stratiform region for a 40 km
radius at 2334 UTC. The following features represent the important structural
aspects: (1) a bright-band signature appears in the reflectivity profile (Fig. 2a); (2) a
jet appears just below the bright band peak (Fig. 2b); (3) the flow is convergent at
middle levels (4-8 kin), divergent below 4 km and weakly divergent above 8 km (Fig.
2d); (4) the divergence profile thus produces weak mesoscale ascent peaking at - 10
cm/s above 6 km and descent of -30 cm/s near 4.5 kin. Such patterns are similar
to VAD measurements of both tropical and midlatitude squall lines.
At the VAD analysis time of Fig. 2, GOES IR temperatures were exhibiting a
warming trend. Thus, the anvil cloud top was sinking while ascent was occurring
within the anvil below. This relationship will be examined in greater detail with
additional analyses, and with the numerical model. Such comparisons between
trends in cloud-top temperature and mesoscale vertical motions within the cloud
below should provide greater insight on physical properties of mesoscale anvils.
5
c) Multiple Doppler analyses
The general objective of this work is to define the kinematic and
precipitation characteristics of deep precipitating convection existing within
low-shear, subtropical (continental) environments. The work on deep convection
involves both observational (primarily Doppler radar) and associated numerical
modeling components. Several cloud systems occurring within and in advance of the
13 July MCS have been examined in some detail. A summary of this work is
presented in Appendix A.
One objective is to define the specific relationships between cloud-scale
flows and mesoscale flows, within evolving mesoscale convective systems. This has
been, and will continue to be, an important but unanswered problem. In particular,
the following questions will be addressed:
(1) What are the instantaneous and time-integrated convective transports
(including entrainment and detrainment) in relation to mesoscale flows within a
MCS?
(2) What are the structural and evolutionary details of the kinematic and
precipitation structures of clouds existing within low-shear, moist environments?
Work under this category will include the development of new conceptual models
and/or refinement of existing models.
(3) In relation to both items above, how do clouds dissipate, i.e., what are the
characteristics of updrafts, downdrafts and precipitation once the cloud becomes
decoupled from low levels?
d) Miscellaneous precipitation analyses
Radar measurement of precipitation is fundamental in the observational
analyses. The C-band CP-4 radar was the primary source of surveillance scans
(acquired every 6 min) during COHMEX. However, attenuation can be significant
6
at C-band wavelengths in heavy rain. Therefore, some effort to partially correct for
attenuation has been expended. The method used applies an attenuation correction
factor to the raw data prior to interpolation to Cartesian coordinates. This method
has been thoroughly tested on the 13 July data and appears to provide a much
improved estimate of C-band attenuation. Such corrected values will be used for
both precipitation estimation and compositing techniques discussed in previous
reports under this grant.
4. Preliminary analysis of the 15 July MCS
The 15 July MCS exhibited appreciable differences in structure and
evolution from the 13 July system. It developed during the early morning hours
(-0800 UTC) and dissipated by midafternoon (-2000 UTC), a cycle which is
completely opposite of normal for the summer months in the Southeast. Such a
behavior suggests that synoptic-scale forcing was appreciable in this case. As shown
in Fig. 3, this MCS at maturity was quasi-circular in shape and approximately 100
km in diameter. In contrast to the 13 July case, deep convection was less intense,
was more randomly distributed and occupied a smaller fraction of the total
precipitation area. Thus the relative contribution from stratiform precipitation was
greater. The stratiform region displayed an echo top of - 10 km, as well as a
significant bright band. An inflow jet (-5 m/s relative speed) centered near a
height of 3 km entered the system from the northwest. The initial inspection of
kinematic structural features indicates weak mesoscale updrafts and downdrafts
within the stratiform region.
A complete analysis of this system is underway and should be completed (in
publication draft form) by late 1989. Three general objectives have been defined:
(1) Define the kinematic and precipitation structure of this MCS, using all
available data sets, and develop a conceptual model of its structure.
(2) Define the relationship of this MCS to its ambient environment.
7
(3) Define and infer the precipitation characteristics within the system (both
in convective and stratiform regions), and compare with similar results from the 13
July case.
5. Future research efforts
Planned work for the latter half of 1989 includes the following:
(1) Complete the descriptive analysis of the 13 July MCS and submit the
manuscript to Monthly Weather Review.
(2) Complete the analyses of the 15 July MCS and put together a journal
manuscript. Three students are currently conducting analyses of this case.
(3) Begin numerical modeling efforts for the 13 July case, including modeling
of cloud-scale modeling activities and mesoscale modeling activities. This work will
be carried out by a graduate student (C.L. Wu).
(4) Begin a detailed analysis of precipitation characteristics within a variety
of precipitation systems, including the 28 June, 13 July and 15 July cases. This work
is conditional upon the anticipated arrival of a post-doc (Y. Golestani) with
expertise in multi-parameter radar analysis.
8
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-120
MCS DEVELOPING STAGE, 2000 UTC (to + 2.5 h)
0_K_ _ 10
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x (kin)
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MCS MATURE STAGE, 2300 UTC (to + 5.5 h)
-120
INFLOW JET
I
-80 -40 0 40
x (kin)
8O
Figure 1. Schematic illustrating the observed cloud-scale and mesoscale features
within the 13 July MCS during(a) the developing st_tge, 2.5 h after first echo; and (b)
the mature stage, 5.5 h after first echo. Arrows denote flow magnitudes in m/s, and
vertical hatching represents precipitation.
9
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Figure 2. Results from a CP-4 VAD
analysis (40 km radius) at 2334 UTC 13
July 1986 showing profiles of (a)
reflect ivity factor averaged around the
circle; (b) mesoscale h _riz_+_:al wind
speed; (c) wind direction; (d) divergence
and (e) vertical motion.
BNA BEGIN 0=660715 T=160000 A= 0.0 E= 0.5 FIX RNG= 0.5 (DZ)X 0.0
END D=BB0715 T=160000 R=35B.O E= 0.5 FIX RNG= 0.5 EVERY 99 RNG X [ RAY
RNF=1,8-,23 3(,KM1200. , , , , , , , , , , , , , , , , , , , , , , , ,
m
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X [KM] CONTOURED I GRTES
Figure 3. Contour analysis of RADAP data from the Nashville WSR-57 radar,
located at the center of the small circle at the upper right. Contours correspond to
18, 30 and 43 dBZ. Values greater than 30 and 43 dBZ are hatched and blackened,
respectively.
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Appendix A.
Reprint of paper from the 24th Conferenceon Radar Meteorology
12


Analysis and modeling of summertime convective cloud and precipitation structure over the Southeastern United States

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